Stardust Surprise

When NASA’s Stardust spacecraft flew by Comet Wild 2, the probe saw
something that surprised astronomers.

January 16, 2004: On Jan. 2nd, 2004, NASA’s Stardust spacecraft approached
Comet Wild 2 and flew into a storm. Flurries of comet dust pelted the craft.
At least half a dozen grains moving faster than bullets penetrated
Stardust’s outermost defenses. The craft’s 16 rocket engines struggled to
maintain course while a collector, about the size of a tennis racquet,
caught some of the dust for return to Earth two years hence.

All that was expected.

Then came the surprise. It happened when Stardust passed by the core of the
comet, only 236 km distant, and photographed it using a navigation camera.
The images were intended primarily to keep the spacecraft on course. They
also revealed a worldlet of startling beauty.

At the heart of every comet lies a "dirty snowball," a compact nucleus of
dust and ice that the sun vaporizes, little by little, to form the comet’s
spectacular tail. These nuclei are hard to see. For one thing, most are
blacker than charcoal; they reflect precious little sunlight for cameras.
Plus they’re hidden deep inside a cloud of vaporizing gas and dust, called
"the coma." Stardust’s plunge into Wild 2’s coma allowed it to view the
nucleus at close range.

Previous flybys of Comet Halley by the European Giotto probe and Comet
Borrelly by NASA’s Deep Space 1 revealed lumpy cores without much
interesting terrain–as expected. These comets have been sun-warmed for many
thousands of years. Solar heating has melted away their sharpest features.

Comet Wild 2, however, looks different. "We were amazed by the feature-rich
surface of the comet," says Donald Brownlee of the University of Washington,
the mission’s principal investigator. "It is highly complex. There are
barn-sized boulders, 100-meter high cliffs, and some weird terrain unlike
anything we’ve ever seen before. There are also some circular features," he
adds, "that look like impact craters as large as 1 km across."

"The high cliffs tell us that the crust of the comet is reasonably strong,"
notes Brownlee. It’s probably a mixture of fine-grained rocky material held
together by frozen water, carbon monoxide and methanol. Certainly a lander
could touch down there, or an astronaut could walk across the surface
without worrying too much about the ground collapsing.

An astronaut standing on Comet Wild 2 would see a truly fantastic landscape,
speculates Brownlee. "I imagine them inside one of the craters, surrounded
by deep cliffs." Icy spires, as tall as a person, might rise out of the
crater floor. "These would be be the comet-equivalent of ‘snow spikes’ on
Earth–those little jagged ridges that form when snow is exposed to sunlight
and melts."

Getting out of the crater would be easy. "Just jump," says Brownlee, "but
not too hard." The comet’s gravity is only 0.0001-g, so "you could easily
leap into orbit."

Some of the photos from Stardust reveal gaseous jets. "The jets come from
active regions on the comet’s surface, fissures or vents probably, where the
ice is vaporizing and rushing into space," Brownlee says. This is how mass
is transferred from the comet’s nucleus to its tail.

Viewed from the surface, the jets would be nearly transparent. But an
astronaut could spot them by looking for "dust entrained with the gas. Dust
grains glinting in the sunlight would look like tracer bullets shooting out
of the ground."

A careful explorer could survey the entire 5-km nucleus in only a few hours,
leaping high above the surface, dodging the occasional jet. "What an
experience that would be," he says.

There are billions of comets in the solar system. "We’ve gotten a close-up
look at only three," says Brownlee. And one of the three, Comet Halley,
presented its night side to the cameras. So it’s too soon to say whether
Comet Wild 2, among comets, is truly unusual.

Unlike comets Halley and Borrelly, notes Brownlee, "Wild 2 is a very recent
arrival to the inner solar system." For billions of years it orbited in the
cold deep space beyond Jupiter, until 1974 when it was nudged by Jupiter’s
gravity into a sun-approaching orbit. Since then the comet has passed by the
Sun only five times; solar heating is only beginning to mold its surface.

And, according to Brownlee, that might be the key to the comet’s appearance.
"Wild 2’s surface is a mixture of young and old that we haven’t see before,"
he explains. Young features include possible sinkholes collapsing as the
terrain is warmed. Impact craters and their ejecta, on the other hand, are
old scars from time spent in the outer solar system.

The old parts of Wild 2 are what make the comet an attractive target for the
Stardust probe, which captured a thousand or more grains of comet dust
during the flyby. Such material, little altered since the formation of the
solar system, could tell us a great deal about our origins.

The craft’s payload will return to Earth in 2006 for analysis by scientists.
If a single picture from the navigation camera can surprise researchers,
just imagine what’s in store when they get their hands on a thousand pieces
of the comet itself.