Around and around and around it goes...

This morning, at 7:42am EDT, the Hubble Space Telescope reached a new milestone by completing it's one hundred thousandth (100,000) orbit around the Earth.

The Hubble orbits the Earth in what is known as a "low-Earth orbit," or "only" 600 kilometers (375 miles) above Earth's surface. While this sounds very high, it's not, in space terms. The space station and space shuttle also orbit this low. In low-Earth orbit, it takes a satellite about 90 minutes to orbit the Earth. But at these altitudes, the Earth's atmosphere is still present (although very, very tenuous), and if you don't put a booster rocket on your spacecraft, it will fall back to Earth in just a few years or decades.

In order to stay in orbit, the Hubble has to move at a speed of about 7.5 kilometers per second, or almost 17,000 miles per hour. When the space shuttle goes to repair Hubble, it is also moving this fast. How can the astronauts safely catch the Hubble? It's all relative speed. Although both spaceships are moving at 17,000 miles per hour, compared to one another, they are moving only a few miles per hour (more when the shuttle is moving in to catch it, and much less when the shuttle sticks out its arm to grab Hubble). It's like passing a car going slightly slower than you on the road. Although you are both moving at 65 miles per hour, you can take several minutes to pass each other. Plenty of time to see what their kids are watching on the DVD, get a good look at the driver, and perhaps even try to pass some Grey Poupon.

In the 18 1/3 years it took Hubble to go around the Earth 100,000 times, it covered a distance of 2.7 billion miles. That sounds like a lot, especially if you are moving at 17,000 miles per hour! But, in space, 2.7 billion miles is only about the distance from the Earth to Neptune, and only 1/10000th the distance to the nearest star. We have four space probes (Pioneer 10 and 11, and Voyagers 1 and 2) that have travelled much further. And many other satellites around the Earth have been longer-lived, and have logged many more miles than Hubble.

Basically, Hubble hasn't set any records. It's just reached a nice, round number that is kinda fun to celebrate. So, in that line of celebration, NASA has released some colorful new pictures of a star-forming region in the Large Magellanic Cloud (LMC), one of the Milky Way's own satellite galaxies. And the LMC has only completed one or two orbits around the Milky Way in the 13 billion years its been around. So, our Hubble has an entire galaxy beat! (Of course, instead of an altitude of 600 kilometers, the LMC is at a distance of about 160,000 light-years, and it is travelling about 40 times faster than Hubble).

Also, NASA has a contest where you can win a nice print of a picture from the Hubble; look on the Hubble's home page for that contest (which ends after this week).

Today's Total Eclipse Video

For those of you who weren't in Siberia today, here is a video of the entire totality, when the sun was completely covered by the moon. The video was taken by Ivan Komarov of Manjerok, Siberia, and sent to cnn.com.

The video starts in the final seconds before totality, when the last bits of the sun are disappearing behind the moon. This is often called the "diamond ring" effect, as one brilliant sliver of the sun shines through a lunar valley.

In the instant the sun is completely covered, the sun's outer atmosphere, or corona, comes into view. What you are seeing is a very tenuous, very hot (millions of degrees!) gas. The light we see is not from the gas itself, but light from the sun's surface (called the photosphere) being reflected toward us. The sun's corona is always there, but is completely obscured unless something (like a 2000-mile diameter rock) blocks the sun's photosphere.

After about 2 minutes, you can see the diamond ring happen in reverse. Notice how fast the sun gets bright again! Its photosphere is so brilliant, that even the tiniest sliver overwhelms cameras (and can damage your eyes). During the totality, it is completely safe to look at the sun without protective equipment, even through telescopes and binoculars. But the instant the photosphere re-emerges, you must use solar filters again, or you will permanently damage your eyes.

Unlike lunar eclipses, total solar eclipses have scientific value, though not as much as they used to. Solar astronomers flock to total eclipses with all sorts of expensive equipment for two minutes of intense data collection. With the sun's photosphere out of view, astronomers can study the very outermost layers of the sun's atmosphere from the ground. Before satellite telescopes, total eclipses were the only time this could be done! But, these days, satellites can use fancy optics to create their own eclipses whenever they want, so the science value of a given eclipse has gone down somewhat. But, it is far, far cheaper to run new experiments on the ground during a total eclipse than to launch a satellite, so there is still valuable and groundbreaking science that is done during total solar eclipses.

