Tuesday, December 30, 2008

Into the Fire

Closer to the protosun, in the hottest regions of the forming solar system, it was the heaviest elements, not ices and gases, that survived to form the planets. Thus the terrestrial planets are rich in the elements silicon, iron, magnesium, and aluminum. The dust grains and then planetesimals from which these planets were formed were rocky rather than icy. It is fortunate that water ice and organic compounds later rained down on the early Earth, or the present-day planet would be as lifeless as the moon.

Out of the Frying Pan

As the solar nebula contracted and flattened into its pancake-like shape, gravitational energy was released in the form of heat, increasing its temperature. Due to the inverse-square law of gravitational attraction, matter piled up mostly at the center of the collapsing cloud. The density of matter and the temperature were highest near the center of the system, closest to the protosun, and gradually dropped farther out into the disk.
At the very center of the nascent solar system, where heat and density were greatest, the solar mass coalesced. In this very hot region, the carefully assembled interstellar dust was pulled apart into its constituent atoms, while the dust in the outer regions of the disk remained intact. Once the gravitational collapse from a cloud to a disk was complete, the temperatures began to fall again, and new dust grains condensed out of the vaporized material toward the center of the solar system. This vaporization and recondensation process was an important step in the formation of the solar system, because it chemically differentiated the dust grains that would go on to form the planets. These grains originally had a uniform composition. In the regions nearest the protosun—where temperatures were highest—metallic grains formed, because metals survived the early heat. Moving farther out, silicates (rocky material), which could not survive intact close to the protosun, were condensed from the vapor. Farther out still, there were water ice grains, and, even farther, ammonia ice grains. What is fascinating to realize is that the heat of the protosun depleted the inner solar system (which is home to the earth) of water ice and organic carbon compounds. These molecules, as we will see, survived in the outer solar system and later rained onto the surfaces of the inner planets, making one of them habitable.
The composition of the surviving dust grains determined the type of planet that would form. Farthest from the sun, the most common substances in the preplanetary dust grains were water vapor, ammonia, and methane, in addition to the elements hydrogen, helium, carbon, nitrogen, and oxygen—which were distributed throughout the solar system. The jovian planets, therefore, formed around mostly icy material. And in the cooler temperatures farthest from the protosolar mass, greater amounts of material were able to condense, so the outer planets tended to be very massive. Their mass was such that, by gravitational force, they accreted hydrogen-rich nebular gases in addition to dust grains. Hydrogen and helium piled onto the outer planets, causing them to contract and heat up. Their central temperatures rose, but never high enough to trigger fusion, the process that produces a star’s enormous energy. Thus the jovian worlds are huge, but also gaseous.

Whipping Up the Recipe

While there is substantial variety among the nine planets, they tend to fall into two broad categories: the large gaseous outer planets, known as the jovians, and the smaller rocky inner planets, the terrestrials. Why this particular differentiation?
As with just about any recipe in any kitchen, part of the difference is caused by heat.

Tuesday, December 16, 2008

Accretion and Fragmentation

The preplanetary clumps grew by accretion from objects that might be imagined to be the size of baseballs and basketballs to planetesimals, embryonic protoplanets several hundred miles across. The early solar system must have consisted of millions of planetesimals. While smaller than mature planets, the planetesimals were large enough to have sufficiently powerful gravitational forces to affect each other. The result was near misses and collisions that merged planetesimals into bigger objects, but also fragmentation, as collisions resulted in chunks of some planetesimals breaking off. As we saw in the last chapter, the formation of the moon likely happened at this point in the history of the solar system.
The larger planetesimals, with their proportionately stronger gravitational fields, captured the lion’s share of the fragments, growing yet larger, while the smaller planetesimals joined other planets or were “tossed out.” A certain number of fragments escaped capture to become asteroids and comets.
Unlike the planets, whose atmospheres and internal geological activity (volcanism and tectonics) would continue to evolve matter (the earth, for example, has rocks and minerals that vary greatly in age), asteroids and comets remained geologically static, dead; therefore, their matter, unchanging, marks well the date of solar system birth.

Birth of the Planets

Let’s put the nebular theory and the condensation theory together, as most current astronomers do.
Here is a possible portrait of the formation of our solar system: A cloud of interstellar dust, measuring about a light-year across, begins to contract, rotating more rapidly the more it contracts. With the accelerating rotation comes a flattening of the cloud into a pancake-like disk, perhaps 100 A.U. across—100 times the current distance between the earth and the sun.
The original gases and dust grains that had formed the nebular cloud have contracted into condensation nuclei, which begin to attract additional matter, forming clumps that rotate within the disk.
The clumps encounter other clumps and more matter, growing larger by accretion. Accretion is the gradual accumulation of mass, and usually refers to the building up of larger masses from smaller ones through the mutual gravitational attraction of matter.

Pearls the Size of Worlds

Beginning in the 1940s, astronomers returned to the idea of the solar nebula to create a modification of it called the condensation theory.
There were critics of the nebular theory in the nineteenth century, among them James Clerk Maxwell, who had figured out the fundamentals of electromagnetic radiation. What Maxwell and the other critics of the Kant-Laplace theory didn’t know about was interstellar dust. Microscopic dust grains—ice crystals and rocky matter—formed in the cooling atmosphere of dying stars, then grew by attracting additional atoms and molecules of various gases. These dust grains served two purposes in the formation of planets:
  1. The presence of grains hastened the collapse of the nebular cloud by promoting the radiation of heat from it. This radiation of heat cooled the cloud, accelerating its collapse.
  2. Each grain acted as a condensation nucleus, like the grain of sand in an oyster that eventually becomes a pearl. These grains eventually grew into pearls the size of worlds. In effect, these grains were planetary seeds.

Angular Momentum Explained

Most importantly, Laplace introduced conservation of angular momentum to the discussion of planetary formation. He demonstrated mathematically that the solar nebula—the gaseous mass that would become the solar system—would spin faster as it contracted. Anyone who has watched an ice skater spinning knows this is true. As a skater pulls in his arms, bringing his mass closer to his axis of rotation, he will spin faster. If he were to put his arms out at his side, his rotation would slow. Newton described how all objects with mass were mutually attracted. As the cloud of gas that eventually formed the solar system started to collapse, it would have to rotate faster and faster to conserve angular momentum. And, as the speed of rotation increased, the shape of the solar nebula would change, becoming the pancake-like disk Kant had first pictured. Think of that the next time you watch the local pizza maker throw dough in the air, making it spin, flatten, and strech all at once.
Laplace theorized that as the spinning disk contracted, it would form concentric rings, each of which would clump together into a “protoplanet” (a sort of embryonic planet), which ultimately developed into a mature planet. The center of the disk (in this picture) would coalesce into a hot, gaseous “protosun,” which ultimately became the sun.

Sunday, November 30, 2008

From Contraction to Condensation

In 1755, the great German philosopher Immanuel Kant (1724–1804) theorized that the solar system had begun as a nebula—a cloud of dust and gas—that slowly rotated, gradually contracting until it became flattened into a spinning disk that variously coalesced into the sun and planets.
Later, in 1796, the French astronomer and mathematician Pierre-Simon Laplace (1749–1827) suggested a similar hypothesis, though he thought the planets formed before the sun.

What Do We Really Know About the Solar System?

