Asteroids and Comets and things, oh my!

The past couple of weeks have been excellent for those interested in asteroids and similar objects. Since I've been slow on the updates recently, I feel I owe you readers a three-for one deal, focusing on the smaller bodies of the solar system, asteroids and comets.

Number One is asteroid 2867 Steins, a three-mile lump of rock wandering around in the relatively asteroid-rich region between Mars and Jupiter known as the "asteroid belt". In itself it's nothing special, but the neat news is that the Rosetta spacecraft from the European Space Agency recently flew close to Steins, coming within 500 miles. (Hey, for a spacecraft, that is close.) Some of the pictures are shown above, and featured in today's Astronomy Picture of the Day

Follow up:

The asteroids are bodies that formed when the solar system was very young, as bits of rock stuck to each other and accumulated into ever-bigger bodies. In the inner solar system, temperatures were too high for ice to form, so unlike comets, asteroids are made almost entirely of rock and metal. Steins is a great example - it's clearly a big rock! (Although it is possible that it's more of a rubble-pile of smaller rocks held together by gravity.) It's too small for gravity to have pulled it into a spherical shape, though, which takes it out of the running to ever be called any kind of planet.

The scarred face of Steins testifies to what a dangerous place the asteroid belt can be - in the long run. It's not that it's crowded in there, since there's an average of over a million miles between any two notable asteroids. Notice that Rosetta is even now cruising through there. However, when you're going round for several billion years, accidents will happen. Clearly it took a pretty big crash to make the large crater on "top" of the asteroid, and if you look closely, you can see a "crater chain" where a number of rocks hit all in a line, like machine-gun-fire. Read more about Steins at ESA's website (with video and 3D image!)

Number 2 is a comet, of sorts. Specifically, a ball of ice and rock that orbits beyond the orbit of Neptune, known as the "Kuiper belt". The icy bits orbiting here are known, unsurprisingly, as "Kuiper Belt Objects", or KBOs. Have I mentioned that astronomers are not generally that creative with names? Anyhow, technically even the dwarf planets Pluto, Eris, and Makimaki are extra-large KBOs... they're icy and they orbit in the Kuiper Belt, which is really all it takes.

This particular KBO is a weirdo nicknamed Drac. Scientists like weirdos - they challenge what we think we know, and force us to re-evaluate our hypotheses and theories. You see, most of the KBOs move peacefully around the Sun in the same direction as the planets do... counter-clockwise, if you were looking down at the solar system from a spot far above Earth's north pole. Drac, however, has an orbit tilted at a very high angle, so that it appears to be orbiting backward with respect to the other KBOs. The name Drac, you see, is short for Dracula, who was supposed to have the ability to walk on walls, at right angles to normal people... have I mentioned that scientists are often also tremendous geeks?

Anyhow, most KBOs seem to be objects that formed right around where we see them today. Out beyond the Kuiper Belt, though, is a scattered distribution of ice-balls that were flung out of the planet-dominated parts of the solar system by gravitational slingshots from the giant planets - it's called the Oort Cloud. Researchers believe Drac had been out in the cold depths of the nearest part of the Oort cloud. Then something - a gravitational tug from a more-distant star, perhaps, or a close encounter with another such object - sent it spinning to a new orbit in the Kuiper Belt.

Actually, Drac should consider itself lucky. Some objects nudged out of the Oort Cloud that way end up zooming as comets into the inner solar system, and repeated close encounters with the Sun can be bad for the health of a giant ball of ice and dust. Drac's current position, compared to that, is pretty stable. You can read more about Drac and its origins at Universe Today.

Number Three is ... er... well, we thought it was an asteroid, but apparently it's a comet. (You'd be surprised at how often this happens, actually.) This Near-Earth Object (NEO) had never earned a name, just a catalog number: NEO 2001 OG108. Because of its position in the solar system (see Steins, above) everyone assumed it was a rocky critter - an asteroid. However, recently, space.com reports that it decided to remind us of the dangers of assumptions. It started to give off a bright coma - the haze of gas that comes from icy material being vaporized as the body is heated by sunlight. Apparently it was a comet in disguise, all along!

But how did it get here? It couldn't have formed in the inner solar system, where was too hot in the early days for ice to condense and stick together in newly-formed bodies. (Indeed, the fact that ice starts to vaporize as such bodies get close suggests it's still too warm around here for their comfort!) Instead, it's probably an extreme case of what happened to Drac, above. It may have started life as a KBO or Oort-Cloud comet, then something out there gave it a gravitational shove, and it was pushed into an orbit that brought it to our part of town. As long as it was out beyond Mars, it looked deceptively asteroid-like, but as it got closer in, it couldn't stand the heat, and developed the characteristic comet coma.

Scientists are now pursuing a study of these impostor objects, to find out where they come from, what they're made of, and how they get to the inner solar system. These could turn out to be vital questions - remember, this example is a Near Earth Object, which means someday we may need to know enough to deflect or destroy such an invader from outer space.