Museum of the Galaxies

"The comet is largely composed of stardust–literally grains sampled from millions of other stars–that got together to form the planets. ..Comets are the museum of the galaxies." –Don Brownlee, principal investigator for Stardust mission

On January 2, the spacecraft Stardust entered a region ‘between reality and never-never land–an unknown environment’, said Stardust Project Manager Tom Duxbury of NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

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Stardust spacecraft beaming back its images soon after surviving its Wild- 2 comet encounter.
Credit: NASA/JPL

The mission highlights the first space sample return since the Apollo moon landings, the first comet dust to be collected in its most pristine condition when still rich in methane- and water-ice, and in two years, the first time any human-made object will attempt a safe-return at such high entry speeds. "The launch was exciting, we were just cruising for awhile, we flew by the asteroid Annefrank, and in 2006, we will try to land at the Utah Test Range. For the Mars rover team, their descent is the beginning of their mission, but for Stardust, the descent to Earth will be the end of ours."

Having weathered its out-of-this-world sandblasting by cometary particles hurtling toward it at about six times the speed of a rifle bullet, NASA’s Stardust spacecraft begins its two-year, 1.14 billion kilometer (708 million mile) trek back to its planet of origin, Earth. "Comet Wild 2 gave up its particles but it did not do so without a fight," Duxbury said "Our data indicates we flew through sheets of cometary particles that jostled the spacecraft and that on at least 10 occasions the first layer of our shielding was breeched. Glad we had a couple more layers of the stuff."

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Comet Halley imaged by European flyby.
Credit: ESA

Stardust entered the comet’s coma – the vast cloud of dust and gas that surrounds a comet’s nucleus – on December 31, 2003. From that point on it kept its defensive shielding between it and what scientists hoped would be the caustic stream of particles it would fly through. And fly through cometary particles Stardust did, but not in the fashion the team envisioned while designing the mission.

"We thought we would see a uniform increase in the number of particles the closer we came to the comet’s nucleus and then a reduction," said University of Washington scientist Dr. Don Brownlee, Stardust’s Principal Investigator. "Instead, our data indicate we flew through a veritable swarm of particles and then there would be almost nothing and then we would fly through another swarm."

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Comet Borrelly imaged by Deep Space I.
Credit: NASA/JPL

Stardust scooped up these cometary particles, impacting at 6.1 kilometers per second (3.8 miles per second), for almost instantaneous analysis from onboard instruments and stored other particles for later, in-depth analysis, here on Earth. Within the grid of its particle collector, one hundred cells of an ultra-low-density silica foam, called aerogel, brought these bullet-speed bits of microdust to a stop in about the width of two human hairs (100 microns). Most of the comet dust was approximately 100 times smaller than these tracks (1-2 microns); a few were in the twenty micron range; and larger ones (one millimeter) penetrated the first layer of the spacecraft’s bumper, called the Whipple Shields, after American astronomer Fred Whipple. This shield protects the ‘wind-ward’ side of Stardust’s high-speed trajectory through the comet’s sandblasting jets.

Stardust showcased four instruments in close proximity to the comet (near a few hundred miles). Its aerogel collector, which is about the size of a tennis racket, will return the first space samples to earth since Apollo. "Like lunar or meteor samples", said Brownlee, "a large number of scientists around the world will be using their best instruments to understand" these samples. "Wild-2 formed around Pluto [at the edge of our solar system] 4.5 billion years ago, in the Kuiper Belt [an outer haven for primitive solar system bodies]. The comet is largely composed of stardust–literally grains sampled from millions of other stars–that got together to form the planets. ..Comets are the museum of the galaxies".

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Jets and pockmarked surface of comet Wild-2, viewed from within a few hundred miles
Credit: NASA/JPL

Along with this cosmic taste testing, the spacecraft’s camera also took some remarkable images of comet Wild 2’s five-kilometer wide (3.1-mile wide) nucleus. To get a good sense of the comet’s density would have required a more dangerous approach to about five times closer. "It’s great we could see features on the comet," said Brownlee. "It could have been a smooth, boring surface." The camera components are not unlike what digital enthusiasts are now familiar with–a 200 millimeter lens and a 1.4 megapixel image density. But like taking an image of the ocean surface from a submarine, the camera viewed its hostile particle stream from the safety of a periscope.