In a very real sense, then, we do have—in meteorites and moon rocks—“witnesses” to the creation of the solar system. These geological remnants are relatively unchanged from the time the solar system was born. But how do we make up for an absence of precedents from which to draw potentially illuminating analogies? Why can’t we just go find another planetary system forming (around a star younger than the sun) and draw our analogies from it? Well, that has been one of the main goals of astronomers in the past decade or so. In fact, NASA has defined one of its primary missions in terms of this search, called the Origins program. We are just now starting to see the results of these searches. The Hubble Space Telescope, in particular, has given us tantalizing clues about the formation of planetary systems. Around the star Beta Pictoris, astonomers have imaged a disk of material larger than the orbit of the most distant planet in our solar system, Pluto. Are the inner reaches of this disk even now taking shape as planets around that star?
The truth is, even with the best instruments that we have today, we can still learn a lot more about how our solar system formed by looking closer to home. There are a number of fundamental things that we know about our solar system, and any explanation that we come up with must, at the very least, account for what we observe. Here are some undeniable facts that the last 300 years of planetary exploration have given us:
  • Most of the planets in the solar system rotate on their axis in the same direction as they orbit the sun (counterclockwise as seen from the North Pole of Earth), and their moons orbit around them in the same direction.
  • The planets in the inner reaches of the solar system are rocky and bunched together, and those in the outer part are gaseous and widely spaced.
  • Most of the planets (with the exception of Pluto) orbit the sun in elliptical paths that are very nearly circles.
  • Except for the innermost planet (Mercury) and the outermost (Pluto), the planets orbit in approximately the same plane (near the ecliptic), and they all orbit in the same direction.
  • Asteroids and comets are very old, and are located in particular places in the solar system. Comets are found in the Kuiper Belt and Oort Cloud, and asteroids in the asteroid belt between Mars and Jupiter.
In addition, it is clear that the asteroids we have examined are some of the oldest unchanged objects in the solar system, and that comets travel in highly elliptical orbits, originating in the far reaches of the solar system. The most important conclusion we can draw from these observations is that the solar system appears to be fundamentally orderly rather than random. It doesn’t appear that the sun formed first, and then gradually captured its nine planets from surrounding space.
Although there are important exceptions, the counterclockwise”
(as viewed from above the North Pole of the Earth) aspect of so many properties in the solar system suggests that the planets fragmented and formed from a large rotating cloud of material. That the orbits of the remaining planets are very nearly circular suggests that the solar system has settled down, as it were. Any planets or planetesimals that were on highly elliptical orbits have been cleared out in the last 4.6 billion years. The inclined, markedly elliptical orbit of Pluto is one of the arguments for its being an escaped moon from an outer planet. The physical differences in planets that are related to their distances from the sun suggests that the sun influenced the formation of the planets; that is, the sun must have formed first.

The Biggest Problem: We Weren’t There

Of course, there was no one around to record the series of events that created the solar system. But there are a few fragments that survive from those early moments, like the years-old crumbs from behind the sofa, that give us clues to how the planets took shape around the youthful sun. The most important clues to the origin of the solar system are to be found not in the sun and planets, but in those untouched smaller fragments: the asteroids, meteoroids, and some of the planetary moons (including our own). These objects make up the incidental matter, the debris of the solar system, if you will.
This solar system debris, though rocky, is not mute. As the English Romantic poet William Wordsworth (1770–1850) wrote of hearing “sermons in stones,” so modern astronomers have extracted eloquent wisdom of a different kind from meteorites as well as “moon rocks.”
Atoms are made up of three basic particles, protons and neutrons in the nucleus, orbited by electrons. Most elements exist in different atomic forms, which, while identical in their chemical properties, differ in the number of neutral particles (neutrons) in the nucleus. Deuterium and Tritium are famous radioactive isotopes of the more familiar hydrogen.
For a single element, these atoms are called isotopes. Through a natural process called radioactive decay, a specific isotope of one atom is converted into another isotope at a constant and known rate, often over many millions of years, depending on the element and isotope involved.
Using a device called a mass spectrometer, scientists can identify the “daughter” atoms formed from the “parent” atoms in a sample, such as a meteorite. If the decay rate of the elements in the sample is known, then the ratio of daughter atoms to parent atoms (called isotopic ratios), as revealed by the instrument, betrays the age of the sample. The results of this dating process have been remarkably consistent. Most meteors and moon rocks (which are the only “bits” of the solar system other than the earth we have been able to study exhaustively ) are from between 4.4 and 4.6 billion years old, which has led scientists to conclude with a high degree of confidence that the age of the solar system is about 4.6 billion years.

Thursday, November 13, 2008

Solar System History

Historians of human events generally enjoy two advantages over would-be historians of the solar system. Those who chronicle human history often have records, even eyewitness reports, and they have the availability of precedent events and subsequent events. For example, a historian of the American Civil War not only has a wealth of eyewitness accounts to draw on, but may also look to civil wars both before and after 1861 to 1865 to help explain the War between the States. Comparing and contrasting the American Civil War to the English civil wars of the seventeenth century or the Russian Civil War of the twentieth may help illuminate analysis and make explanations clearer.
The solar system historian lacks both witnesses and precedents. But she has several advantages as well. Human history is complicated by the infinite depths of the human mind. But if we understand the fundamental laws of physics, and make good observations of our solar system today, a recounting of the early history of the solar system should be within our grasp.

And What’s Inside the Moon?

Geologically, the moon is apparently as dead as it is biologically. Astronauts have left seismic instruments on the lunar surface, which have recorded only the slightest seismic activity, barely perceptible, in contrast to the exciting (and sometimes terribly destructive) seismic activity common on Earth and some other bodies in the solar system. It is believed, then, that the interior of the moon is uniformly dense, poor in heavy elements (such as iron) but high in silicates. The core of the moon, about 250 miles (402 km) in diameter, may be partially molten. Around this core is probably an inner mantle, perhaps 300 miles (483 km) thick, consisting of semisolid rock, and around this layer, a solid outer mantle some 550 miles (885 km) thick. The lunar crust is of variable thickness, ranging from 40 to 90 miles (64 to 145 km) or so.
The moon is responsible for everything from the earth’s tides to the length of our day, and perhaps the presence of seasons. Most astronomers think that the moon is with us today because of a gigantic collision early in the life of the solar system.
The moon’s gravity pulls tides across the earth’s surface, and its presence has slowed the rotation of the earth from a frenetic 6 hours to our current 24. Think of that next time you see the moon shining peacefully over your head.

Pocked Face Moon

Look at the moon through even the most modest of telescopes—as Galileo did—and you are impressed first and foremost by the craters that pock its surface. Most craters are the result of asteroid and meteoroid impacts. Only about a hundred craters have been identified on Earth, but the moon has thousands, great and small. Was the moon just unlucky? No. Many meteoroids that approach Earth burn up in our atmosphere before they strike ground. And the traces of those that do strike the ground are gradually covered by the effects of water and wind erosion as well as by plate tectonics. Without an atmosphere, the moon has been vulnerable to whatever comes its way, and preserves a nearly perfect record of every impact it has ever suffered.
Meteoroid collisions release terrific amounts of energy. Upon impact, heat is generated,
melting and deforming the surface rock, while pushing rock up and out and creating an ejecta blanket of debris, including large boulders and dust. It is this ejected material that covers the lunar surface.
It is believed that the rate of meteoroid impact with the moon (and with other objects in the solar system) was once much higher than it is now. The rate dropped sharply about 3.9 billion years ago— at the end of the period in which it is believed that the planets of the solar system were formed—and, some time later, lunar volcanic activity filled the largest craters with lava, giving many of them a smooth-floored appearance.

Moon, The Green Cheese?

Even with the naked eye, the moon doesn’t look particularly green. And Neil Armstrong confirmed that the surface of the moon was more dusty than cheesy. On any night that the moon is visible, the large, dark maria are clearly visible. These are vast plains created by lava spread during a period of the moon’s evolution marked by intense volcanic activity. The lighter areas visible to the naked eye are called highlands.
Generally, the highlands represent the moon’s surface layer, its crust, while the maria consist of much denser rock representative of the moon’s lower layer, its mantle. The surface rock is fine-grained, as was made dramatically apparent by the image of the first human footprint on the moon. The mare resemble terrestrial basalt, created by molten mantle material that, through volcanic activity, swelled through the crust.
The mass of the moon is insufficient for it to have held on to its atmosphere. As the sun heated up the molecules and atoms in whatever thin atmosphere the moon may have once had, they drifted away into space.
With no atmosphere, the moon has no weather, no erosion other than what is caused by asteroid impacts—and no life. While it was thought that the moon had absolutely no water, recent robotic lunar missions have shown that there may be water (in the form of ice) in the permanent shadows of the polar craters.

