Normally, a spacecraft slamming into a planet’s surface at the speed of sound is considered a bad thing, but the European Space Agency (ESA) plans to do just that. As part of its Core Technology Programme for Cosmic Vision, the agency fired a pair of experimental surface penetrators from a rocket sled at a test facility at the UK Military of Defence Pendine site in Wales last July. The goal is to find ways of delivering instruments beneath the ground or ice of alien worlds without drilling.
Landers have come a long way since the days when all they could do is passively take pictures and temperature readings. Now, they can rove about and the NASA Curiosity rover has conducted the first robotic drilling operation on another planet. It works, but such drilling is complicated, slow and limited in how deep it can go or how great an area it can cover. It would be much simpler to just drop instrument packages from orbit over a wide area and let them burrow under the sand or ice using their own momentum. The trick is having them dig in instead of smashing to bits on impact.
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Building a penetrator for the job is actually very simple and the technology has been around since Barnes Wallis worked on the problem during World War II as a way of taking out German bunkers. The Americans had a similar problem during the 1991 Gulf War and in less than a month came up with penetrator bombs made out of surplus 8-inch (203 mm) artillery gun barrels that could plow through reinforced concrete. Something like these would be perfect for dropping on Mars or the moons of Jupiter, but no launch vehicle or probe known could carry a 2-ton penetrator such a distance, so a compromise needs to be made between strength and weight.
The problem is, a penetrator is the opposite of how space engineers think. It can’t be made out of the usual light alloys used in spacecraft as it needs to be made of relatively thick steel. Also, the instrument package needs to withstand massive g-forces on impact instead of the usual five g’s and below, so they need to be designed along the lines of an anti-aircraft shell’s electronic proximity fuse.
The ESA penetrator program is being conducted by a consortium of agencies and companies led by Astrium UK. The current experimental design is intended to test the feasibility of using a penetrator to deliver an instrument package beneath the surface of a planet or icy moon to a depth of 3 m (10 ft).
Weighing 20 kg (44 lb) and measuring 400 mm (15.7 in) long and 200 mm (7.8 in) in diameter, it consists of a steel shell with spring-mounted aluminum instrument bays on the inside. Between the two is a vacuum to insulate the instruments against heat. The rear is an additional hollow section to provide aerodynamic stability. The payload is accelerometers to record the g-forces and a dummy sampling mechanism. If this were operational, it would be able to collect a few grams of samples 10 to 20 cm (3.9 to 7.8 in) from the shell.
For the tests, 12 solid rockets shot the penetrator down a 300 m (984 ft) rocket sled track. As it left the track 1.5 seconds after ignition, it was going at 341 m/s (762 mph, 1,227 km/h) and struck its target with the force of 24,000 g’s.
There were two impact tests, both of which used targets housed in a steel and concrete bunker. The first on July 11 was fired into about ten tonnes of ice to simulate the surface of moons like Europa. The second on July 16 used sand as a stand in for Martian soil.
When the the penetrator hit, the ice shattered into small crystals and the shell was dented when it struck a steel roof beam. On hitting the sand, the penetrator burrowed to a depth of one meter (3.2 ft) in and one meter up. Unlike the ice test, it wasn't dented, but the surface was sand blasted. One unfortunate result was that the penetrator struck at an angle of 22° instead of 8°, though this was attributed to aerodynamic lift during horizontal flight, which wouldn't occur when dropped from space.
The next step of the program will be to study the effect of the impact on the penetrator and to work on developing a battery and communication system robust enough to survive a very unpleasant landing.
The video below by study participant QinetiQ shows the impact tests.