Total solar eclipses were also the first independent test of Einstein's General Theory of Relativity. Einstein predicted that the sun's gravity should bend the light from stars ever so slightly; careful measurements of stars (which become visible during totality!) taken during eclipses found that Einstein's predictions were spot on. Here is an article discussing some of those first historic measurements. It is important to remember that this was a pure prediction -- nobody had the slightest idea that gravity could bend light, so no other theory predicted any bending, and Einstein's theory got not just the bending right, but the the amount of bending right. It was an amazing prediction, and an even more amazing confirmation of Einstein's theory.

As I said, satellites can now make artificial total eclipses whenever they want; this has led to a revolution in understanding how are sun works, and how it interacts with the Earth. The SOHO spacecraft releases images and movies of the sun and its corona daily; go here and click on the image to look at, or the link to movies (at the bottom of the page). The "false eclipses" are the pictures from the "LASCO C2" and "LASCO C3" instruments, where the white circle indicates the size of the sun. The central dark area (much bigger than the sun) is a disk used to block the sun's light. Right now, you can see the planet Mercury passing the sun. And this page has movies from throughout the SOHO mission's storied life.

Looking For Gamma Rays

Image Credits: NASA / CBS

Yesterday, NASA launched a new telescope into orbit, The Gamma-Ray Large Area Space Telescope, or GLAST. I've been hearing about the preparation of this mission for at least a decade, so it is great to see it underway! My congratulations to the team.

But why would astronomers want to look at gamma rays? And don't we already have telescopes looking at gamma rays? Gamma rays are the most energetic form of light in the Universe. The wimpiest gamma rays are about 20,000 times more energetic than visible light, and there is no theoretical limit to how strong they can become (though there are many practical limits). This makes gamma rays very dangerous to living organisms; unsafe levels of exposure can cause all kinds of cancers and other nasty effects. (It was an overdose of gamma ray radiation that turned mild-mannered Bruce Banner into the Incredible Hulk at least one evening a week back in the late 1970s and early 1980s.)

Most gamma rays people encounter come from nuclear reactions and radioactive decay; gamma rays are the most dangerous form of radiation from nuclear waste. There are also gamma rays flying around you all the time from naturally-occurring radioactive elements. But, unless you are involved in a nuclear accident, the numbers of gamma rays on Earth are too low to cause much harm.

Many objects in space also produce gamma rays. Our atmosphere is opaque to gamma rays, so we are protected from this potentially dangerous radiation. But since gamma rays are produced by some of the most energetic and mysterious astronomical objects, like black holes, neutron stars, exploding stars, and the radioactive remnants of these exploding stars, we astronomers would like to study them. So we have to launch telescopes into space to look at gamma rays.

So, why don't we use the Hubble to look at gamma rays? Why spend lots of money on a totally different telescope? It's because gamma rays are so energetic, we can't look at them with normal mirrors. Gamma rays just pass right through the Hubble's mirror. So GLAST uses a very clever technique that relies on Einstein's most famous equation, E=mc².

Behind that famous equation is the idea that matter (electrons, protons, atoms, rocks, hamsters, etc.) is just another form of energy, like light, heat, and motion. And it is possible to change energy from one kind into another. Our car engines convert chemical energy from gasoline into the motion energy of travel, as well as into heat energy (which is why the engine gets hot!). On a sunny day, the interior of that same car converts the light energy from the sun into heat energy. Nuclear reactors change some of the matter in the atomic fuel into light and heat energy. And the GLAST telescope changes the light energy of gamma rays into matter: two or more subatomic particles (and any leftover energy is turned into motion energy of the particles). The telescope then tracks the position and speed of these particles, which, through some complex but well-understood physics, lets us surmise the original energy and direction of the gamma ray.

But gamma rays are rare, and gamma ray telescopes aren't very efficient at converting light into matter. So, it is important to make the telescope big ("large area") so we can detect as many gamma rays as possible.

GLAST is NASA's second big gamma ray telescope, after the Compton Gamma Ray Observatory, launched by the space shuttle in 1991. NASA has another gamma-ray telescope in orbit, the Swift Telescope, but Swift just looks for the mysterious flashes of gamma ray light called gamma ray bursts. GLAST can detect gamma ray bursts, but its primary mission is to look for other, steady sources of gamma rays.

