Gunning for a Crash: Rehearsing Deep Impact's Comet Collision
The Ames Vertical Gun Range at NASA Ames Research Center in Moffett Field, California.
Credit: NASA/Ames.

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.

  • Deep Impact: Special Report