[meteorite-list] Spacecraft Swansong: DS1's Surprising, Puzzling Final Comet Encounter

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:55:39 2004
Message-ID: <200201021654.IAA15573_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/solarsystem/ds1_swansong_020102.html

Spacecraft Swansong: DS1's Surprising, Puzzling Final Comet Encounter
By Robert Roy Britt
space.com
02 January 2002

As NASA engineers waved an ethereal goodbye to the Deep Space 1 spacecraft
Dec. 18, communicating with the scrappy robot for the final time across 10
light-minutes of space, astronomers back home were just saying hello to the
spacecraft's prized catch, the crazy comet Borrelly.

Data and imagery sent back by Deep Space 1 after a Sept. 22 flyby of the
comet show mysterious jets of material shooting into space with unexpected
force in strange directions, like the shocks of white emanating from a mad
scientist's head.

Borrelly's head, meanwhile, is not screwed on straight.

While the comet isn't quite driving scientists mad, it's certainly got them
scratching their own noggins.

Off kilter

As a comet approaches the Sun, water ice and other chemicals, along with
dust, boil off its rock-hard nucleus, generating a cloud of debris called a
coma, or head. The head is what sometimes makes a comet visible from Earth,
when sunlight reflects off the material.

During the flyby, Deep Space 1 measured interaction of all this comet stuff
with the solar wind -- charged particles that race outward from the Sun. As
expected, the solar wind flowed around the comet.

But the nucleus was not at the center of the flow. It was like watching the
wake of a boat spread farther and faster on one side than on the other.

"The formation of the coma is not the simple process we once thought it
was," said David Young of the University of Michigan, Ann Arbor. Young led a
team that measured the wake.

Another instrument on the spacecraft, called PEPE (Plasma Experiment for
Planetary Exploration), also examined the coma. Beth Nordholt, a researcher
at the Los Alamos National Lab, which helped build the instrument, said the
PEPE data have yet to be fully analyzed. But already, she said, it confirms
the offset coma.

Answers may come

PEPE data may also show a relationship between the distorted head and the
comet's crazy jets.

"It appears as if there's a jet that is driving a tremendous amount of mass
away from the nucleus of the comet," Nordholt said in a telephone interview.

Simple enough. But there's a big problem: The jet that would be needed to
create the offset does not match the jet seen in Deep Space 1 pictures,
Nordholt said.

The visible jet shoots out about 37 miles (60 kilometers) from the
5-mile-long (8 kilometers), potato-shaped comet. Oddly, material emanates
mostly from the middle of the comet, whereas scientists had expected a more
even distribution. Adding to the perplexity, the primary jet does not point
toward the Sun, as expected based on observations of other comets.

The comet's activity may come with a price.

Borrelly dishes out so much material from its midsection -- some 2 tons
every minute -- that it will likely break in half within 10,000 years, says
Laurence Soderblom, U.S. Geological Survey researcher who led the imaging
team.

There are other Borrelly enigmas to ponder.

Researchers announced earlier this month that Borrelly is darker than any
other known object in the solar system, reflecting less than 3 percent of
the sunlight that hits it and absorbing the rest.

As black as photocopier toner, they say. Yet the brightest minds don't fully
understand how anything in space can be so dark. The finding points to a
surface made of carbon and iron, but experts say they aren't sure of this.

And no one yet knows what's inside a comet.

Like Halley

While Deep Space 1 has generated many puzzles, it has also contributed to a
new level of confidence among comet researchers.

Until the flyby, a huge chunk of knowledge about these frozen relics of the
solar system's earliest years relied on what was known of comet Halley,
which was examined in a 1986 flyby.

Since then, scientists have observed comets through telescopes using
assumptions and constraints based on their knowledge of Halley. No one knew
for sure if these baselines were correct, and thus how accurate various
studies of other comets have been.

Nordholt says that while Halley and Borrelly are quite different in terms of
their exact composition and behavior, they are all-in-all very much alike,
confirming suspicions that most comets likely formed in a similar manner and
at a similar time -- back when the solar system was gathering itself
together some 4.6 billion years ago.

"Borrelly seems to come from the same primordial stuff that Halley comes
from," she said.

Deep Space 1 findings therefore help confirm a host of other studies.

"The observational work that has been done telescopically has led us to the
correct conclusions," said Nordholt.

So while Deep Space 1 has laid down an abundant foundation of data for comet
researchers, future missions will have plenty of glory to claim as they seek
to determine the contents of comets and help unravel their strange behavior.
Received on Wed 02 Jan 2002 11:54:10 AM PST