47 moons and counting: a space update

Every year, the Jet Propulsion Laboratory in Pasadena, Calif., holds Open House weekend, the only time the public is free to come in and wander around.

We JPL-ers brace for it. Usually, about 50,000 people show up, and it never ceases to amaze me how interested and cheerful everyone seems, even after having to wait much too long for parking. I spend most of my time at the Ask a Scientist booth, where four or five bonafide scientists and engineers sit ready to answer any question a member of the public would like to ask. We get all sorts, from conspiracy buffs who accuse us of hiding alien spaceships, to young children curious about how astronauts use the bathroom in space. One man even who stood there seriously talking to me for more than an hour as I tried to explain string theory.

Of course, none of us can answer all the questions. We are all assured ahead of time that 'I dont know' is a legitimate answer, hopefully followed by some suggestions as to where the information might be obtained. So I wasn't feeling too bad when a grade-school age girl asked us how many moons the planet Saturn had.

Honestly, I had no idea. I knew it was a lot, but with the Cassini spacecraft cruising around the Saturn system, making new discoveries as we spoke, I figured the number was probably subject to change anyway. Then something wonderful happened. The scientist sitting next to me at the booth picked up her cellphone and called a friend of hers who happened to be on the Cassini team. Answer: for the moment, the official count was 47 moons. The team was still arguing about three other objects, which they weren't sure were big enough to qualify as moons.

This simple exchange reminded me how much I love astronomy for how changeable everything is. I'm only in my mid-30s, but things I learned in graduate school, or even taught my own students, are now woefully out of date. With that in mind, I thought I would provide a bit of an astronomy up-date, especially for a few subjects on the cutting edge of discovery. There might be some good trivia questions in here, but beware: chances are the answers will change in a month or so.

One question I like to pose to the public is, how many planets are there? Most people guess nine, as that's the official count in our own solar system (more about that later). But what about planets going around other stars? How many of them do we know about?

As of now, and yes, I did just check this moment, we know of 154 extrasolar planets. Thirty-nine of those planets are orbiting stars you can see with your naked eye. We haven't just found solitary planets, either; we know of 97 other planetary systems with more than one planet orbiting the star. Some of the planets even orbit binary stars, giving their solar systems two suns instead of one. One planet was even found in a triple-star system. For the latest on extra-solar planets, give the Planetquest website a regular check at http://planetquest.jpl.nasa.gov.

So, planets. Let's get back to the ones we're most familiar with. How many planets are there in our solar system? Yes, historically there are nine planets, with Pluto being the farthest from the sun, at least during most of its orbit. And yes, there may be good reason not to call Pluto a planet, as its off-center orbit, low mass and icy composition make it much more like an icy body than a planet.

We've found other large icy things out there, like Quaoar, which is much like Pluto (about half the size), only a bit farther out from the sun (a billion miles farther than Pluto). Currently, the farthest planet-like object we know of in our own system is Sedna. At the moment, Sedna is 8 billion miles away from the sun, and that's about the closest its ever going to get. Sedna has a weird, elongated orbit that takes it way out into space, about 84 billion miles at its farthest point, and takes about 10,000 years to make one pass around the sun. We don't have a great measurement of Sedna's size yet, but we know its bigger than Quaoar, probably about three-quarters the size of Pluto.

Since we're talking about things that are very far away, why not take it to the limit? Another dynamically changeable subject: the farthest object we can see. The farthest object you can see with the naked eye is the Andromeda Galaxy, which looks like a dim smudge just above the middle of the V shape that makes up the constellation Andromeda (assuming you can get to a very dark place). That smudge is really the combined light of about 100 billion stars shining over 2 million light years away (one light year is 5,865,696,000,000 miles).

But hey, we've got much better things than the human eye to observe with. What's the most distant object that our giant, painfully sensitive telescopes can see? For this question, I couldn't find a single, exact answer. There are several objects that are about 13 billion light years away, but there are some decent errors associated with estimating the distance to something that far away. At the moment, there are a couple of young, bright galaxies vying to be the farthest observable object. Sometimes these galaxies are called quasars, although again, astronomers are arguing about definitions.

A quasar is the unimaginably bright core of a young galaxy (remember, when you look billions of light years away, you're also looking billions of years into the past), probably powered by huge amounts of matter getting pulled down a massive black hole. Quasars are hundreds of times as bright as our Milky Way Galaxy, which, simply put, is why we can see them from really far away. Here, as you might guess, the time lag is astonishing. When we look at these distant quasars, were seeing the universe as it looked 13 billions years ago, or when the universe was only about half a billion years old.

If quasars are lit up by vast amounts of matter streaming in toward a massive black hole, how massive are we talking about here? We're not sure about what's lurking in the heart of the brightest quasars, but in nearby active galaxies (galaxies that almost certainly were quasars when they were younger), we have observations of stars whipping around central black holes of up to 3 billion times the mass of our sun. A black hole gains mass by swallowing material, so you guessed it, these monsters have devoured the equivalent of billions of suns.

There are so many other great things going on right now. Did you know that the Spirit Rover on Mars has taken a movie of a Martian tornado spinning by? Although the winds were weak by our standards (Mars does have a very thin atmosphere), the diameter of the funnel was about a mile across! Check out the movie at http://science.nasa.gov/headlines/y2005/14jul_dustdevils.htm Or did you miss the drama when the other rover, Opportunity, got stuck in the sand for weeks? You can see the mess we made in the Martian sand trying (and finally succeeding) to get out of Purgatory Dune, as well as other recent images at http://www.jpl.nasa.gov/missions/mer.

On the topic of messes, we just made a big one out of comet Tempel 1, smashing a football field-sized crater out of it on July 4. You can down-load images taken from the doomed impactor spacecraft as it hurtled toward its target at http://deepimpact.jpl.nasa.gov/gallery/images-impactor.html .

For closure, maybe we should head back to Saturn, with its 47 moons and counting.

The Cassini mission has already been a huge success, returning spectacular images of Saturn, its moons, and its rings. One of my favorite images is of a small moon orbiting inside two rings, surfing along a wave in the ring caused by its gravity: http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=570 .

I am also blown away by what the Huygens probe found on Saturn's largest moon, Titan. As planned, Huygens parachuted through the thick atmosphere of the moon in a surprisingly turbulent descent and landed on the surface. Scientists didn't know whether to expect a hard landing or a splash-down on a super-chilled lake; the spacecraft was designed to accommodate either scenario. As it was, they got a slush-down in icy mud. I use the term ice broadly, as the surface of Titan is so cold that any water ice would be frozen as hard as steel. In fact, the Cassini team has found evidence of volcano on Titan, but the lava oozing out of the crater isn't made of molten rock, but water.

Water lava? Judging from the descent images, Huygens may have come to rest on the border of a lake, but again, not the sort of lake you usually think of. This lake, if it really was liquid, was probably made of liquid ethane. From high above, the orbiting Cassini spacecraft has just made another startling discovery - this time of a much larger lake, something about the size of Lake Ontario. The ink-black color of the lake suggests that it might be full of liquid hydrocarbons, a super-cooled brew of organic material.

From Great-Lake sized pools of organic goo to giant black holes at the edge of the visible universe, is all changing out there. And far from being confident, staid experts, we scientists are just going along with the ride like everyone else. Lets all stay tuned together.

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