Earth & Moon: Give and Take

Newton proposed that every object with mass exerts a gravitational pull or force on every other object with mass in the universe. Well, the earth is much more (80more) massive than the moon, which is why the moon orbits us, and not we it. (If you want to get technical, we both actually orbit an imaginary point called the center of mass.) However, the moon is sufficiently massive to make the effects of its gravitational field felt on the earth.
Anyone who lives near the ocean is familiar with tides. Coastal areas experience 2 high and 2 low tides within any 24-hour period. The difference between high and low tides is variable, but, out in the open ocean, the difference is somewhat more than 3 feet. If you’ve ever lifted a large bucket of water, you know how heavy water is. Imagine the forces required to raise the level of an entire ocean 3 or more feet! What force can accomplish this?
The tidal force of gravity exerted by the moon on the earth and its oceans. The moon and the earth mutually pull on each other; the earth’s gravity keeping the moon in its orbit, the moon’s gravity causing a small deformity in the earth’s shape.
This deformity results because the moon does not pull equally on all parts of the earth. It exerts more force on parts of the earth that are closer, and less force on parts of the earth that are farther away. Just as Newton told us: Gravitational forces depend on distance. These differential or tidal forces are the cause of the earth’s slightly distorted shape—it’s ovoid rather than a perfect sphere— and they also make the oceans flow to two locations on the earth: directly below the moon, and on the opposite side. This flow causes the oceans to be deeper at these two locations, which are known as the tidal bulges. The entire Earth is pulled by the moon into a somewhat elongated—football—shape, but the oceans, being less rigid than the earth, undergo the greatest degree of deformity.
Interestingly, the side of the earth farthest from the moon at any given time also exhibits a tidal bulge. This is because the Earth experiences a stronger gravitaional pull than the ocean on top of it, and the Earth is “pulled away” from the ocean on that side. As the Earth rotates beneath the slower-moving moon, the forces exerted on the water cause high and low tides to move across the face of the earth.
The tides of largest range are the spring tides, which occur at new moon, when the moon and the sun are in the same direction, and at full moon, when they are in opposite directions. The tides of smallest range are the neap tides, which occur when the sun and the moon are at 90 degrees to one another in the sky. Tides affect us every day, of course, especially if you happen to be a sailor or a fisherman. But even if you live high and dry in Kansas or Nebraska, say, tides (and the moon) still affect you. Every day, the earth is spinning a little slower on its axis because of the moon. The earth’s rotation is slowing down at a rate that increases the length of a day by approximately 2 milliseconds (2/1,000 of a second) every century. Over millions of years, though, this slowing effect adds up. Five hundred million years ago, a day was a little over 21 hours long, and a year (1 orbit of the sun) was packed with 410 days. When a planetesimal plowed into the earth early in the history of the solar system, it was rotating once every 6 hours. (And you think there aren’t enough hours in the day now!)
How does this happen? Well, let’s think again of why tides occur. The moon’s gravity causes two bulges to form in the earth’s oceans, and the earth rotates (once every 24 hours) beneath that bulge. As the earth spins, friction between it and the oceans tends to pull the high tide ahead, so that the “bulge” actually leads the position of the moon overhead.
With the ocean’s bulge thus slightly ahead of the moon’s position, the moon’s gravity exerts a force that tends to slow rotation. Eventually, the earth’s rotation will slow sufficiently to become synchronized with the orbit of the moon around the earth. When that happens, the moon will always be above the same point on Earth, and the earth’s rotation period will have slowed (billions of years from now) from its present 24 hours to 47 days.
But that is only half of the picture. The earth can’t be slowing down without something else speeding up (as a result of one of the fundamental conservation laws of physics). What’s speeding up? The moon. And what does that mean? That it’s spiraling away slowly, and getting smaller and smaller in the sky.

Thursday, October 30, 2008

Tidal Forces and Moon

Newton proposed that every object with mass exerts a gravitational pull or force on every other object with mass in the universe. Well, the earth is much more (80more) massive than the moon, which is why the moon orbits us, and not we it. (If you want to get technical, we both actually orbit an imaginary point called the center of mass.) However, the moon is sufficiently massive to make the effects of its gravitational field felt on the earth.
Anyone who lives near the ocean is familiar with tides. Coastal areas experience 2 high and 2 low tides within any 24-hour period. The difference between high and low tides is variable, but, out in the open ocean, the difference is somewhat more than 3 feet. If you’ve ever lifted a large bucket of water, you know how heavy water is. Imagine the forces required to raise the level of an entire ocean 3 or more feet! What force can accomplish this?
The tidal force of gravity exerted by the moon on the earth and its oceans. The moon and the earth mutually pull on each other; the earth’s gravity keeping the moon in its orbit, the moon’s gravity causing a small deformity in the earth’s shape.
This deformity results because the moon does not pull equally on all parts of the earth. It exerts more force on parts of the earth that are closer, and less force on parts of the earth that are farther away. Just as Newton told us: Gravitational forces depend on distance. These differential or tidal forces are the cause of the earth’s slightly distorted shape—it’s ovoid rather than a perfect sphere— and they also make the oceans flow to two locations on the earth: directly below the moon, and on the opposite side. This flow causes the oceans to be deeper at these two locations, which are known as the tidal bulges. The entire Earth is pulled by the moon into a somewhat elongated—football—shape, but the oceans, being less rigid than the earth, undergo the greatest degree of deformity.
Interestingly, the side of the earth farthest from the moon at any given time also exhibits a tidal bulge. This is because the Earth experiences a stronger gravitational pull than the ocean on top of it, and the Earth is “pulled away” from the ocean on that side. As the Earth rotates beneath the slower-moving moon, the forces exerted on the water cause high and low tides to move across the face of the earth.
The tides of largest range are the spring tides, which occur at new moon, when the moon and the sun are in the same direction, and at full moon, when they are in opposite directions. The tides of smallest range are the neap tides, which occur when the sun and the moon are at 90 degrees to one another in the sky. Tides affect us every day, of course, especially if you happen to be a sailor or a fisherman. But even if you live high and dry in Kansas or Nebraska, say, tides (and the moon) still affect you. Every day, the earth is spinning a little slower on its axis because of the moon.

Impact theory of moon

The favored theory today combines elements of the daughter theory and the capture theory in something called an impact theory. Most astronomers now believe that a very large object, roughly the size of Mars, collided with the earth when it was still molten and forming. Assuming the impact was a glancing one, it is suggested that shrapnel from the earth and the remnant of the other planetesimal (a planet in an early stage of formation) were ejected and then slowly coalesced into a stable orbit that formed the moon.
This model is also popular because it can explain some unusual aspects of the earth (the “tip” of its rotational axis, perhaps) and the moon. In the impact model, it is further theorized that most of the iron core of the Mars-sized object would have been left behind on the earth, eventually to become part of the earth’s core, while the material that would coalesce into the moon acquired little of this metallic core. This model can explain why the earth and moon share similar mantles (outer layers), but apparently differ in core composition.

Captive theory of moon

A third theory suggests that the moon was formed independently and far from the earth, but was later captured by the earth’s gravitational pull when it came too close.
This theory can account for the differences in composition between the earth and the moon, but it does not explain how the earth could have gravitationally captured such a large moon. Indeed, attempts to model this scenario with computer simulations have failed. Moreover, while the theory accounts for some of the chemical differences between the earth and moon, it does not explain the many chemical similarities that also exist.

A sister theory of moon

Another theory holds that the moon formed separately near the earth from the same material that formed the earth. In effect, the earth and the moon formed as a double-planet system.
This theory seemed quite plausible until lunar rock samples were recovered, revealing that the moon differs from Earth not only in density, but in composition. If the two bodies had formed out of essentially the same stuff, why would their compositions be so different?

Daughter theory of moon

The oldest of the four theories speculates that the moon was originally part of the earth, and was somehow spun off a rapidly spinning, partially molten, newly forming planet.
Once prevalent, this theory (sometimes referred to as the fission theory) has largely been rejected, because it does not explain how the proto-Earth could have been spinning with sufficient velocity to eject the material that became the moon. Moreover, it is highly unlikely that such an ejection would have put the moon into a stable Earth orbit.

