[meteorite-list] Deep Impact: Probing A Comet's Inner Secrets

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:27:53 2004
Message-ID: <200311251607.IAA19603_at_zagami.jpl.nasa.gov>

http://www.space.com/businesstechnology/technology/deep_impact_031125.html

Deep Impact: Probing A Comet's Inner Secrets
By Leonard David
space.com
25 November 2003

BOULDER, Colorado -- All spacecraft missions aim to be a smashing success.
But in the case of NASA's Deep Impact, such desire takes on new meaning in
the form of a head-on collision with a comet.

Scientists and engineers are taking a crash course in understanding what's
inside Comet 9P/Tempel 1 -- an effort to help deduce how the solar system
was formed. Comets are time capsules. They consist of chunks of ice, gas and
dust - ancient scraps from the earliest and coldest period of our solar
system 4.5 billion years ago.

At a clean room here at Ball Aerospace & Technologies Corporation, both the
Deep Impact mission's "flyby" spacecraft and a comet "impactor" are being
built, tested, and readied for launch late next year.

Celestial fireworks

Slated for launch in December 2004, Deep Impact is a two-part hit-and-run
mission.

After a six-month cruise to Tempel 1, the combined spacecraft approaches the
object imaging the icy drifter before the impact. Twenty-four hours before
impact, the flyby spacecraft points high-precision tracking telescopes at
the comet and releases the impactor into the comet's path.

Now on its own, the impactor -- a specially equipped autonomous craft --
takes a premeditated plunge into the sunlit side of the speeding comet.

The resulting crater could range in size from that of a house to a football
stadium. The hole in the comet might be from two to 15 stories deep. In the
excavation process, the pristine interior of a comet is to be studied --
truly, in depth. This all takes place while the flyby spacecraft has
maneuvered itself to a telescopic front-row seat, but at a safe distance,
speeding by the show-and-tell at closest approach some 300 miles (500
kilometers) away.

After special shielding guards against high-velocity particles from the
comet's dust tail passing overhead, the flyby spacecraft turns to look at
the comet again. The flyby spacecraft takes additional data from the other
side of the comet's nucleus.

Images from both the flyby spacecraft and the impactor will be sent back to
distant Earth as data in near real-time.

Seemingly in a blend of orbital mechanics, celebratory patriotism and
celestial fireworks, Deep Impact's smack down with Tempel 1 is set for July
4, 2005 -- Independence Day for the United States.

"Mini-Me" Impactor

"There's a lot of newness in this program," said Monte Henderson, Deputy
Program Manager on the Deep Impact project at Ball Aerospace. "This is our
company's first program that sends us into deep space," he told SPACE.com .

Garbed in white smocks, technicians are busily checking and re-checking Deep
Impact hardware and software, moving closer to a series of essential
confidence-building tests in the coming months.

Sitting side-by-side in the clean room is the flyby spacecraft and the
smaller, "Mini-Me" impactor for the Deep Impact Mission.

"In a sense, this program is building two 100-percent capable spacecraft.
And that has been a big challenge," Henderson said. The two craft share a
large number of parts, such as electrical components and control units. The
impactor uses simpler versions of the flyby spacecraft's hardware and
software, but sports fewer backup systems.

Often, the impactor gets treated as the little brother to the flyby
spacecraft - just the rock that's going to go out and smack into the comet,
Henderson stated. "The impactor has become a very smart, fully-autonomous
spacecraft. It's capable of maneuvering and taking care of its own
positioning and targeting completely independent of what's going on with the
flyby spacecraft," he added.

The impactor totes a "cratering mass" -- 220 pounds (100 kilograms) of pure
copper.

Pushing the envelope

Work on the Deep Impact mission has been underway since November 1999. It is
a NASA Discovery-class mission, the eighth in a series of low-cost, highly
focused space science investigations. Total contract value for Deep Impact
is $300 million.

It has been an uphill battle wrestling with several technical and
cost-growth issues. A year ago, those troubles forced a cancellation review
of the project at NASA Headquarters. Issues were eventually resolved, but
led to a change in launch date that would have been next month. A year slip
to a target liftoff of December 2004 permitted more ground testing of
tough-to-master technologies. But that also meant an infusion of extra money
to keep engineers on tap for rounds of pre-flight work on spacecraft
hardware

Deep Impact has involved numerous cutting-edge technology developments. "We
are pushing the envelope in several areas on this mission," Henderson noted.

For one, a new space-based processor was necessary to handle high data rates
at comet encounter. On the flyby spacecraft, lightweight shielding had to be
created using layers of aluminum sheets to thwart particles encountered as
the craft zooms through Comet Temple 1's tail. Also, to hold a pinpoint lock
onto the speeding comet, precision-pointing technology was advanced.

