[meteorite-list] Deep Impact: NASA's Crash Course in Comet Science

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Jan 7 15:43:45 2005
Message-ID: <200501072043.MAA19952_at_zagami.jpl.nasa.gov>

http://www.spaceflightnow.com/delta/d311/050106preview.html

Deep Impact: NASA's crash course in comet science
BY JUSTIN RAY
SPACEFLIGHT NOW
This story first appeared in the January issue of Astronomy Now
magazine

NASA launches a space mission in January to blast a hole in the side of
a comet and learn more about the make up of these icy bodies.

As ancient wanderers of the solar system laden with primordial material
frozen in time, comets offer humanity clues to some of the most
fundamental questions about conditions when the planets were forming
more than four billion years ago.

Buried inside the hearts of these rocky snowballs are the pristine
building blocks that hold the chemical records from the solar system's
creation. Comets likely peppered the young Earth, possibly delivering
the organic materials needed for the rise of life, the water for our
oceans and even playing a role in generating the atmosphere.

To capture an unprecedented glimpse at this preserved material, NASA's
Deep Impact spacecraft is scheduled for launch January 12 carrying a
copper bullet that will be fired into the heart of Tempel 1 next July 4,
carving out a stadium-sized crater.

"We're doing this to discover the comet's structure and makeup," said
Rick Grammier, NASA's Deep Impact project manager. "This is like
swinging an 820-pound slug of copper at this thing and seeing what
happens."

Sophisticated instruments on the Deep Impact's mothership will record
the blast and peer into a comet's interior for the first time.
Observatories around the globe, plus the Hubble and Spitzer space
telescopes, will be watching the aftermath to collect crucial
information about the dusts and gases blown out of Tempel 1.

Conquering the mysteries

"My interest in comets all along has been trying to understand the
chemical composition and to use that to put constraints on our theories
of what conditions were like 4.5 billion years ago when the whole solar
system was forming," said Michael A'Hearn, astronomer from the
University of Maryland and the Deep Impact principal investigator.

"What we see coming out of comets as gas and dust is stuff that has been
modified because it is very near the surface, and every time the comet
goes around the sun the surface gets heated. So there have been changes
in the surface layers... What I really want to do is figure out how
different the surface is from what's inside.

Discovered on April 3, 1867 by Ernst Wilhelm Leberecht Tempel in
Marseilles, France, Comet 9P/Tempel 1 currently circles the sun every
5.5 years. Its orbit lies between Mars and Jupiter, providing the Deep
Impact mission a perfect target for reaching with a modest launch
vehicle, striking at high speed and being visible from Earth at impact
about 130 million kilometres away.

The exact size and shape of the comet's nucleus is unclear from
observations made to date. It is thought to be elongated and six
kilometres in diameter. How Tempel 1 will react to the impact is also a
mystery, but scientists do not believe the comet will shatter apart.

"It has turned out that the physics of how the impact occurs is also a
large unknown because we know so little about fragility or strength of
the cometary nuclei generally. We certainly know nothing about this
particular comet," A'Hearn said.

"There is an outside chance that we could break the comet. We don't
think that will happen... We don't think that the comet can propagate a
shockwave through from one side to the other so that you can break it
because we don't think it's that strong and cohesive everywhere."

Starting two months before the encounter, Deep Impact commences its
science observations in earnest, painting a picture of what the
spacecraft should expect at arrival and giving ample time to change the
approach strategy if necessary. Specifically, mission planners want to
pin down how the comet nucleus rotates and examine the jets of gas and
dust streaming away from Tempel 1.

Demolition day

One day before the big bang, the mothership releases the impactor. This
one-metre diameter, 0.8-metre tall projectile is equipped with an
autonomous navigation computer, cameras and a propulsion system to guide
itself toward a suitable impact point that is well lit. The mothership
performs an evasive manoeuvre, plotting a trajectory to fly past the
comet shortly after the impact.

"Early images from the impactor are mainly for navigation... to make
sure that it hits in an illuminated area and not in a dark area. As we
get closer, those images become important for science because as we get
closer and closer we will get higher and higher spatial resolution. We
will directly see the change in texture as you change spatial scale.
Assuming the camera on the impactor survives until very shortly before
impact, these will be the highest resolution pictures ever of a cometary
nucleus, much higher than we will get from the flyby [spacecraft],"
A'Hearn said.

The flyby craft will be using its spectrometer to identify and quantify
the materials across the comet's dust- and gas-filled coma head and
taking images in a wide variety of different colours. Since the nucleus
is believed to have a 41-hour rotational period, less than half will be
seen at good resolution.

"Shortly before the time of impact, the flyby spacecraft determines how
fast it is having to rotate to keep the high-resolution camera pointed
at the nucleus. It uses that to calculate when it will be at closest
approach and then knowing the difference velocity, when the impactor
will impact. It sends that information up to the impactor so the
impactor can optimise the imaging sequence at the expected time of
impact. Flyby uses it internally, also, to optimise the imaging sequence
for the time of closest approach," A'Hearn said.

Information from both craft is fed back to Earth in real-time in case
comet shrapnel fatally wounds the mothership during the encounter.

Hit and run

The 370-kilogramme impactor and comet collide at 10.2 km/s, releasing
the energy equivalent of 4.4 tonnes of exploding TNT and creating the
ultimate fireworks display for Americans on the Fourth of July.