Friday, October 17, 2008

Understanding Moon Phases

Take the time to observe the moon through all of its phases. When the moon is about three or four days “old,” Mare Crisium and other vivid features—including the prominent craters Burckhardt and Geminus—become dramatically visible, assuming it’s a clear night. You can also begin to see Mare Tranquilitatis, the Sea of Tranquility.
At day seven, the moon is at its first quarter. At this time, mountains and craters are most dramatically visible. Indeed, this is the optimum night for looking at lunar features in their most deeply shadowed relief.
As the moon enters its waxing gibbous beyond first quarter phase, its full, bright light is cheerful, but so bright that it actually becomes more difficult to make out sharp details on the lunar surface. An inexpensive “moon filter” or variable polarizing filter fitted to your telescope can help increase contrast on the bright lunar surface. As the moon verges on full, we do get great views of the eastern maria, the lunar plains. Past day 14, the moon begins to wane as the sunset terminator moves slowly across the lunar landscape. At about day 22, the Apennine Mountains are clearly visible. It was these mountains that Galileo studied most intensely, attempting to judge their height by the shadows they cast.
During the late waning phase of the moon, moonrise comes later and later at night, as the moon gradually catches up with the sun in the sky. By the time the moon passes day 26, it is nothing but a thin crescent of light present in the predawn sky. The new moon follows, and as the moon overtakes the sun, the crescent reappears (on the other side of the moon at sunset), and it begins to wax again. Here are some cold, hard facts about a cold, hard place. The moon is Earth’s only natural satellite, and in fact a very large satellite for a planet as small as the earth. The planet Mercury is only slightly larger than the moon. The mean distance between the earth and moon, as it orbits our planet from west to east, is 239,900 miles (386,239 km). The moon is less than one-third the size of the earth, with a diameter of about 2,160 miles (3,476 km) at its equator. Moreover, it is much less massive and less dense than the earth—1⁄80 as massive, with a density of 3.34 g/cm3, in contrast to 5.52 g/cm3 for the earth. If the earth were the size of your head, the orbiting moon would be a tennis ball 30 feet away.
Because the moon is so much less massive than the earth, and about a third as big, its surface gravity is about one-sixth that of our planet. That’s why the Apollo astronauts could skip and jump like they did, even wearing those heavy space suits. If you weigh 160 pounds on the earth’s surface, you would weigh only 27 pounds on the moon. This apparent change would give you the feeling of having great strength, since your body’s muscles are accustomed to lifting and carrying six times the load that burdens them on the moon. Of course, your mass—how much matter is in you—does not change. If your mass is 60 kilograms (kg) on the earth, it will still be 60 kg on the moon. the moon is in a synchronous orbit around the earth; that is, it rotates once on its axis every 27.3 days, which is the same time that it takes to complete one orbit around the earth. Thus synchronized, we see only one side of the moon (except for the tantalizing peek at the far side that libration affords).

What You Can See On The Moon?

Even if you don’t have a telescope, there are some very interesting lunar observations you can make. Have you ever thought that the moon looks bigger when it’s closer to the horizon? It’s just an optical illusion, but you can test it out. The angular size of the moon is surprisingly small. A circular piece of paper just about 0.2 inches in diameter held at arms’ length should cover the moon. At the next full moon, cut out a little disk of that size and prove to yourself that the moon stays the same size as it rises high into the sky.
The telescope through which Galileo Galilei made his remarkable lunar observations was a brand-new and very rare instrument in 1609; but you can easily surpass the quality of his observations with even a modest amateur instrument.
Why is it so exciting to point your telescope at the moon?
Because no other celestial object is so close to us. Being so close, the moon provides the most detailed images of an extraterrestrial geography that you will ever see through your own telescope.
When should you look at the moon?
The easy answer is: anytime the sky is reasonably clear. But if you’re thinking that you should always wait until the moon is full, think again. When is the best time to view a rugged Earthly landscape at its most dramatic? When the sun is low, early in the morning or late in the afternoon, and the light rakes across the earth, so that shadows are cast long and all stands in bold relief.
The same holds true for the moon. When you can see the sunrise or sunset line (the terminator), and the moon is not so full as to be blindingly bright, that is when the topography of the moon will leap out at you most vividly. This characteristic means that you’ll get some very satisfying viewing when the moon is at one of its crescent phases, and probably not at its full phase.

What Galileo Saw on the moon?

It is possible to observe many features of the moon without a telescope. One of the first things you should try is to track its daily motion against the background stars. Since the moon travels around the earth (360 degrees) in
27.3 days, it will travel through about 13 degrees in 24 hours, or about half a degree (its diameter) every hour.
Galileo was the first person to look at the moon through a telescope; indeed, its mottled gray face was one of the first celestial objects on which he trained his new instrument in 1609.
What he saw conflicted with existing theories that the surface was glassy smooth; it was instead rough and mountainous. He closely studied the terminator, or the boundary separating day and night, and noted the shining tops of mountains. Using simple geometry, he calculated the height of some of the mountains based on the angle of the sun and the estimated length of shadows cast. Galileo overestimated the height of the lunar mountains he observed; but he did conclude rightly that their altitudes were comparable to Earthly peaks.
Noticing mountains and craters on the moon was important, because it helped Galileo conclude that the moon was fundamentally not all that different from the earth. It had mountains, valleys, and it even had what were called seas—in Latin, maria though there is no indication that Galileo or anyone else maintained after telescopic bservations that the maria were water-filled oceans. Conten-ding that the moon resembled the earth in 1609 was not a small thing. This statement implied that there was nothing supernatural or special about the moon or perhaps the planets and the stars, either. Followed to its conclusion, the observation implied that there was perhaps nothing divine or extraordinary about the earth itself. The earth was a body in space, like the moon and the other planets.

Monday, September 22, 2008

Lunar Looking

While the world greeted Jules Verne’s 1865 book De la Terre à la Lune (translated in 1873 as From the Earth to the Moon) with acclaim and wonder, it was hardly the first fictional speculation about a voyage to our nearest cosmic neighbor. The Greek satirist Lucian had written about such a flight as early as the second century C.E. and the moon, our constant companion, has always been an object of intense fascination.
Its reflected silvery glow bathes the Earth with romance and mystery. Its changing face, as it travels through its monthly cycle, has always commanded our attention, as have its peculiarly human qualities: Unlike the stars, it is pocked, mottled, imperfect. Almost all cultures at all times have seen some sort of face or figure in the features of the moon. Only rather recently have we realized just how important the moon has been in the evolution of our planet. The sun is so intensely brilliant that to gaze at it is to go blind. But the moon, coincidentally the same size in the sky as the sun, shines with harmless reflected light that invites us to gaze and gaze—to become lunatics.

What If We Had No Moon?

It seems like a reasonable question to ask. What if we had no moon? Would it matter? What has the moon done for me lately?
It turns out that the presence of such a large moon as we have is unusual for a terrestrial planet. Mercury and Venus have no moons, and Mars has two tiny moons, Phobos and Deimos. To have a moon roughly 1⁄3 the size of the planet is unique in the inner solar system. Our Moon, for example, is as large as some of the moons of the giant gas planets in the outer solar system. If there were no moon, we would have no ocean tides, and the rotation rate of the earth would not have slowed to its current 24 hours. It is thought that early in the life of the Earth, it rotated once every 6
hours. The moon also appears to stabilize the rotational axis of the Earth. The Moon, in periodically blocking the light from the Sun’s photosphere gives us a view of the outer layers of the Sun’s atmosphere, and it also gave early astronomers clues to the distribution of objects in the solar system.

Our Closest Neighbor : The Moon

It has been more than 30 years since Neil Armstrong stepped from the Apollo 11 Lunar Lander onto the surface of the moon. The moon is still the only celestial body other than the Earth where humans have stood. But why did we go there? Columbus had sailed to a place promising great riches to exploit. The moon, in contrast, was and is a lifeless orb, devoid of water (mostly!), air, sound, weather, trees, or grass. While Columbus’s voyages had their tight moments (he once had to “predict” a solar eclipse to impress the natives), on his return from the fourth and final voyage to the New World, Columbus announced that he had indeed found an Earthly paradise.
But the moon?
From the pictures we’ve all seen, the lunar landscape is one of rock, dust, and desolation. And although the astronauts were seen skipping across its surface, they were clearly happy to return to mother Earth. Why on earth did our nation expend such effort, treasure, and risk to send astronauts to the lunar surface? What have we

Friday, September 12, 2008

The Demise of Mir

After several years of mishaps and close calls, the decision was made to discontinue
use of the Mir Space Station and to concentrate on the collaboration with the international community on the International Space Station. Early in 2001, the Mir Space Station was de-orbited and allowed to crash into the South Pacific Ocean. At 12:55 A.M. EST on March 22, 2001 (05:55 Greenwich Mean Time), the Mir station was 50 km (31 mi) above Earth’s surface. At 12:58 A.M. EST (05:58 GMT, 8:55 A.M. Moscow time) fragments of the station hit the ocean.
Alix Bowles, Project Coordinator for MirReentry.com watched the space station break into pieces as it streaked through the sky from a beach in Fiji. “It was a stunning blue steak followed by a sonic boom,” he said. “The pieces had a blue incandescence to them. There was something very peaceful about it,” he added. In its later years, the Mir station had become the butt of late-night television jokes, but, in fact, it was a productive scientific instrument and an important test bed for technology used on the International Space Station. Mir lasted years far longer than its designers had envisioned.


