In what at first glance seems like a terrible sense of direction, in March students from the University of Washington fired rockets from kites and balloons at an altitude of 3,000 ft (914 m) straight into the ground at Black Rock, Nevada: a dry lake bed in the desert 100 mi (160 km) north of Reno. This may seem like the ultimate in larking about, but it's actually a serious effort to develop new ways of collecting samples from asteroids.
The test was part of the “Sample Return Systems for Extreme Environments” project. The idea is to find cheaper, more efficient ways of collecting samples from asteroids and hazardous areas on Earth, such as volcanoes and nuclear disaster zones, by using penetrators instead of soft landers or ground crews to hammer out sample cores.
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According to the team, this would result in lower cost than soft landing techniques by reducing the velocity and vehicle mass needed to gather the sample, minimizing damage on impact, as well as being mechanically simpler. In the Nevada test, the penetrators were fired at the ground using rocket boosters to provide as much speed as possible at impact.
“We’re trying to figure out what the maximum speed is that a rocket can survive a hard impact,” says Robert Winglee, a UW professor of Earth and space sciences.
The penetrator used in the test is 6 ft (1.8 m) long and 6 in (15.2 cm) in diameter. A carbon fiber airframe makes up the main body of the penetrator, which has a hardened solid tip with three sample ports. The tip must be harder than the material it penetrates, so it consists of aluminum or steel, depending on the target.
Behind the tip and lining the airframe is a crumple zone filled with Hexcel: a honeycomb aluminum material that collapses in an orderly fashion to absorb the force of impact. This protects the central collecting tube, which is attached to a tether developed in collaboration with Robert Hoyt of Tethers Unlimited Inc. of Bothell, Washington. The purpose of the tether is to control the impact speed of the penetrator and allow for sample return while helping to minimize the onboard electronics needed.
The way the penetrator works is that it is either dropped from space or, if used on Earth, fired into the ground using a rocket called a "sustainer" from a balloon or kite. When the penetrator hits, it's at 100 m/s (330 ft/s) and a force of 1,000 g’s on Earth and 1 km/s (0.6 mi/s) and 10,000 g’s if aimed at an asteroid.
On impact, the penetrator burrows several feet into the surface and the force of impact rams sample material through the ports on either side of the nose, which funnels it to the interior sample return capsule that is attached by tether to a balloon or a spacecraft. The tether attached to the capsule is then used to reel in the capsule to the balloon or mothership to recover the sample.
The ultimate goal of the project is an unmanned sample collector spacecraft for studying asteroids or small moons. Using a solar-electric ion propulsion system, it would cruise to its target to conduct either a flyby or orbital survey. Once on station, the craft would fire a penetrator, which would burrow into the surface, then a tether would immediately haul the sample container back into the craft for return to Earth. The spacecraft would carry multiple penetrators for multiple sample collection.
According to Winglee, this year's test were successful, but the impact wasn't fast enough for a proper test, but it still won US$500,000 over two years from NASA Innovative Advanced Concepts. A second phase of testing in California is scheduled for next Northern Hemisphere summer and the year after with the goal of hitting the ground at Mach 2 (1,500 mph, 2,400 km/h).
The video below shows a rocket impact test.