[meteorite-list] Rendezvous With a Comet (Deep Impact)

[Ron Baalke]

http://rockymountainnews.com/drmn/state/article/0,1299,DRMN_21_3882379,00.html

Rendezvous with a comet

Boulder craft seeks solar-system data in cosmic collision

By Jim Erickson
Rocky Mountain News
June 25, 2005

BOULDER - It's the ultimate July 4 bash.

After cruising 268 million miles through space, NASA's Boulder-built
Deep Impact spacecraft is on course for a holiday weekend collision with
a jet black, pickle-shaped comet named Tempel 1.

A copper-fortified "impactor" the size of a washing machine will blast a
crater in the side of the comet, dredging up primordial ices that should
help scientists refine theories about the solar system's formation 4.6
billion years ago.

The energy released at impact will be roughly equivalent to detonating 4
1/2 tons of TNT and could gouge a pit the size of Invesco Field. The
light flash produced at the moment of impact could be visible in
telescopes on Earth, 83 million miles away.

"Normally, we build spacecraft that do things that are a bit more
mundane than this," said Charlie Schira, a Ball Aerospace & Technologies
engineer on the Deep Impact team.

"You don't get to go smash into something all that often," Schira said.
"I believe that this is the coolest mission I'll probably ever work on.
It's certainly been the hardest."

The Boulder company built Deep Impact and its four science instruments.
At peak staffing, 340 Ball employees worked on the $333 million project.

A team of 20 Ball engineers will help NASA control the Deep Impact
spacecraft next week at the Jet Propulsion Laboratory in Pasadena, Calif.

Other spacecraft have flown through comet tails or through the coma, the
glowing cloud of dust and gas that surrounds a comet's icy core. But
Deep Impact will be the first to attempt a direct hit on a comet's core,
known as the nucleus.

It's a daunting challenge, roughly equivalent to firing a BB gun at a
charcoal briquette 6 miles away.

At night.

With a gun that holds just one BB.

"This is a tough shot. It's a very tough shot," Ball spacecraft system
engineer Tom Bank said.

Ball engineers say there's a 90 percent probability that the 820-pound
impactor will hit its target.

For the Deep Impact mission to be a total success, however, a bull's-eye
hit isn't good enough. The impactor needs to hit the comet's sunlit half.

Tricky choreography

Deep Impact is actually two spacecraft: the impactor and a Ford
Explorer-size mother ship, known as the flyby spacecraft.

The impactor will target Tempel 1's sunny side so the flyby spacecraft
can photograph the collision and analyze the composition of the icy
debris thrown from the crater.

It's a tricky piece of cosmic choreography.

"Hitting the comet is definitely the most challenging part of this
mission, but right on its tail is getting the pictures of the impact
from the flyby," Bank said.

The impactor will be released into the path of the oncoming comet 24
hours before the collision. It's a bit like placing a penny on a
railroad track as the locomotive bears down - except there's no track to
guide the train onto the penny.

Instead, the impactor will rely on an onboard camera, a navigation
system and thrusters to refine its position autonomously, like a smart
bomb homing in on its target.

As the impactor executes its suicide mission, it will be buffeted by
dust and small chunks of debris. Its camera lens may get sandblasted,
and engineers say there's at least a 50-50 chance that data transmission
will be knocked out during the final 10 seconds before impact.

Flyby craft to record event

Meanwhile, the flyby spacecraft will record the event from a safe
distance, then beam pictures back to Earth.

"It's a bullet trying to hit a second bullet, with a third bullet in the
right place at the right time, watching the first two bullets and
gathering the scientific data from the impact," said Rick Grammier, Deep
Impact project manager at NASA's Jet Propulsion Laboratory.

The impact is scheduled to occur at 11:52 p.m. MDT, give or take a few
minutes, on July 3. The first pictures from a camera on the impactor
should arrive on Earth less than 10 minutes later.

Those initial images will be posted on the space agency's Web site:
www.nasa.gov .

NASA TV, also available through the Web site, will carry live coverage
of the event.

