[meteorite-list] Gunning for a Crash: Rehearsing Deep Impact's Comet Collision

[Ron Baalke]

Gunning for a Crash: Rehearsing Deep Impact's Comet Collision
space.com
22 June 2005

When it comes to slamming space probes into comets, astronomers - like
Boy Scouts - prefer to be prepared.

As part of that preparation, a team of researchers has been using a
giant gun to hurl projectiles at comet-like substances, then pore over
the results. By doing so repeatedly, the astronomers hope to ready
themselves for what they might see on July 3-4, when NASA's Deep Impact
mission is slated to hurl its own projectile at a comet known as Tempel 1.

"We look at a wide range of possible scenarios here," said planetary
geologist Peter Schultz, a Deep Impact co-investigator at Brown
University, in telephone interview. "The hope is that, with the short
period of time in which we start getting back images, we can understand
what we're seeing."

Schultz and his fellow researchers have been using NASA's Ames Vertical
Gun Range to fire small, bead-sized projectiles at sand, ice and a host
of other materials designed to mimic what might be waiting for Deep
Impact at Tempel 1. During its mission, Deep Impact will release a
copper-tipped,  820-pound (372-kilogram) probe that will smash into the
Tempel 1's surface while the mission's flyby spacecraft  and a swarm of
orbital and ground-based telescopes look on.

Astronomers hope the impactor probe will blow a hole in Tempel 1 large
enough to peer through its outer surface into the pristine layers
beneath, which should be a prime source of some of the Solar System's
earliest material. Deep Impact launched in January 2005.

Bring out the big guns

Originally built for lunar impact studies during NASA's Apollo moon
program, the Ames Vertical Gun Range is a vital room-sized tool for
researchers studying planetary geology.

The Ames vertical gun relies on a 0.30 caliber light-gas gun and a
powder gun to hurl projectiles up to about four miles per second (seven
kilometers per second). Its angle of impact can be swiveled up to 90
degrees to observe the effects of changing conditions.

"We're firing six millimeter beads into same at a velocity of around
four miles per second for our tests," Schultz said.

While the Deep Impact team is primarily using beads, the vertical gun
can fire a wide variety of small particles ranging from simple spheres
and cylinders to irregular shapes, and even clusters of objects. The
target chamber itself spans about 2.5 meters square and can record
impact events with either high-speed film or particle image velocimetry.

Fluffy, crusty, rigid or crunchy: Predicting a comet's surface

Deep Impact has only a 55-minute window to crash its impactor into
Tempel 1.

Researchers hope to be able to quickly determine what type of material
sits beneath the icy wanderer's outer skin based on the resulting
crater. Mission scientists have estimated that Deep Impact's crater
could stretch from just 10 meters across to the length of a football
stadium.

"We know we're going to impact, and we know how big our projectile is,"
Schultz said. "But what we really don't know is the nature of the
comet's surface."

Tempel 1's surface could have the consistency of sand or fluffy snow. It
could be icy and hard, or merely covered in a crunchy crust. Schultz
wants to be ready for as many surface types as possible and his team has
outlined two general scenarios that could guide Deep Impact's mission.

The first, a gravity-controlled case, depends on Tempel 1's local tug to
limit the size of the resulting crater from the impactor probe. The
stronger the local gravity, the slower Deep Impact's crater would grow,
Schultz said.

The strength of the impact, and how well the impactor slams into its
comet target, could also shape the resulting blast. If Tempel 1 is
coated in fluffy material, Deep Impact's impactor could crash through
the surface and compress the material in front as it submerges deeper
into the comet.

"From experiments, we've found that if that happens, [Deep Impact] goes
down deep and explodes," Schultz said. "So we'd get an enormous crater."

Prepare for the unexpected

Despite the preparation by Schultz and other Deep Impact researchers to
ready themselves for the first post-crash images from flyby, there is
always the chance that something new might pop up.

"One thing we've learned here is that something unexpected can always
happen," Schultz said. "And I suspect that Deep Impact won't be any
different."

The mission's swift schedule, just about six months from launch to
impact, has also made the need to be accurate and adequately prepared
paramount for the mission team, researches added.

"I really do think we can learn a lot from these types of active probes
and I think we'll see some surprises here that will prepare us for
future missions," Schultz said.