Gamma rays are produced by matter about to fall into a black hole. The matter gets sped up to high speeds by the black hole's gravity, and before it falls into the black hole's event horizon it can emit gamma rays that we can detect here on Earth. Energetic jets spewing from the regions around black holes can also act as atomic particle accelerators, which can create all kinds of subatomic particles that then collide and release gamma rays. The remnants of exploding stars, such as Cassiopeia A, also glow in gamma rays from radioactive elements created in the giant explosion that destroyed a dying star.

Don't expect many spectacular pictures from GLAST. It's just not possible to make sharp, focused pictures. While the Hubble Space Telescope can see details as fine as 0.05 arcseconds (an angle something like the size of a penny seen from 50 miles away), GLAST can only see as sharp as 1 arcminute (600 times worse than Hubble). GLAST would have trouble resolving details the size of a penny about 380 feet away, a feat that sharp-eyed people can do in excellent conditions. But it is still much better than the Compton Gamma Ray Observatory, which could only resolve the said penny about 40 feet away, something anyone with normal vision can easily do.

But what GLAST can't do in sharpness, it can make up for in its field of vision. While Hubble can only look at a sliver of sky about 1/80th the size of the full moon, GLAST can look at 20% of the entire sky at once!

It'll probably be a year or two before the first GLAST science not dealing with gamma ray bursts comes out. Until then, GLAST will be staring hard, catching elusive gamma rays from deep space. Let's just hope that the scientists are nice to their telescope and don't make it angry. You wouldn't like the telescope when it gets angry.

The dangers of shooting down satellites

Several news accounts suggest that the U.S. military is going to try and "shoot down" a non-functional spy satellite in the next few days. I think this is a dangerous idea, both from a scientific and a political standpoint.

Last month, I blogged about how I thought the chance of the satellite endangering a human was small. I still think that is the case.

Make no bones about it, the hydrazine fuel that is being used as a justification for shooting down the satellite is very nasty stuff. Hydrazine has properties that are great for rocket thruster fuel, but it is a very hazardous chemical. However, it is also quite volatile, which is why many people suspect the fuel tank on the satellite would not survive re-entry, but would explode due to the heat of re-entry. Even if the tank were to survive intact, the chances of it hitting a populated area are amazingly tiny.

If NASA or the military were simply launching an operating thruster that could attach itself to the spy satellite and purposefully de-orbit the satellite into the ocean, I would have no qualms with the operation. We de-orbit satellites like this a lot, and an ocean splashdown would not only protect humans, but it would successfully hide any sensitive equipment that we do not want our enemies to have.

But what it appears the military is doing is actually launching a missile to collide with the satellite. To some degree, this will also serve the purpose. The collision will slow the spy satellite, causing it to de-orbit. If the collision is timed properly, the satellite would fall safely into the ocean.

However, if you've ever been in a car wreck, or seen wrecks on TV, you know what happens when two big things collide. Yes, the main bodies of the cars pretty much stop, but small pieces go flying in every direction, some at very high speed. Now, image that the collision is at thousands of miles per hour. The same thing will happen, with small pieces flying off at even faster speeds.

In space, some of those pieces will stay in orbit, forming an ever-expanding cloud of debris. If some of the small chunks of debris, even something as small as a bolt that we cannot detect with radar, were to hit the space shuttle or space station, it could puncture the spacecraft and endanger the entire crew, not to mention millions of dollars of investments. Or, the debris can hit commercial satellites in low-earth orbit, including many different communications systems.

Last year, the Chinese shot down one of their aging weather satellites to test anti-satellite capabilities. The resulting debris field now threatens satellites of every space-faring nation.

The problem is worse than it looks. The more debris that is up in space, the more likely that some chunk of it will collide with another dead satellite that can't move out of the way. That collision creates more debris, increasing the likelihood of debris collisions, and threatening a runaway reaction that could clog the space near Earth with shrapnel and make low-Earth orbit too dangerous for satellites or people.

I don't want to be too alarmist here -- we are unsure how much debris it takes for such a runaway to occur. Some scientists think we are near that point, others don't. If there is one saving grace, it is that most of the debris in low-Earth orbit will slowly fall out of orbit in the next several years. But if the debris is replenished at a rate faster than it falls, the problem slowly gets worse.

