Stardust Surprise

When NASA's Stardust spacecraft dipped inside the coma of Comet Wild 2
last week, the probe saw something that surprised astronomers.

On Jan. 2^nd^, 2004, NASA Stardust spacecraft flew into a storm.
Flurries of comet dust pelted the craft. At least a dozen bullet-fast
grains penetrated Stardust's outermost defenses. The craft's 16 rocket
engines fired constantly, struggling to maintain course while a
collector, the size of a baseball mitt, caught some of the comet dust
for return to Earth two years hence.

All that was expected. The surprise came when Stardust got a clear look
at Comet Wild 2's nucleus.

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. The hearts of comets are very hard to see. For
one thing, most are blacker than charcoal. They reflect precious little
sunlight for cameras. And, worse, they're hidden deep inside a
Jupiter-sized cloud of evaporating gas and dust.

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---not too surprising. After all, these comets have
been "melting" for thousands (perhaps millions) of years. Try this: Drop
a jagged ice cube in a cup of warm tea. It gets smooth and featureless
pretty quickly. Sun-warmed comets get smooth for similar reasons.

Comet Wild 2 was different, though.

"We were amazed by the feature-rich surface of the comet," says Donald
Brownlee, 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. 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 on Wild 2 would see a truly fantastic landscape. "I imagine
them inside one of the craters, surrounded by cliffs," speculates
Brownlee. "The ground nearby might be spiky---like snow on Earth exposed
to sunlight for many days." The spikes on Wild 2 could be as big as a
person.

Getting out of the crater would be easy. "Just jump," says Brownlee. But
not too hard. The comet's gravity is only 0.01-g so "you could easily
leap into orbit."

Another hazard would be jets. "There are active regions on the comet's
surface, fissures or vents, where the ice is melting and jetting into
space," explains Brownlee. "These jets, would be mostly invisible except
for the dust entrained with the gas. Dust grains glinting in the
sunlight would look like tracer bullets."

A careful astronaut 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," says Brownlee.

Among comets, the rich terrain of Comet Wild 2 might or might not be
unusual. Spacecraft have only photographed three comet cores---"not a
big sample," notes Brownlee---and one of the three, Comet Halley,
presented its night side to the cameras.

Comet Wild 2 might be typical of comets newly arriving in the inner
solar system.

"Wild 2 is a new arrival to the inner solar system," explains Brownlee.
Before 1974 it orbited the Sun in the outer solar system where the sun
did little to warm it. But then it had an encounter with Jupiter that
nudged the comet inward. Closer to Earth and closer to the Sun.

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Surprises:

We were expecting to see particles impacting the particles, to come in
at a uniform rate, instead

Expecting a nucleus with subtle surface features (Halley 100m
resolution, picture taken from the dark site, Borrelly best 45 m
resolution it showed a smooth surface, some variations of terrain, dark
object.)

What we found: dust flux came in spikes, a bunch of impacts at slosest
approach, 10 minutes later more, we went through some jets. Very
dramatic! Quite a number of particles hit the spacecraft, including
quite a few large ones, at least hald a dozen penetrated the first
bumper of the whipple shield, honeycomb structure about 1cm think. 1mm
required to penetrate.

1 minute wide flurries, structures in the jet. We actually have several
ways to detect particles. Microphone elements + the Chicago instrument,
torques on the spacecraft. Thrusters on the craft were firing all the
time. 16 rocket engines, going all the time to keep the craft level and
straight. Laser gyros detect the movement. Solar panels at 16 feet lomg,
and they have meteor bumpers. Maintain spacecraft attitude with +- 2
degrees. We did +- 0.1 degrees.

20 meters for Wild 2.

What really surprised us: there were very few definitive features in
Borrely.

We were amazed by the feature rich surface of the comet. It's remarkable
feature rich. If you loo at Mars in a single picture, most of its
features you can't see in a single picture. Ditto for comets. In comet
Wild 2. What are those thing? The surface is highly complex with feature
producing shadows.

