Thursday, December 21, 2006

Traveler's Tales

When you know a thing, to hold that you know it; and when you do not know a thing, to allow that you do not know it—that is knowledge. ~Confucius

Scientists had high hopes for the Stardust mission, and, so far, they aren't being disappointed.

Stardust spent five years catching up to comet Wild 2 (pronounced Vilt, for those of you who wish to be precise), caught a myriad of tiny particles in its aerogel-filled collectors, and high-tailed it back to Earth, landing in the Utah wilderness softly (unlike its friend Genesis, who pancaked in the desert). After an exciting hour or two of searching, the team recovered Stardust, and the scientists have been going slightly crazy ever since.

There are so many impacts in the aerogels that the public has been enlisted to search through the collector images to try to identify promising particles. Stardust will be yielding information for years to come.

It has already served up some treats. One particle has been discovered that is apparently not from our solar system. This is determined by the oxygen isotope content. This tiny bit has 150% more of the heavy oxygen isotope than it should have. Comet Wild seems to have collected some detritus from outside our immediate neighborhood.

Most of the samples studied so far would indicated that Wild, like many such objects, formed out in the Kuiper belt and has only lately wandered into the inner part of the solar system. At least, that should be the case, but one of the samples returned isn't from the icy reaches beyond Pluto. It seems that it is something called osbornite, which forms in temperatures of 3000 degrees Kelvin.

That's hot, and how something heated like that got into the comet is a mystery. Scientists can only speculate that the nebula in which the Sun and planets formed was more active than previously imagined. Then there's the organic material.

"We didn't expect any organics to survive" from the impact and heat during the collection process, said Livermore researcher Sasa Bajt (see Resources for the Science Daily article). "But we found a rich variety of organics that were both oxygen-rich and nitrogen rich compared to organics previously found in meteorites.” Finding organic materials lends considerable credence to the theory held by some that life on Earth was “seeded” from extraterrestrial sources.

Even the idea of what a comet is has come under scrutiny. For years, the general theory of cometary makeup was best described as the “dirty snowball” scenario. All comets were formed away out there somewhere out of gases and light materials left over from the creation of the Sun and planets. Away out there was where they stayed until perturbed by something or other, at which point they would come into the inner planetary area, outgassing geysers of some sort of melting matter as they neared the Sun. The biggest question was whether they were relatively solid or whether they were just jumbles of rubble loosely held together.

Now, it seems that every time we get near a comet, we find out something different. Wild 2 has been both very hot and very cold. Tempel I, target of Deep Impact had its own surprises.

Deep Impact was a remarkably ambitious attempt to send a probe (if one can call something the size of a washing machine a “probe”) crashing into a comet to study the ejecta and possibly see into the comet. The Deep Impact team was both delighted and disappointed by the result. They were elated that they actually hit the bloody thing. After all, you've got Deep Impact hurtling headlong toward a large fast object; at full tilt, you push off a probe which has its own navigation system and swing the main satellite out and around so it won't hit Tempel. By some miracle, the probe actually gets where it's supposed to go, and the result is a spectacular outburst of light.

That was also the bad part. As Deep Impact whizzed by, it couldn't see into the comet at all because the glare was terrific. But, the Swift X-Ray telescope saw something, and that was odd, because it wasn't supposed to. No one is quite sure yet why there was an outpouring of X-rays from Tempel I and why it should have lasted for 12 days. In fact, optical telescopes only saw optical flaring for about 5 days, so the mystery of what was going on in the remaining 7 is perplexing.

Amazingly for the amount of vaporization, it also turns out that Tempel is only 6% water, with the rest being a rather fragile agglomeration of dust. So where did the X-rays come from?

One of the Deep Impact scientists put it best, soon after the successful probe impact. At the time, we had taken a close look at three comets, Halley, Wild (photographic returns; the samples were still in route), and now Tempel. Here we were, expecting comets to be pretty similar in makeup, yet all three were very different. It seems like the more we were learning about comets, the less we knew. It also turns out that we know a lot less about the early solar system than we thought.

Carl Sagan once talked of the Voyager satellites sending back 'traveler's tales.” Apparently, Stardust, Deep Impact, and even Genesis (whose sample collectors are still being diligently cleaned) have new tales to tell us.

We're listening.

Resources:
Stardust's Comet Clues Reveal Early Solar System
"Deep Impact" Comet Spewed Tons of Water, Study Finds
Comet grain confirms early solar system mixed it up
Space Probe Brought Real Stardust Back to Earth
Comet Particles Tell New Story About Birth Of Solar System

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