On May 14, 1973, the United States launched its first orbiting space station, Skylab, designed to accommodate teams of astronauts to conduct a variety of experiments in geography, engineering, Earth resources, and biomedicine. Such work was carried out during 1973 and the beginning of 1974. In 1974, the craft’s orbit was adjusted to an altitude believed sufficient to keep Skylab in orbit until 1983, when a visit from the Space Shuttle was contemplated. At that time, the orbit would again be adjusted. Unfortunately, Skylab wandered out of orbit prematurely, in June 1978, and ultimately disintegrated and fell into the Indian Ocean on July 11, 1979.

Space Shuttles and Space Stations

The flight of Apollo 17 in 1972 was the last manned lunar mission, but not the end of the U.S. manned space program. On April 12, 1981, the first Space Shuttle, a reusable spacecraft (the previous space capsules had been one-shot vehicles) was launched. The Shuttle was intended to transport personnel and cargo back and forth from a manned space station, planned for Earth orbit. So far, Shuttle missions have carried out a variety of experiments, have delivered satellites into orbit, and have even repaired and upgraded the Hubble Space. In 1999, it started its most ambitious mission: the construction of an international space station, to be built in conjunction with Russia, Japan, and the European Space Agency (ESA). The realities of politics and economics mean that, in the twenty-first century, countries will be much more likely to cooperate in the race to space.

Tuesday, August 26, 2008

A More Distant Voyager

The Cassini-Huygens mission, a joint undertaking of NASA, the European Space Agency (ESA), the Italian Space Agency (ASI), and several other organizations, was sent on its way October 15, 1997, to investigate Saturn as well as Titan (one of Saturn’s moons). Some scientists believe that Titan might support life or, at least, offer conditions in which life could develop. The mission was named Cassini, in honor of the seventeenth-century French-Italian astronomer Jean Dominique Cassini, who discovered the prominent gap in Saturn’s main rings; and Huygens, after the Dutch scientist Christiaan Huygens, who discovered the Saturn moon Titan in 1655, as well as the rings of Saturn. It recently transmitted dramatic images of Jupiter as it sped past on its way to Saturn.

Mars Observer, Surveyor, and Pathfinder

Mars Observer, launched on September 25, 1992, was to conduct extensive imaging work while orbiting Mars, but contact was lost with the spacecraft on August 22, 1993, as the satellite was establishing an orbit around the red planet. It is possible that a fuel tank exploded, destroying the spacecraft. Mars Global Surveyor was launched on November 7, 1996, and is continuing a long project of (among other things) detailed low-altitude mapping of the Martian surface. Unexpected oscillations in its solar panels while coming into a circular orbit around the planet caused the start of the major surface mapping program to be delayed by almost a year.

Although the Global Surveyor project is extraordinarily ambitious, the public may have been more excited by the mission of the Mars Pathfinder. The craft was launched on December 4, 1996, and landed on Mars the following summer, using a combination parachute and rocket-braking system, as well as an air bag system to ensure a soft, upright landing. A “micro-rover” vehicle was deployed, which began transmitting extraordinary panoramic and close-up pictures of the Martian landscape. It is little wonder that Pathfinder has caused such a stir. We’ve always been fascinated by Mars, which, of all the planets, seems most like Earth and has often been thought of as possibly harboring life—even civilization.

Magellan, Galileo, and Ulysses

More recent U.S. planetary probes have been increasingly ambitious. Magellan was launched in May 1989 and ultimately placed into orbit around Venus. Using high-resolution radar imaging, Magellan produced images of more than 90 percent of the planet, yielding more information about Venus than all other planetary missions combined.
The spacecraft made a dramatic conclusion to its four-year mission when it was commanded to plunge into the planet’s dense atmosphere on October 11, 1994, in order to gain data on the planet’s atmosphere and on the performance of the spacecraft as it descended.
On October 18, 1989, Galileo was launched on a journey to Jupiter and transmitted data on Venus, the earth’s moon, and asteroids before reaching Jupiter on July 13, 1995, and dropping an atmospheric probe, which gathered data on Jupiter’s atmosphere. After an extended analysis of the giant planet, Galileo began a mission to study Jupiter’s moons, beginning with Europa. The so-called Galilean moons were discovered by the mission’s namesake, Galileo Galilei in 1610. The Ulysses probe was delivered into orbit by the shuttle Discovery on October 6, 1990. A joint project of NASA and the European Space Agency (ESA), Ulysses gathers solar data and studies interstellar space as well as the outer regions of our own solar system. Much of the spacecraft’s instrumentation is designed to study x-rays and gamma rays of solar and cosmic origin.

Pioneers and Voyagers

In the fall of 1958, Pioneer 1 was launched into lunar orbit as a dress rehearsal for the planetary probes that followed. The rest of the Pioneer craft probed the inner solar system for planetary information, and Pioneers 10 (1972) and 11 (1973) explored Jupiter and Saturn, the giants at the far end of our solar system. Later, in 1978, Pioneer Venus 1 and Pioneer Venus 2 orbited Venus to make surveys of that planet’s lower atmosphere and, using radar imaging, penetrated thick gaseous clouds in order to reveal the spectacular and forbidding landscape below.

Mariners and Vikings

The U.S. Mariner program launched probes designed to make close approaches to Mars, Venus, and Mercury. Mariner 2 (1962) and Mariner 5 (1967) analyzed the atmosphere of Venus. Mariner 4 (1964) and 6 and 7 (both 1969) photographed the Martian surface, as well as analyzed the planet’s atmosphere. Mariners 6 and 7 also used infrared instruments to create thermal maps of the Martian surface, and, in 1971, Mariner 9, in orbit around Mars, transmitted television pictures of the planetary surface. Mariner 10, launched in 1973, was the first spacecraft to make a close approach to Mercury and photograph its surface.
But even more exciting were the two Viking missions, launched in 1975. The following year, both made successful soft landings on Mars and conducted extensive analysis of the Martian surface.

Thursday, August 14, 2008

The Apollo Missions

The data from the unmanned probes and orbiters was overwhelming in its volume and detail. Some critics continued to argue: Why send human beings? The manned missions clearly captured public attention, beginning with the Soviet Vostok series (1961–1963, including Vostok 6, which carried the first woman into space, Valentina V. Tereshkova) and the U.S. Mercury series (1961–1963). The Mercury series included two suborbital flights and the first U.S. manned flight in orbit, Friendship 7, commanded by John H. Glenn Jr., and launched on February 20, 1962.

(On October 29, 1998, 77-year-old Senator John Glenn boarded the Space Shuttle Discovery and became the oldest man in space. He returned from the mission on November 7.) The U.S. Gemini program came next, twelve two-man spaceflights launched between 1964 and 1967. The Gemini flights were intended very specifically to prepare astronauts for the manned lunar missions by testing their ability to maneuver spacecraft, to develop techniques for orbital rendezvous and docking with another vehicle—essential procedures for the subsequent Apollo Moon-landing program—and to endure long spaceflights. The eight-day Gemini 5 mission, launched August 21, 1965, was the longest spaceflight to that time. The Soviets also developed larger launch vehicles and orbiters. Voskhod 1, launched on October 12, 1964, carried three “cosmonauts” (as the Russians called their astronauts) into Earth orbit.

The U.S. Apollo lunar missions not only made up the most complex space exploration program ever conceived, but were perhaps the most elaborate scientific and technological venture in the history of humankind. Today, even if we had the desire, we no longer have the launch vehicles required to bring astronauts to the moon. According to the mission plan, a Saturn V multistage booster (rocket) would lift the 3-man Apollo spacecraft on its 21⁄4-day voyage to the moon, leaving behind the launch stages in pieces as it left the earth. After its journey, the small remaining piece of the initially launched craft would become a satellite of the moon, and the Lunar Module, with two men aboard, would separate from the orbiting Command Module and land on the moon. After a period of exploration on the lunar surface, the astronauts would climb back into the Lunar Module, lift off, and dock with the orbiting Command Module, which would fire its rockets to leave its lunar orbit and carry the three astronauts back to Earth.

