Getting to Earth from the International Space Station (ISS) by jumping out of an airlock with a parachute may seem daft, but a group of students are trying just that with a CubeSat. According to NASA, TechEdSat-4, which was jettisoned from the space station on March 3, has reached its designated orbit, where it will use a parachute-like "exo-brake" to slow it down enough to safely re-enter the Earth's atmosphere.
Getting into space is difficult, but getting home again is no picnic either. True, it's all downhill, but velocity still needs to be reduced if a spacecraft is going to leave orbit. Usually, that means using rockets to slow it down enough to re-enter the atmosphere. It also means that getting samples back from the ISS involves waiting months for a crew or cargo craft going in the right direction.
What many scientists would like is a cheaper, simpler system that can send back samples more often or even on demand. That could be done with small reentry capsules rigged with retro rockets, but NASA takes a dim view of keeping rockets inside the space station, so a safer alternative would be a system that doesn't use any propellant at all.
The principle behind the exo-brake is the same one that causes orbital decay and sends old satellites plummeting back to Earth. Though space officially starts 62 mi (100 km) up, the atmosphere actually extends hundreds of miles above that as the exosphere. It's extremely tenuous, but there are still enough air molecules present to gradually slow down satellites until they succumb to the pull of gravity, re-enter and burn up.
Exo-braking exploits the exosphere by deploying a drogue chute with an area of about one square meter (10.7 sq ft) per kilogram (2.2 lb) of satellite. The technique was first tested by TechEdSat-3 in 2013 and TechEdSat-4 is carrying a second-generation version.
"The exo-brake is a self-stabilizing exospheric deorbiting mechanism that will allow us to return a payload to Earth fairly rapidly from an orbital platform, like the International Space Station," says Marcus Murbach, the TechEdSat-4 principal investigator at NASA's Ames Research Center in Moffett Field, California. "We were able to send commands and receive data to and from the satellite via the onboard modem using only a laptop and email account. This capability may greatly benefit the entire nanosatellite community."
TechEdSat-4 is the fourth in the TechEdSat series developed at San Jose State University (SJSU) in California and the University of Idaho (UI) in Moscow, Idaho. It was built by student interns using off-the-shelf components and cost under US$50,000. It's design is based on a standard CubeSat structure, which is made up cubic units 10 cm (4 in) on a side. In the case of TechEdSat-4, it's made of three cubes for a total length of 30 cm (12 in) and weighs about 5 lb (2.2 kg).
TechEdSat was jettisoned from the ISS using the NanoRacks Deployer and after opening the ex-brake canopy under e-mail command it moved to its present orbit, where it will conduct tests over the next four weeks using satellite-to-satellite communications to provide telemetry.
The next satellite in the series, TechEdSat-5, will have an exo-brake that can reconfigure itself, so it can make a controlled re-entry flight. NASA says that in addition to returning samples to Earth, the technology could be used to land small payloads on Mars or other planets with atmospheres.
"We've already developed a sample canister that during atmospheric re-entry could slip out the back of the satellite and safely be recovered on Earth," says Murbach. "This could also be adapted to future Mars satellites as a piggy-back or ride-along payload that could jettison independently and study the mid-latitude or other scientifically interesting regions of Mars. Currently, it is extremely challenging to access these sites."
The video below discusses the exo-brake experiment.