Though the holiday bash could produce striking pictures and new
scientific insights, most scientists agree that the impactor is simply
too small to shatter Tempel 1 or alter its orbital path. The comet is
about half the size of Manhattan Island.

"In the world of science, this is the astronomical equivalent of a 767
airliner running into a mosquito," said Don Yeomans, a Deep Impact
scientist at JPL.

When the impactor strikes Tempel 1, about 100 professional astronomers
will be watching at 60 ground-based telescopes in 20 countries.

Three NASA-orbiting observatories - Hubble, Spitzer and Chandra - will
be looking too, along with a "small army" of amateur astronomers who've
been recruited to report their observations, Yeomans said.

What they'll see is anybody's guess.

Comet Tempel 1 is an oblong, dirt-encrusted chunk of ice that orbits the
sun once every 5 1/2 years, between the orbits of Mars and Jupiter. Its
nucleus is about 9 miles long and 3 miles wide, and may weigh a billion
tons or so.

The comet is too dim to be seen with the naked eye. A small amateur
telescope reveals a dim, unimpressive fuzzball with a short tail.

How bright will Tempel 1 get after the Deep Impact collision?

That depends on how much sun-catching ice and dust gets ejected, which
in turn depends on the size of the crater excavated.

Estimates of the likely crater size range from a large house to a
football stadium.

"What we don't have control over is what will happen when we do the
impact. We know so little about comets that it's very hard to predict,"
said University of Maryland astronomer Michael A'Hearn, the lead Deep
Impact scientist.

If a lot of sun-reflecting debris is ejected, the comet could become 15
to 40 times brighter than normal - perhaps visible to the unaided human
eye, according to NASA. The brightening could persist for more than a day.

If it's successful, the Deep Impact mission will provide the first
glimpse beneath a comet's dirty outer crust, exposing material that has
remained largely unchanged since the solar system formed 4.6 billion
years ago.

"Comets hold clues to the early solar system," A'Hearn said. "So the
real goal is to understand what the conditions were like that led to
planet formation in the outer solar system."

On impact day, about 2,600 Tempel 1 pictures will be snapped by cameras
on the flyby spacecraft and the impactor.

The highest-priority images will be sent back immediately, but about 90
percent of the pictures will be stored on the flyby spacecraft for later
transmission, said Phil Inslee, lead software engineer on Ball's Deep
Impact team.

To get those pictures back to Earth, the flyby spacecraft must survive
passage through Tempel 1's dense inner coma, a cloud of gas, dust, ice
and pebbly debris swarming around the nucleus.

After photographing the impactor's demise, the flyby spacecraft will
turn so that its dust shields are facing the oncoming debris, which will
be streaking by at 23,000 mph.

Fourteen minutes after the impactor is vaporized, the flyby spacecraft
will pass 311 miles from Tempel 1's nucleus. The craft is designed to
survive the impact of quarter-gram chunks, which are about the size of
one-quarter of an M&M.

Anything bigger could punch a lethal hole in the spacecraft.

If the flyby spacecraft is crippled, most of the comet images and other
science data could get stranded in space.

"That, I think, is what everybody is really feeling," said Monte
Henderson, program manager for the Ball team. "We know we've done
everything we can, but we aren't 100 percent in control of this mission."

Mirror causes glitch

Deep Impact's most powerful camera-telescope is the High Resolution
Instrument which rides on the flyby spacecraft. It was designed to see
surface features as small as 6 feet across on Tempel 1's nucleus.

But because of a focusing problem discovered after Deep Impact's Jan. 12
launch, the raw images are as much as four times fuzzier than expected.

Image-processing software will be used to sharpen the comet photos sent
back to Earth.

The focusing problem was traced to a mirror used when the HRI was tested
at Ball Aerospace in Boulder. While flat at room temperature, the 8-inch
mirror unexpectedly developed a slight concavity during testing at
ultra-cold temperatures.

The mirror deviated from flatness by an infinitesimally small amount:
about one-fortieth the thickness of a human hair, Henderson said.