"Our navigation camera was designed to assist in navigation, not science," said Stardust’s imaging team lead Ray Newburn. "But these are the best images ever taken of a comet and there is a remarkable amount of information in those 72 pictures. Not only did we image the jets of material spewing out from the comet, but for the first time in history we can actually see the location of their origin on the surface of the comet."

Three large comet jets registered on the third Stardust instrument, its dust counter. Three distinct peaks appeared with thousands of particle strikes each. Slightly less than an ounce of comet dust, or about a thimbleful, were collected over the spacecraft’s 12 minute pass through these large jets. "The secret of our mission is that we sample only the volatile material, that which is evaporating into space," said Brownlee. "That’s the way we avoid any contaminants that might have left those impact-like marks on the comet’s surface. So it was better in this case to fly-through the lighter dust stream, than to land on this comet. We’d have to drive around a bit to find just the comet stuff." In just such a science-fiction scenario of landing on a comet, the European mission, called Rosetta, will launch next month and travel to comet Churyumov-Gerasimenko in November 2014.

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During the comet flyby, the closest view yet of a comet at about 149 miles.
Credit: NASA/JPL

The fourth Stardust instrument, a spectrophotometer, will give scientists an idea of the comet’s chemical composition, most likely its precious wealth of volatile ices made from methane and water.

One mystery from the close-views of Wild-2 was its pockmarks. "Last night I looked at the images in stereo view," said Brownlee. "One large depression has a bottom that is flat, with very steep walls [400-500 meters deep]. While any scientific evidence is only two days old," most impact craters are expected to be bowl-shaped with much shallower aspect ratios (0.1-0.2), meaning they are five times wider than they are deep. Some of these depressions are not round, but scalloped and much deeper (aspect ratio, 0.4).

"I am from Washington state", said Brownlee, "and when the comet is viewed in stereo pairs like that, it reminds me of Grand Cooley, with its steep cliffs and run-out areas at the bottom. Like flood areas from the Columbia River, if you were standing at the bottom of one of these comet depressions. But the floor of these comet depressions are incredibly complicated, like balls of clay have been mashed together and then etched."

"The mission scientists with Deep Space I," which flew by comet Borrelly, found surprising "mesas", said Brownlee. "They speculated that these walls can sometimes face sunwards, and volatiles like ice and methane may evaporate or etch that surface. But on Wild-2, we see pits, not mesas. The two comets are quite different. We may have [with Wild-2] a young comet that evolves towards Borrelly, or vice versa."

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The approximately 4 kilometer cometary nucleus to be landed upon by Rosetta, using the European spacecraft’s harpooning landing hooks
Credit: Hubble

At about 11:25 am Pacific Standard Time (2:25pm EST) on Jan. 2, only minutes after its closest approach with the comet, Stardust pointed its high gain antenna at Earth and began transmitting a data stream that took over 30 hours to send but will keep cometary scientists busy for years to come. About six hours later another event took place that goes a long way to increasing the scientists task load exponentially.

"Six hours after encounter we retracted the collector grid, with what we are all confident is an abundance of cometary particles, into the spacecraft’s sample return capsule," added Duxbury. "The next time the sample return capsule is going to be opened is in a clean room at the Johnson Space Center in the days following Earth return in January 2006."

Scientists expect in-depth terrestrial analysis of the samples will reveal much about comets and the earliest history of the solar system. Chemical and physical information locked within the particles could be the record of the formation of the planets and the materials from which they were made. Brownlee is excited about the possibilities of sharing data among many scientists, just as was the case for lunar and meteor samples.

While working at JPL near the Mars team, he has already had productive conversations with those trying to figure out surface features on the red planet. Images taken from orbital cameras have shown some ‘depressions’, particularly in martian polar regions that resemble some parts of the strange pockmarks and steep walls seen on Wild-2. They may share additional features where ‘impacts and volatiles sculpt’ a surface, whether a dense planet or a more delicate comet.


Related Web Pages

Stardust
Live Webcam of Stardust Mission
Early Wild Success for Stardust
Telescopes for Stardust
Harpooning a Comet
Two-Way Asteroid Trip Takes Off
Tale of a Comet
We Are All Made of Stars
Winter Boon From Deep Space