There are hundreds of near-Earth asteroids hurtling through space that are a potential danger to our planet. One way of dealing with the problem is to deflect them with a space probe deliberately set on a collision course. To see how effective such a collision would be, Frank Schäfer of the Fraunhofer Institute for High-Speed Dynamics, Ernst-Mach-Institut, EMI in Freiburg, Germany is looking at what asteroids are made of and how this affects a deflection impact.

It may seem odd to use a space probe the size of a washing machine to fend off an asteroid that weighs in at a couple of hundred tons, but then, a few ounces of lead are enough to bring down an elephant. It’s a matter of putting enough energy behind the bullet. In this case, the bullet is a spacecraft, but even moving at orbital velocity the effect wouldn’t do much to an asteroid large enough to pose a danger to Earth. However, it doesn’t have to, if the impact is properly timed.

“In actual fact, the impact of a space probe would change the speed of the asteroid by just a few centimeters per second. But that’s enough to deflect its course to a significant degree over a longer period. So if we want to stop an asteroid on collision course with the Earth from hitting us, we’ll need to fire at it many years ahead of time,” says Schäfer.

Artist's concept of a space probe impact on an asteroid, with another probe observing (Image: ESA)

The study originally wanted to look at how to deflect asteroids between 100 and 300 m (330 and 990 ft) in diameter by striking them with massive space probes in a manner that was described as similar to two balls colliding in billiards. However, Schafer noticed that something else was going on, other than simple recoil. The substance that the asteroid was made of was also a factor, especially if the impact threw off a plume of debris. In this case, the momentum transfer is up to four times greater than a simple impact.

“During impact, not only does the probe transfer its own momentum to the asteroid, there is also the recoil of detached material from the crater, which is ejected against the direction of the impact,” Schäfer says. “This recoil effect acts like a turbocharger on the deviation of the asteroid.”

Schäfer’s experiments involved taking pendulums and attaching materials similar in consistency to those that make up asteroids, such as dense quartzite, porous sandstone or airy concrete. He fired aluminum projectiles at them at up to 10 km/s (6.2 miles per second) while recording the results with high-speed cameras, interferometers and lasers.

The results of the experiments demonstrated that the asteroid’s substance affects the outcome of a probe impact. A porous substance will absorb the strike, similar to how the crumple zone of a car soaks up the energy of a crash. Conversely, a denser, more elastic substance will enhance the deflection.

Part of the NEOShield program, Schäfer’s work will be used by the project in its plans to try deflecting an asteroid by mid-2015.

Source: Fraunhofer