"Although we have been struggling with a variety of problems on our
spacecraft computer in the last few months, it looks to me as though we have
found most of them and are on our way to fixing them.

Fortunately, we have some schedule margin and are therefore looking ahead to
launch at the very end of 2004," said Michael A'Hearn, principal
investigator for the Deep Impact Mission in response to a separate email
question. He is an astronomer at the University of Maryland in College Park,
Maryland.

Sweaty palms

While spacecraft hardware was daunting, still an unknown is what exactly
Deep Impact will find at Tempel 1, an object discovered in 1867. The comet
has made many passages through the inner solar system orbiting the Sun every
5.5 years. This makes Tempel 1 a good target to study evolutionary change in
its mantle, or upper crust.

In fact, a recent assessment of the comet shows it to be smaller than the
once projected 3 miles (5 kilometers) in diameter.

But will the comet be a solid mass? Perhaps it's a jumble of debris
underneath an ice shell? Could the impactor just shoot right through the
comet? Potato-shaped or dumbbell-shaped? Drawing closer to the object, Deep
Impact onboard telescopes and sensors can provide a detailed look.

"It's an evolving science of what this comet is - and what it's made of,"
Henderson said.

There is a lot of flexibility in the mission over the six months en route to
the target. The flight to the comet is to be run from an operations center
at the Jet Propulsion Laboratory (JPL) in Pasadena, California. They are the
world's leading experts in flying deep space missions and are overall
manager of the Deep Impact mission.

Once the impactor is released from the flyby spacecraft, some last-minute
chance for mission updates and adjustments prior to comet collision 24-hours
later are possible. "But that means sweaty palms making those decisions,"
Henderson admitted.

Big unknowns

There is a significant ground-based component to the comet mission, said
Lucy McFadden, a science team member for Deep Impact. She is a space
scientist at the University of Maryland. The entire Deep Impact team
consists of more than 250 scientists, engineers, managers, and educators.

Telescope observations of the comet are now underway, and others are being
planned, McFadden told SPACE.com in an email response to questions.

"In a year, we'll have small telescope observers measuring the comet's
magnitude," McFadden said. "The advantage to small telescope observations is
that there is potential to observe the comet more frequently and get good
temporal coverage of its brightness variation as it comes back into the
inner solar system."

Deep Impact science team members are engaged in research to better determine
exactly what they will encounter at Tempel 1. "Its rotation rate is know
pretty well. It is rotating slowly, so we won't hit and then lose sight of
the crater due to rotation," McFadden explained.

McFadden pointed out, however, there are big unknowns about Tempel 1.

Are there significant concavities that will cast shadows that will limit our
opportunities for hitting a sunlit area? If it is shaped like Comet Borrelly
-- NASA's Deep Space 1 spacecraft flew by this object on September 22,
2001-- chances are good that there will be plenty of sunlit surface exposed,
McFadden observed. "After that, the biggest uncertainty is in the nature and
structure of the comet itself, and that is why we are doing the experiment.
How big a crater will we excavate and how deep will the crater be?"

"We need the observational science community to make complementary
observations, both prior to encounter -- to characterize the comet and
enable us to plan the experiment -- as well as follow the event from Earth,"
McFadden said.

Planetary defense

Could the flying of Deep Impact offer a glimpse into potential planetary
protection?

Given the fear of harmful-to-Earth comets and asteroids, the technology
mustered by Deep Impact might well find future use in hammering out an
anti-incoming object strategy.

"Yes, it will provide important information on the physical properties that
will be essential to planning any threat diversion," A'Hearn of the
University of Maryland said. "However, it will be applicable only to the
small subset of potential impactors that are comets (or extinct comets)," he
added in response to email queries.

Henderson of Ball Aerospace called Deep Impact a "good learning experiment"
in this regard.

"I personally feel like Deep Impact is a lot more complex. If we just wanted
to blow up a comet, we wouldn't have to deal with the two spacecraft
pointing issues that we've got," Henderson advised. "So impacting a
spacecraft - is something we'll be able to bring the intelligence forward," he
concluded.

Meanwhile, Deep Impact is moving into final testing. It will now be shaked
and baked, and undergo acoustic vibration. Its mission draws closer, with a
shipping date to Cape Canaveral, Florida slated for next October, followed
by mating with a Delta 2 booster.

"We're less than a year from ship," Henderson said. "This is when it gets
really exciting. Everybody has been building individual pieces. Now we put
it all together and say: 'Prove that it works.'"
Received on Tue 25 Nov 2003 11:07:15 AM PST