After several preliminary missions, including Earth- and Moon-orbital flights, Apollo 11 was launched on July 16, 1969. On board were Neil A. Armstrong, Edwin E. “Buzz” Aldrin Jr., and Michael Collins. While in lunar orbit, Armstrong and Aldrin entered the Lunar Module, separated from the Command Module, and landed on the Moon, July 20, 8:17 P.M. Greenwich Mean Time.

“That’s one small step for [a] man,” Armstrong declared, “one giant leap for mankind.” And perhaps that sentence expressed the rationale for the effort, which went beyond strictly scientific objectives and spoke of and to the human spirit. Not that science was neglected. During their stay of 21 hours and 36 minutes, the astronauts collected lunar soil and moon rocks and set up solar-wind

Apollo 12 landed on the Moon on November 19, but Apollo 13, launched April 11, 1970, had to be aborted because of an explosion, and the astronauts, as recounted in a recent film through their great skill, resourcefulness, and courage, barely escaped death.

Apollo 14 (launched January 31, 1971), Apollo 15 (July 26, 1971), Apollo 16 (April 16, 1972), and Apollo 17 (December 7, 1972) all made successful lunar landings. Budgetary constraints, declining public interest, and the improving capabilities of unmanned missions eventually brought an end to the Apollo missions.

Lunar Probes

There were voices raised in protest, both in the political and scientific communities. Why try to put men on the moon, when unmanned probes could tell as much or more—and accomplish the mission with far less expense and danger?
The Russians had successfully launched the first lunar probe, Luna 2, on September 12, 1959, targeting and hitting the moon with it. Luna 3, launched the following month, on October 4, 1959, made the first circumnavigation of the moon
and transmitted back to Earth civilization’s first photographs of the Moon’s mysterious far side.
Another Soviet lunar first would come on January 31, 1966, when Luna 9 made a successful lunar soft landing—as opposed to a destructive impact.
In 1961, the United States launched the first of the Ranger series of nine unmanned lunar probes, hitting the moon with Ranger 4 in 1962 and orbiting it, with Rangers 7, 8, and 9, during 1964–1965. These last three missions generated some 17,000 high-resolution photographs of the lunar surface, not only valuable as astronomy, but indispensable as a prelanding survey.
From 1966 to 1968, seven Surveyor probes made lunar landings (not all successful), took photographs, sampled the lunar soil, and performed environmental analysis. Surveyor 6 (launched on November 7, 1967) landed on the lunar surface, took photographs, then lifted off, moved eight feet, landed again, and took more photographs. It was the first successful lift-off from an extraterrestrial body. The Lunar Orbiter series, five orbital missions launched during 1966–1967, mapped much of the lunar surface in 1,950 wide-angle and high-resolution photographs. These images were used to select the five primary landing sites for the manned Apollo missions.

JFK’s Challenge

On May 5, 1961, about three weeks after the Russians put a man into a single orbit, U.S. Navy commander Alan B. Shephard was launched on a 15-minute suborbital flight into space. Americans were proud of this achievement, to be sure, but the Soviets had clearly upstaged it. Just 20 days later, on May 25, President John F. Kennedy spoke to Congress: “I believe this nation should commit itself to achieving the goal, before the decade is out, of landing a man on the moon and returning him safely to Earth. No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space, and none will be so difficult or expensive to accomplish.”

First Observatories in Space

In 1962, the United States launched its first extraterrestrial observatory, the Orbiting Solar Observatory (OSO). It was the first of a series of solar observatories, designed to gather and transmit such data as the frequency and energy of solar electromagnetic radiation in ultraviolet, x-ray, and gamma ray regions of the spectrum—all regions to which our atmosphere is partially or totally opaque.

The Early Explorers

The first space satellite the United States sent into orbit was Explorer 1, launched on January 31, 1958. While the satellite didn’t beat Sputnik 1 into space, it accomplished considerably more than the Soviet probe. Explorer 1 carried equipment that discovered the innermost of the Van Allen radiation belts, two zones of charged particles that surround the earth. By 1975, when the Explorer series of missions ended, 55 satellites had been launched, including Explorer 38 (July 4, 1968), which detected galactic radio sources, and Explorer 53 (May 7, 1975), which investigated x-ray emission inside and beyond the Milky Way.

Satellites and Probes

Astronomers and other scientists were not always enthusiastically supportive of the manned space program, many of them feeling that it stole both public attention and government funding away from more useful data-gathering missions that could be carried out much more efficiently and inexpensively by unmanned satellites and probes. There is much truth to this sentiment. However, at least in the 1960s, unmanned exploratory missions continued to have high priority, and did not really suffer from the parallel development of the manned space program.

Sunday, July 27, 2008

Early Human Missions

While both the Soviet Union and the United States launched a series of artificial satellites, the major goal quickly became “putting a man in space.” This objective was less scientific than psychological and political. The Soviet communists were determined to demonstrate the superiority of their technology generally and, in particular, the might of their ballistic missiles. At the time, their rockets were more powerful than what the United States had. The Soviets were eager to demonstrate that they were capable of lofting a person (and all of the machinery necessary to support a person) into space—or a warhead onto an American city.

Just as the Soviets had been first into orbit with Sputnik, so, on April 12, 1961, they were first to put a person, Yuri Alekseyevich Gagarin, into space—and into Earth orbit, no less. The first woman, Valentina Tereshkova of the USSR followed in 1963. It took America 20 more years to achieve this landmark. Through the rest of the 1960s, the Soviets and the Americans sent cosmonauts and astronauts into orbit and even had them practice working outside of their spacecraft in what were termed “extravehicular activities” or, more familiarly, space walks.

The Battle Cry of Sputnik

Impressive as the achievements of Piccard and others were, balloons could never move beyond the frontier of space. They needed the earth’s atmosphere to loft them.
After the war, scientists in America and the Soviet Union began experimenting with
so-called sounding rockets developed from the V-2s, in part to probe (sound) the upper atmosphere. While a sounding rocket was accelerated to speeds of up to 5,000 miles per hour, it would run out of fuel by about 20 miles up.

This acceleration gave the rockets sufficient velocity to continue their ascent to about a hundred miles, after which the rocket fell to Earth. Any instrumentation it carried had to be ejected, parachuted to safety and recovered, or the information had to be transmitted to a ground station by radio before the rocket crashed. The goal of rocket science at this point was not only to reach higher altitudes, but to achieve a velocity that could launch an artificial satellite into orbit around the earth. Imagine a rock thrown into the air. The force of gravity causes it to travel in a parabola and return to the earth. If the ball were thrown at a greater and greater velocity, it would travel farther and farther until it returned to the earth. At some velocity, however, the rock would never return to the earth, but continually fall toward it (this is what the moon is doing: orbiting the earth). It was no mean trick to get a satellite going fast enough to make it orbit the earth.

A single-stage rocket, like the V-2, exhausted its fuel supply before it reached sufficient altitude and velocity to achieve orbit. It lacked the necessary thrust. To build a more powerful rocket required a return to Goddard’s idea of a “stepped” or staged rocket. A staged rocket jettisoned large parts of itself as fuel in each lower part—or stage—ran out. Thus the rocket became progressively less massive as it ascended, both by burning fuel and by discarding the empty fuel tanks.

During the early and mid-1950s, there was much talk of putting a satellite into orbit, and both the United States and the Soviets declared their intention to do so. In the Cold War atmosphere of the time, it came as a great shock to Americans when the USSR was the first to succeed, launching Sputnik I (Russian for “satellite”) into orbit on October 4, 1957. The 185-pound (83.25 kg) satellite had been lofted to an altitude of about 125 miles (201 km) and had achieved the required Earth orbital velocity of some 18,000 miles (28,980 km) an hour. The first Sputnik was a primitive device by today’s standards. It did nothing more than emit a radio beep to tell the world it was there. But it didn’t have to do more than that. The point was made, the Space Age was born, and the space race had begun.