Sublimation: On thing that puts comets apart, is that they're coming
apart. They have tails, comas and acvtyivity, losing mass as a ferocious
rate. Lose 0.1% of its mass each time around the sun. These are bodys
that are highly unstable. They are subliming water vapor, CO ice,
methanol ice. CH4-OH. Comets are coming apart. When we took pictures we
toggled exposure time 10 ms 100 ms. The latter were for navigation.
Those longer exposure pictures overexpose the nucleus and show the jets.
There are these fanbtastic jets---at least a half a dozen and maybe
dozens squirting out from active regions. They are highly collimated 100
m-wide emanating from hot spots.

Borrely and Halley showed jets. Halley is a much more active comet. Mass
loss through jets. It's most clear in our images. This is gas being
blasted into a vacuum. Typical comets eject material eject material from
only a few % of their surface. Typical comets are; it's this jetting
phenomenon that makes comets odd. If the active region is near the
sun-pole, the comet can be inactive.

On the surface, we were astounded by the rich ranges of features. We see
craters, big depressions. The 1^st^ order thing is which ones are impact
craters and which are not. There are definitely some that are not. They
are highly noncircular features, flat floors and steep clips, look like
collapse features, similar to a sinkhole. There are some large
structures that are round. They might be impact craters. 0.1 -- 1 km
size. Means the sureface we're seeing is old. Most comets have young
surface. Ancient craterd surface. It's only been in its orbit for

Pre-Jupiter it orbied between Jupiter and Uranus, and before that
probably the Kuiper Belt. These comets that have a close approach to
Jupiter. The abundance of short period coments suggests a reservoir of
comets in the vicinity of Pluto. Starting 10 years ago people started
discovering them---big ones.

It may be even more surprising. Non impact craters must be very young
features 100 meters deep. Could you excavate those in only 30 years?
Wild 2 is active even at the distance from the sun. Some are definitely
not impact craters: they are irregular with flat floors. There are
craters that are round. Small features look like large rocks, barn sized
for us to see. Usually rocks are produced by impacts. We thing they
would be made of grains held together by ice. It's not clear what the
ration of ice to rock is.

Ratio of rock to ice that much grater than one. Frosty rock. Like Pluto
maybe. No similar experience, extremely fragile dirt clod. Made of very
fine dust. Go to a river bank and find some consolidated crumbly
material.

Microporous mixture of dust and ice. Processes could modify this. Some
volatives go up, and some go down. Comets are cold at depth and hot on
the surface. There might be a strong crust, we see cliffs 100 meters
high. We even The surface cannot be a lose powder. It must have some
strength. Condensed water vapor gives the material its strength. Rosetta
will

Astronaut: my vision is that you wpuld likely be inside a hole with deep
cliffs around you. Columbia river we hae basalt cliffs, quite steep, not
basalt. People walk across ice flows, which are complicated structures,
either above or in its. People are good about getting around cliffy
areas. The gravity is not you could leap across craters but you have to
be care fule not to jump into orbit. Astronauts on the moon bouncing
around with leaps amplified by

Low pressure steam vents, what you see is particles entrained in gas.
Just like smoke from a fire, it doesn't take much to see. Glowing
because of scattered sunlight, you might see some individual particles
tracer bullets (cm-sized particles) Walking around Yellowstone.

Something is making those cliffs, my guess is that they are crumbling.
Occasionally on these clifs, you get a little landslide and get a big
flare. One thing that's been mysterious to me: mixture of black rocky
material + volatiles: shine sunlight on it. Sinlight devolitalizes the
surface. Remaining dust shields the ice from sunset. Self-quencing.
That's probably why most of the surface is inactive. Something has to
open up a channels to get heat into the interiol. Black dust is a great
insulator. There's no force involved in this.

We were in the danger zone.

Landing may not be difficult, but roving could be.surface like broken up
ice ridges we see in the arctic. It's possible that these things have
even more fantastic structure at the meter level. In Colorado I wanted
to take a picture of dirty ice ablating on the side of the road. Comples
structures with needlw-like features sticking up.

Only a few truly circular features.

It really blew us away!