"We put the impactor in the comet's path so that the comet overtakes it.
So it is like standing in the middle of the road with semi truck bearing
down on you," Grammier said.

The intense forces vaporize the projectile as the circular crater is
rapidly excavated. "We expect it could put a crater about the size of a
house up to the size of a football stadium and it could be anywhere from
seven to 14 stories deep.

"As a result of forming the crater, it will throw out a bunch of the
surface and interior material that is displaced. It will come up in a
big cloud that will reflect the sunlight. So you will see a large
brightening, and you will see that brightening from telescopes on Earth
as well. Then you will see it slowly dissipate as the material either
settles back down onto the comet itself or becomes part of its coma dust
cloud. What we are hoping to see to from the flyby spacecraft viewpoint
is being able to look all the way down into the interior of the crater
and determine what its materials are made of," Grammier continued.

"Since these are the original remnants of the solar system formation,
not knowing how the exterior of a comet relates to interior, what we are
hoping to do is expose all of that fresh material and see the material
that was actually present at the formation of the solar system."

The impact occurs with the mothership 8,600 kilometres away and closing
fast. The medium-resolution camera will be taking pictures as swiftly as
possible to capture the moment of impact. "We are hoping to catch a
bright flash that will last less than a second by taking four or eight
frames per second," A'Hearn said. The high-resolution camera snaps
pictures at a slower pace.

Scientists expect the materials thrown out of the freshly bored hole
will settle within a few minutes, permitting good visibility into the
crater. The mothership has less than 14 minutes to make its observations
while zooming toward the comet before passing by Tempel 1 at a distance
of 500 kilometres. The craft enters a "shield mode" to protect itself
from the powerful sandblasting during flight through the coma at closest
approach.

"Our baseline is it will take 200 seconds to form the crater, but
uncertainties in the density of the nucleus - something that we just
don't know - the crater could take as long as 600 seconds to form. This
was one of our mission design problems, making sure we had long enough
interval to observe so that we make sure the crater finished forming
before we flew by but keeping the interval small enough that we weren't
so far away at the time of impact that we had no resolution. This what
led us to the 800-second window between impact and the going into our
shield mode through the innermost coma," A'Hearn said.

Surviving the getaway

Deep Impact has just one shot at grabbing scientific data on the
primordial material packed inside the comet.

"We do maps across the nucleus after the impact to try and get spectra
of the crater floor, see how different it is from the neighbouring
terrain that is undisturbed," A'Hearn said. "We take some spectra off
the limb to look at the gases that are coming out of the crater. As we
get very close, we actually have to let the camera drift a little and
take a couple of images to make sure we get crater in the
high-resolution camera."

About 50 seconds before closest approach, the flyby craft orients itself
with protective shielding guarding against a destructive hit by comet dust.

"We've designed extra shielding on certain parts of the spacecraft. So
when I say it turns to shield mode, what that means is it actually
places those shields in the direction of the cometary dust and debris.
That is meant to protect the spacecraft itself from any particle hits.
That shielding was designed based on what we know today of probable
particle sizes, distribution and density at that distance from the
comet," Grammier said.

Despite the added protection, the mothership will be relaying its
pictures and information to Earth live in case the craft doesn't survive
the encounter to tell the tale afterward.

"There are worries, that is why we are transmitting as much as we can in
real-time, as much as the communications system will allow us to,"
A'Hearn said. "The engineers have predicted that the probability of a
fatal hit is down at the one or a couple percent level, given the amount
of shielding we have."

Once through the dangerous region, the departing mothership manoeuvres
to observe the comet's back side a quarter-hour after closest approach.

"We fly through the innermost coma, fly through the orbital plane and
then turn around and look back... to take images of the other side. When
we take pictures of the other side, the crater itself will be hidden,
but we will still be looking to see if we can see ejecta from the
crater. A likely scenario is that after we make the crater, there will
be a lot spontaneous outgasing from the floor of the crater because
there is very volatile ice near the surface that used to be buried deeply.

There is a reasonable chance that we would see a new jet in the coma
coming from the crater - and we would see it where it comes out from
behind the limb of the nucleus," A'Hearn said.

Aftermath

"We also use these look-back images to figure out the three-dimensional
shape of the nucleus since we don't get to see a full rotation. We do
the look-back monitoring for up to a day after impact."

Ground-based telescopes in Hawaii will have prime viewing with the comet
high in the sky at the time of impact, while the southwestern U.S. and
Baja California will have Tempel 1 low in the sky. But a global campaign
is underway to provide thorough monitoring of the comet before and after
the collision with special imaging techniques.

"We may create this new jet that may persist for hours or days or weeks
or even months. So we are looking for observations afterwards," A'Hearn
said.

"We are trying to get complete longitude coverage so we can monitor the
comet continuously from something like four days before the impact - two
rotation periods - until a week after the impact."

The $320 million mission follows NASA's Stardust project that flew past
Comet Wild 2 in January, catching dust particles for return to Earth in
2006. The European Rosetta mission is currently flying to Comet
Churyumov-Gerasimenko where a tiny lander will be dispatched to the
frigid nucleus.

If the Deep Impact mothership remains in good health, NASA could route
the craft to other comets for close-up imaging by the onboard cameras,
A'Hearn said.

Justin Ray is editor of Spaceflight Now. He is based at Cape Canaveral
and has covered the space programme since 1995.
Received on Fri 07 Jan 2005 03:43:36 PM PST