Playing with Balloons

While the V-2 had achieved great altitude by the 1940s, scientists were still a long way from attempting a human ascent. These early rockets were intended to explode at the end of the journey. If an instrument or a human were on board, explosions were to be avoided at all costs. In fact, another technology, the balloon, would be the first to take human beings into the upper stratosphere, the frontier of space. Auguste Piccard (1884–1962), a Swiss-born Belgian physicist, built a balloon in 1930 to study cosmic rays, which the earth’s atmosphere filters out. Piccard developed revolutionary pressurized cabin designs, which supported life at high altitudes, and, in 1932, reached an altitude of 55,563 feet. The following year, balloonists in the Soviet Union used Piccard’s design to reach 60,700 feet, and an American balloonist topped that later in the year at 61,221 feet.

From Scientific Tool to Weapon and Back Again

From the early 1900s through the 1930s, peacetime governments and the scientific community showed relatively little interest in supporting such pioneers as Tsiolkovsky, Goddard, and Oberth. Unfortunately, it took war in Europe, and a desire to launch bombs onto other nations, to spur serious, practical development of rockets. The research and development took place almost exclusively in Germany.
During the late 1930s, under the militaristic regime of Adolf Hitler, two rocket weapons were created. The first, known as the V-1, was more a pilotless jet aircraft than a rocket. About 25 feet long, it carried a 2,000-pound bomb at 360 miles per hour for a distance of about 150 miles. It was a fairly crude device: When it ran out of fuel, it crashed and exploded. Out of about 8,000 launched, some 2,400 rained down on London from June 13, 1944, to March 29, 1945, with deadly effect. In contrast to the V-1, the V-2 was a genuine rocket, powered not by an air-breathing jet engine, but by a rocket engine burning a mixture of alcohol and liquid oxygen.

The V-2 had a range of about 220 miles and also delivered 2,000 pounds of high explosives to its target. From September 8, 1944, to March 27, 1945, about 1,300 V-2s were launched against Britain. Scientists of every stripe spent the years from 1939 to 1945 directing their energies toward the defeat of the enemy. Many of the techniques developed during the war (radar technology and rocket engines, to name two) would become crucial to astronomy in the decades after WWII.
During the last days of the war in Europe, as U.S. forces invaded Germany from the west and Soviet forces invaded from the east, both sides scrambled to capture V-2s and, with them, German rocket scientists, such as Wernher von Braun. Both sides saw the potential in being able to deliver bombs over long distances. These rockets and the scientists who made them were at the center of the Cold War and the Space Race—a period of competition in politics and high technology between the two superpowers that dominated the postwar world.

Sunday, July 13, 2008

This Really Is Rocket Science

While spaceflight was the subject of many centuries of speculation, three men worked independently to lay its practical foundation. Konstantin Eduardovich Tsiolkovsky (1857–1935) was a lonely Russian boy, almost totally deaf, who grew up in retreat with his books. He became a provincial schoolteacher, but his consuming interest was flight, and he built a wind tunnel to test various aircraft designs. Soon he became even more fascinated by the thought of space travel, producing the first serious theoretical books on the subject during the late nineteenth and early twentieth centuries.

Another quiet, introspective boy, this one a New Englander, Robert Hutchings Goddard (1882–1945), was captivated by H. G. Wells’s science-fiction novel War of the Worlds, which he read in an 1898 serialization in the Boston Post. On October 19, 1899 (as he remembered it for the rest of his life), young Goddard climbed a cherry tree in his backyard and “imagined how wonderful it would be to make some device which had even the possibility of ascending to Mars.” From that day, the path of his life became clear to him. Goddard earned his Ph.D. in physics in 1908 from Clark University in his hometown of Worcester, Massachusetts, and, working in a very modest laboratory, he showed experimentally that thrust and propulsion can take place in a vacuum (this follows from Newton’s Laws of motion—the expelled gases pushing forward on the rocket). He also began to work out the complex mathematics of energy production versus the weight of various fuels, including liquid oxygen and liquid hydrogen. These are the fuels that would ultimately power the great rockets that lofted human beings into orbit and to the moon—and still power the launch of many rockets today. Goddard was the first scientist to develop liquid-fuel rocket motors, launching the inaugural vehicle in 1926, not from some governmental, multimillion-dollar test site, but from his Aunt Effie’s farm in Auburn, Massachusetts. Through the 1930s and 1940s, he tested increasingly larger and more powerful rockets, patenting a steering apparatus and the idea of what he termed “step rockets”—what would later be called multistage rockets—to gain greater altitude.

Goddard’s achievements were little recognized in his own time, but, in fact, he had single-handedly mapped out the basics of space-vehicle technology, including fuel pumps, self-cooling rocket motors, and other devices required for an engine designed to carry human beings, telecommunications satellites, and telescopes into orbit. Hermann Oberth (1894–1989), born in Austria, was destined for a medical career, like his father, but his medical studies were interrupted by World War I. Wounded, he studied physics and aeronautics while recovering. While he was still in the Austrian army, he performed experiments to simulate weightlessness, and designed a longrange, liquid-propellant rocket. The design greatly impressed Oberth’s commanding officer, who sent it on to the War Ministry, which summarily rejected it. After the war, University of Heidelberg faculty members likewise rejected Oberth’s dissertation concerning rocket design. Undaunted, Oberth published it himself—to great acclaim—as The Rocket into Interplanetary Space (1923). In 1929, he wrote Ways to Spaceflight, winning a prize that helped him finance the creation of his first liquidpropellant rocket, which he launched in 1931.
During World War II, Oberth became a German citizen and worked with Wernher von Braun to develop rocket weapons.

The Space Race

While countless human beings have gazed up at the sky with wonder, a few were never content just to look. They didn’t want to wait for the information to get here, they wanted to go there. In the second century C.E., the Greek satirist Lucian wrote the first account we have of a fictional trip from the earth to the moon. Doubtless, someone had thought about such a trip before Lucian, and certainly many contemplated space travel after him. It was not until the eighteenth century that people were first lofted into the air by hot-air balloons. And while the airplane made its debut in 1903, human spaceflight—in which a human ventured beyond the earth’s protective atmospheric blanket—did not come about until the 1961 flight of a Soviet cosmonaut Yuri Alekseyevich Gagarin.

Chandrasekhar and the X-Ray Revolution

Electromagnetic radiation at the highest end of the spectrum can now be studied. Since x-rays and gamma rays cannot penetrate our atmosphere, all of this work must be done by satellite. Work began in earnest in 1978 when an x-ray telescope was launched, called the High-Energy Astronomy Observatory (later, the Einstein Observatory). The Röentgen Satellite (ROSAT) was next launched by Germany in 1990. The Chandra X-ray Observatory (named for astronomer Subrahmanyah Chandrasekhar) was launched into orbit in July 1999 and has produced unparalleled high-resolution images of the x-ray universe. The Chandra image of the Crab Nebula, home to a known pulsar, showed never before seen details of the environment of an exploded star. For recent images, go to www.chandra.harvard.edu. X-rays are detected from very high energy sources, such as the remnants of exploded stars (supernova remnants) and jets of material streaming from the centers of galaxies. Chandra is the premier x-ray instrument, doing in this region of the spectrum what the Hubble Space Telescope has done for optical observations.

In 1991, the Gamma Ray Observatory (GRO) was launched by the space shuttle. It is revealing unique views of the cosmos, especially in regions where the energies involved are very high: near black holes, at the centers of active galaxies, and near neutron stars.

New Infrared and Ultraviolet Observations

Telescopes need to be specially equipped to detect infrared radiation—the portion of the spectrum just below the red end of visible light. Infrared observatories have applications in almost all areas of astronomy, from the study of star formation, cool stars, and the center of the Milky Way, to active galaxies, and the large-scale structure of the universe. IRAS (the Infrared Astronomy Satellite) was launched in 1983 and sent images back to Earth for many years. Like all infrared detectors, though, the ones on IRAS had to be cooled to low temperatures so that their own heat did not overwhelm the weak signals that they were trying to detect. Although the satellite is still in orbit, it has long since run out of coolant, and can no longer make images. The infrared capability of the Hubble Space Telescope provided by NICMOS (Near-Infrared Camera and Multi-Object Spectograph) yielded spectacular results while in operation. The Next-Generation Space Telescope (NGST) will be optimized to operate at infrared wavelengths, and will be cooled passively (by a large solar shield).