In my opinion (and let me stress that this is an opinion; I have no information on this than what is in the news), this shoot-down is not as advertised. It is not an attempt to protect people from the fuel or other hazardous components of the satellite. It's not even an attempt to ensure that our enemies don't get their hands on the satellite. There are other ways of meeting these goals without creating an expanding field of sometimes-undetectable debris that threatens both our astronauts and our vital commercial satellites.

No, I think the obvious purpose of this test is to demonstrate that the U.S. can shoot down satellites. We've proven this in the past -- both the U.S. and the Soviet Union performed several such tests through the mid-1980s. The technology required for a "shoot-down" doesn't really change, so I am skeptical that any test is necessary. In other words, we're just proving to other countries that may have recently shot down satellites that we can do the same (though we've proven that many times in the past), and at the same time we are risking setting off yet another space arms race, all the while endandering our astronauts and multi-million dollar investments in space hardware. The gain is small, and the risk is large.

The (miniscule) threat of a US spy satellite

Image Credit: Chris Peat, Heavens-Above GmbH

This weekend, I opened the newspaper to see a story with the title "Disabled Spy Satellite Threatens Earth." I was curious, since I didn't know that a single satellite, especially a defunct satellite, could threaten the entire planet. So, I read the article, and I still know that I was right -- Earth is not threatened by this satellite.

First, let's figure out what the story is about beyond the over-hyped headline. The if you read the story, you'll see that the facts are that a U.S. spy satellite has lost power. That also means it has lost the use of its thrusters. And that's about it for details, with some knowledgeable sources saying the satellite will re-enter Earth's atmosphere in late February or in March.

Our satellites in low-Earth orbit (those satellites orbiting less than about 1200 miles above Earth's surface) are not in a pure vacuum of space. Sure, there isn't exactly a lot of air around, but there are tiny bits of Earth's atmosphere up there. As a satellite orbits, that tiny bit of atmosphere drags on the satellite, slowing it down and causing it to slowly inch toward the Earth. As it gets closer to the Earth, the thickness of the atmosphere increases, so the drag increases, so the rate of slowing increases, so the satellite's descent speeds up, and the atmosphere continues to get thicker, and so on until the satellite falls back to Earth. It doesn't matter whether a satellite has electricity or not, or whether it is under control or not. All Earth satellites in low-Earth orbit will fall back to Earth in a timescale of months to decades, depending on how high up the satellite is.

There is one way to keep from falling back, and that is to use a rocket engine to give a satellite a little acceleration to counter-act Earth's atmospheric drag. As long as the engine still works and has fuel, the satellite can orbit as long as it wants.

The graph at the top of this post is a plot of the height of the International Space Station over time. You can see that, most of the time, the space station is slowly sinking back toward Earth, and every once in a while it very quickly moves further away again. These boosts in the station's altitude are caused by rocket engines on visiting spacecraft (Russian Progress supply rockets and the Space Shuttle). These boosts keep the station in orbit.

The Hubble Space Telescope doesn't have a rocket engine, but it is higher than the space station and so can survive longer. Every time a shuttle visits the Hubble, they use the shuttle's engines to boost Hubble back up to a safe orbit.

But satellites without rockets, or satellites that lose power and go out of control (like the US spy satellite) will eventually fall back to Earth. Most satellites disintegrate high in the atmosphere and completely burn up; larger satellites (like NASA's Skylab or Russia's Mir space station) can have pieces that survive re-entry; pieces of Skylab were found across Australia.

For that reason, countries now try and de-orbit useless satellites by using a rocket engine to cause the satellite to fall into the ocean. Astronauts on the next Hubble repair mission will attach some equipment to the Hubble that will allow a future robotic spacecraft to attach an engine to Hubble to bring it down. But, if a satellite doesn't have an engine, or if the engine isn't working, then the spacecraft could fall anywhere.

As you might imagine, some satellites have some pretty nasty stuff. Rocket fuel can be hazardous, and some satellites have radioactive power supplies (these are built to withstand re-entry intact so as not to spread radioactive particles over the Earth). If those satellites fall in a populated area, people could get sick. But most of the Earth is water, and most of the rest is unpopulated, so the chances of a noxious rogue satellite part hitting a city are very tiny.

So, the real story of the US spy satellite is, if a piece lands in your backyard, don't touch it. The chances are it wouldn't harm you, but it might make you sick. And, more to the point, there are parts of that satellite that the government doesn't want anyone to see. So picking up a piece of the satellite for decoration is probably not a good idea, unless you'd like an inside look at Guantanamo Bay.