When president Trump released his budget proposal earlier this year, space enthusiasts were dismayed to learn that it didn't include funding for NASA's mission to place a lander on Europa. Although it is just one of Jupiter's 67 moons, Europa is unique in that it is thought to have an ocean of liquid water under the icy, red-veined crust that covers its surface, which makes it the best candidate in the solar system for hosting alien life.
All hope is not lost for the subsurface exploration of Europa. Just last week, a French researcher suggested that NASA and the European Space Agency could pursue a joint Europa lander mission in lieu of the NASA's solo mission. A spokesperson for NASA said that neither agency is developing such plans at this time.
This is particularly good news for the folks at Stone Aerospace, who have spent the last few years developing ARTEMIS, an autonomous submarine that is pioneering the technology that they hope will one day be used to explore Europa's ocean. In 2015, ARTEMIS was given its first field test off the coast of Antarctica and the results of these test runs were presented at NASA's astrobiology conference last week. By all accounts, the mission was a huge success.
The submarine developed by Stone as part of a multi-million-dollar NASA grant is nearly 14 feet long, weighs over 2,800 pounds, and is capable of traveling a little over 3 miles on its own before returning and docking itself at its pickup point. Obviously, such a large craft would be prohibitive for any space mission to Europa. ARTEMIS was made from off-the-shelf parts and meant to test autonomous navigation and sample retrieval systems. The actual sub used on Europa would have to be entirely custom made to lower its bulk.
The systems on ARTEMIS were designed with the particularly challenging environment of Europa in mind. The moon has no atmosphere, which means using parachutes to land a craft on its surface is a no-go. It has a surface temperature that never rises above -260 F and is covered with an icy crust of uncertain thickness (although NASA estimates it to be between 10 and 15 miles deep). Moreover, no one will know the chemical composition of the ocean beneath the icy shell until the Europa Clipper mission makes its flybys in the late 2020s.
The first technical problem faced by a lander trying to get to Europa's liquid ocean is how to get through the thick layer of ice that covers the moon's surface. To this end, Stone is developing an autonomous cryobot called SPINDLE. The cryobot will basically be a large, nuclear-powered soldering iron that will house the submarine and melt a borehole through the ice to the ocean below using powerful lasers.
Thus far, an early prototype of the SPINDLE cryobot called VALKYRIE has made two successful trips to test the penetration technology on an Alaskan glacier. According to Evan Clark, a field roboticist at Stone, getting permission to drop a nuclear reactor into a glacier in Alaska is "basically impossible" so the VALKYRIE bot made use of a 5 kilowatt laser to melt through the ice.
"There's only one way to energetically to get through the ice shell of Europa and that's nuclear," Clark said at the NASA astrobiology conference. "You may get your energy from nuclear, but how are you going to use that? There's contact melt, using it to run a drill, or as the SPINDLE project has discovered, you can just fire the laser directly into the ice."
So far, the maximum penetration rate achieved by Stone's cryobot is about 72 feet of ice per hour, but figuring out how to penetrate Europa's ice shelll is just half the problem. Given the freezing temperatures on the moon's surface, the borehole will continuously reseal behind the SPINDLE as it melts its way deeper into the crust. Electromagnetic waves don't propagate well through ice, which would make retrieving the valuable data from the alien-hunting submarine being towed by a cryobot impossible.
To circumvent this issue, Kristof Richmond, the ARTEMIS project manager, said a cryobot on Europa would likely deposit radio receiver buoys into the ice from its rear as it makes it descent. Although the receivers will become encased in ice, they will be close enough together to allow radio signals to hop from receiver to receiver until they reach the surface, at which point they can be transmitted to an orbiter and sent back to Earth.
Once the cryobot ice penetrator reaches liquid water, it will deploy an autonomous submarine. The sub must be autonomous because the lag time between Jupiter and Earth (between 30 minutes and an hour) and the difficulties of communicating with a craft submerged in water. So Richmond and his colleagues at Stone are developing a sophisticated autonomous guidance system which will allow the craft to safely navigate the ocean beneath the crust while also taking samples from the ice and surrounding water.
Since radio waves do not propagate well underwater, an autonomous submarine on Europa will not have recourse to standard radio communications or GPS satellites. To get around these restraints, the Stone team turned to a navigation system that uses a gyroscope to determine the submarine's direction and a Doppler velocity log to measure the craft's speed relative to the ice ceiling. This navigation method works well enough, but due to drift in the ocean small navigation errors add up over the course of the journey so that the submarine can only return to within about a kilometer of its starting point—a small hole in a massive sheet of ice.
At this point, the second part of the ARTEMIS navigation system kicks in: an acoustic beacon on the ARTEMIS docking station which allows the craft to navigate within 20-30 feet of the docking station. It will be able to see a white light bar attacked to the docking station, even in the pitch black of the under-ice ocean, and use a machine vision algorithm to navigate itself to the light.
For now, the ARTEMIS team is still working through the data from the Antarctic mission which ended in December 2015. There are still a number of kinks to work out in the development of a future space-borne submarine, but they've got some time to figure it out. The most recent Europa lander mission only made it to the initial planning phase, so even if NASA and the European Space Agency go forward with their plans for a joint mission to the surface, it's unlikely that the mission will launch before the late 2020s. And Richmond hopes that their submarine will be along for the ride.
"We are trying to push to have all these things at basic levels of development and technology readiness," Richmond told me. "Then when we find all the missing information from the Europa flyby and lander missions, we can pull the trigger and be ready to go."
Correction: An earlier version of this piece stated that NASA and the European Space Agency announced a joint mission proposal for a new Europa lander. In fact, the announcement was from an independent researcher suggesting this collaboration could happen, but such a plan is not currently being developed by either agency. Motherboard regrets the error.
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