Ultraviolet radiation, which begins in the spectrum at frequencies higher than those of visible light, is also being studied with new telescopes. Since our atmosphere blocks all but a small amount of ultraviolet radiation, ultraviolet studies must be made by high-altitude balloons, rockets, or orbital satellites. The Hubble Space Telescope, for instance, has the capability to detect ultraviolet (UV) photons as well as those with frequencies in the visible and infrared. Ultraviolet observations provide our best views of stars, and stars with surface temperatures higher than the sun’s.

What is SETI?

If you’ve seen such sci-fi movies as The Arrival, Independence Day, or Contact, you already know about an organization called SETI (Search for Extra-Terrestrial Intelligence). It is an international group of scientists and others who, for the most part, use radio telescopes to monitor the heavens in search of radio signals generated not by natural phenomena, but broadcast artificially by intelligent beings from other worlds. So far, no clearly artificial extraterrestrial radio signals have been confirmed, but SETI personnel keep searching. The SETI project got a large boost recently when Paul Allen, one of the co-founders of Microsoft, committed $12.5 million to the project. The new instrument to be built exclusively for SETI will be called the Allen Telescope Array.
If you are interested in the search for extraterrestrial intelligence, you don’t have to just read about it, you can actually participate in it. A few highly committed amateur radio astronomers have built SETI-capable radio telescopes and spend time searching for artificial signals of extraterrestrial origin. If you’re not up to making such a commitment, the SETI Institute is developing an alternative. In a project called SETI@home, a special kind of screensaver program (a program that, typically, puts up a pretty, animated picture on your PC monitor when the computer is idle) has been installed on over 1.5 million computers in 224 countries. When the computer is idle, this program will use the time to go to work analyzing data from four million different combinations of frequency bandwidth and drift rate recorded by the world’s largest radio telescope at Arecibo, Puerto Rico. With thousands of computers crunching this data, the SETI Institute believes that it can analyze data more quickly, thereby increasing the chances of ferreting out a radio signal from an intelligent source. Information on SETI@home can be found on the Net at www.seti-inst.edu.

Solar Flares and Meteor Events

Solar flares, are explosive events that occur in or near an active region on the sun’s surface. Flares can be detected with a very low frequency (VLF) receiver operating in the 20- to 100-KHz (kilohertz, or thousand hertz) band. (This is below the region of the spectrum where AM radio stations broadcast.) Such a receiver can be homemade, using plans supplied by such organizations as SARA. Solar flares can also be monitored in regular shortwave radio bands with a standard shortwave receiver. Why do solar flares create radio signals? They generate x-rays that strike a part of our atmosphere called the ionosphere, greatly enhancing the electron count in this atmospheric region. These electrons generate the noise picked up by the radio.

Saturday, June 28, 2008

Amateur Radio Astronomy: No-Cost and Low-Cost Approaches

A decent optical telescope costs at least $300 to $400. For free optical astronomy, all you need are your eyes. You can also do some radio astronomy for free—if you own an FM radio or a television set. We thank Tom Crowley of Atlanta Astronomy Club for many of the following ideas. Even they are affordable, don't drop it, because it breaks easily. If that's happen then I'll send you a nice condolence letter.
Have you ever witnessed a meteor shower? The streaks of light in the night sky can be quite spectacular.
Meteors are the bright trails of ionized atmosphere behind tiny bits of cometary debris that enter the earth’s atmosphere. Most meteorites are no larger than a pea. What if we told you that there was another way to watch a meteor shower—using a radio telescope otherwise known as an FM radio?
Meteor counts by radio are about ten times more accurate than visual observation—and, as with any radio observations, you can observe during the day or through clouds. It doesn’t have to be dark or clear outside. You may want to supplement your optical meteor gazing with your radio on cloudy nights.
Recall that the earth’s atmosphere is transparent to some forms of electromagnetic radiation and opaque to others. The upper atmosphere normally reflects low-frequency AM radio signals. In contrast, the atmosphere is transparent to higher frequency FM radio waves, which, as a consequence, have a shorter range. They usually penetrate and are not reflected by the atmosphere.
But something happens when a meteorite enters the atmosphere. Each piece of debris that tears into the atmosphere (at up to 40 miles per second), heats up the air around it and creates a tiny ionized (electrically charged) vapor trail in the upper atmosphere. These columns of charged particles can reflect even higher frequency FM radio waves. This temporary condition means that previously out-of-range FM broadcasts can (for a moment) be heard. During periods of known meteoric activity, stay at the low end of the FM dial and try to find FM radio stations that are from 400 to 1,300 miles away. You might call a distant friend to get the broadcast frequency of a few stations. When a distant station fades in for a second or two, you are indirectly observing a meteor. The trail behind it has momentarily reflected a distant radio signal into your receiver. It helps if you can hook up your radio to an outdoor antenna, but if the meteor shower is fairly intense, you should detect many events even without such an antenna. You can also try tuning your TV set to the lowest unused VHF channel. Again, when a distant station, normally out of range, fades in and the signal becomes strong for a second or two, you know a meteor has entered the atmosphere. (Note that this works only with a television receiving signals from an outdoor antenna—not via cable or satellite!)
Just by tuning in your radio, you can do some meaningful radio astronomy. You can make it more interesting by recording the events on tape, or keeping a written record of the number of events you detect per hour.
But amateur radio astronomy need not be limited to listening for distant FM radio or TV stations. If you are an amateur radio operator—a ham—you already have much of the equipment required for more serious radio astronomy. If you aren’t into amateur radio, you can get started for a highly variable but modest cost. The first step to take is to log onto SARA’s World Wide Web site (www.bambi.net/sara.html) for overview information.
Essentially, amateurs can use either nonimaging or imaging radio astronomy techniques. Nonimaging techniques (which monitor radio emissions without pinpointing locations) require a simple shortwave receiver, usually modified to receive a narrow band of frequencies, and a simple antenna system. With such equipment, you can track radio emission from Jupiter, solar flares, and meteor events. Imaging techniques (which provide more detailed information on the location and nature of the signal) require a more serious commitment of resources, including a much larger dish-type antenna, more sophisticated receiving equipment, reasonably elaborate recording equipment, and (probably) a rural location removed from most sources of radio interference. For purposes of this blog, we’ll restrict ourselves to the more approachable nonimaging techniques, which are more appropriate for beginners.

You Can Do This, Too!

Building a huge radio telescope like Arecibo or the Green Bank Telescope (GBT) takes a great deal of money, and so does operating one. Even if you had the cash, your neighbors (not to mention the local zoning board), might frown on your building even a modest 30-foot-diameter dish antenna in your backyard. However, remember that radio astronomy originated with non-astronomers, and there is still plenty of room in the field for amateurs, including amateurs of modest means. You can see the sky just like an ancient Aztec astronomer. A small but committed group of enthusiasts have formed the Society of Amateur Radio Astronomers (SARA). Most books for budding astronomers don’t discuss amateur radio astronomy, though it is a fascinating and rewarding subject.

What Radio Astronomers “See”

Insomnia is a valuable affliction for optical astronomers, who need to make good use of the hours of darkness when the sun is on the other side of the earth. But as Karl Jansky discovered so many years ago, the sun is not a particularly bright radio source. In consequence, radio astronomers (and radio telescopes) can work night and day.
The VLA, for example, gathers data (or runs tests) 24 hours a day, 363 days a year. Not only is darkness not required, but you can even make radio observations through a cloud-filled sky. The senior author of this book even observed a distant star-forming region in the midst of a storm during which lightning struck near the VLA and disabled it for a few minutes.
As the Dutch astronomer Jan Oort realized after reading Reber’s work in the 1940s, radio waves opened new vistas into the Milky Way and beyond. Radio astronomers can observe objects whose visible light doesn’t reach the earth because of obscuration by interstellar dust or simply because they emit little or no visible light. The fantastic objects known as quasars, pulsars, and the regions around black holes—all of which we will encounter later in this book—are often faint or invisible optically, but do emit radio waves.
The spiral form of our own Galaxy was first mapped using the 21 cm radio spectral line from neutral (cold) hydrogen atoms, and the discovery of complex molecules between the stars was made at radio frequencies.
The very center of our own Milky Way Galaxy is hidden from optical probing, so that most of what we know of our galactic center has come from infrared and radio observation. Since radio interferometers are detecting an interference pattern, radio data has to be processed in ways different from optical data. But the end result is either a radio image, showing the brightness of the source on the sky, or a radio spectrum, showing a spectral line or lines.