Salto has been one of our favorite robots since we were first introduced to it in 2016 as a project out of Ron Fearing’s lab at UC Berkeley. The palm-sized spring-loaded jumping robot has gone from barely being able to chain together a few open-loop jumps to mastering landings, bouncing around outside, powering through obstacle courses, and occasionally exploding.
What’s quite unusual about Salto is that it’s still an active research project—nine years is an astonishingly long life time for any robot, especially one without any immediately obvious practical applications. But one of Salto’s original creators, Justin Yim (who is now a professor at the University of Illinois), has found a niche where Salto might be able to do what no other robot can: mid-air sampling of the water geysering out of the frigid surface of Enceladus, a moon of Saturn.
What makes Enceladus so interesting is that it’s completely covered in a 40 kilometer thick sheet of ice, and underneath that ice is a 10 km-deep global ocean. And within that ocean can be found—we know not what. Diving in that buried ocean is a problem that robots may be able to solve at some point, but in the near(er) term, Enceladus’ south pole is home to over a hundred cryovolcanoes that spew plumes of water vapor and all kinds of other stuff right out into space, offering a sampling opportunity to any robot that can get close enough for a sip.
“We can cover large distances, we can get over obstacles, we don’t require an atmosphere, and we don’t pollute anything.” —Justin Yim, University of Illinois
Yim, along with another Salto veteran Ethan Schaler (now at JPL), have been awarded funding through NASA’s Innovative Advanced Concepts (NIAC) program to turn Salto into a robot that can perform “Legged Exploration Across the Plume,” or in an only moderately strained backronym, LEAP. LEAP would be a space-ified version of Salto with a couple of major modifications allowing it to operate in a freezing, airless, low-gravity environment.
Exploring Enceladus’ Challenging Terrain
As best as we can make out from images taken during Cassini flybys, the surface of Enceladus is unfriendly to traditional rovers, covered in ridges and fissures, although we don’t have very much information on the exact properties of the terrain. There’s also essentially no atmosphere, meaning that you can’t fly using aerodynamics, and if you use rockets to fly instead, you run the risk of your exhaust contaminating any samples that you take.
“This doesn’t leave us with a whole lot of options for getting around, but one that seems like it might be particularly suitable is jumping,” Yim tells us. “We can cover large distances, we can get over obstacles, we don’t require an atmosphere, and we don’t pollute anything.” And with Enceladus’ gravity being just 1/80th that of Earth, Salto’s meter-high jump on Earth would enable it to travel a hundred meters or so on Enceladus, taking samples as it soars through cryovolcano plumes.
The current version of Salto does require an atmosphere, because it uses a pair of propellers as tiny thrusters to control yaw and roll. On LEAP, those thrusters would be replaced with an angled pair of reaction wheels instead. To deal with the terrain, the robot will also likely need a foot that can handle jumping from (and landing on) surfaces composed of granular ice particles.
LEAP is designed to jump through Enceladus’ many plumes to collect samples, and use the moon’s terrain to direct subsequent jumps.NASA/Justin Yim
While the vision is for LEAP to jump continuously, bouncing over the surface and through plumes in a controlled series of hops, sooner or later it’s going to have a bad landing, and the robot has to be prepared for that. “I think one of the biggest new technological developments is going to be multimodal locomotion,” explains Yim. “Specifically, we’d like to have a robust ability to handle falls.” The reaction wheels can help with this in two ways: they offer some protection by acting like a shell around the robot, and they can also operate as a regular pair of wheels, allowing the robot to roll around on the ground a little bit. “With some maneuvers that we’re experimenting with now, the reaction wheels might also be able to help the robot to pop itself back upright so that it can start jumping again after it falls over,” Yim says.
A NIAC project like this is about as early-stage as it gets for something like LEAP, and an Enceladus mission is very far away as measured by almost every metric—space, time, funding, policy, you name it. Long term, the idea with LEAP is that it could be an add-on to a mission concept called the Enceladus Orbilander. This US $2.5 billion spacecraft would launch sometime in the 2030s, and spend about a dozen years getting to Saturn and entering orbit around Enceladus. After 1.5 years in orbit, the spacecraft would land on the surface, and spend a further 2 years looking for biosignatures. The Orbilander itself would be stationary, Yim explains, “so having this robotic mobility solution would be a great way to do expanded exploration of Enceladus, getting really long distance coverage to collect water samples from plumes on different areas of the surface.”
LEAP has been funded through a nine-month Phase 1 study that begins this April. While the JPL team investigates ice-foot interactions and tries to figure out how to keep the robot from freezing to death, at the University of Illinois Yim will be upgrading Salto with self-righting capability. Honestly, it’s exciting to think that after so many years, Salto may have finally found an application where it offers the actual best solution for solving this particular problem of low-gravity mobility for science.
The post “A Tiny Jumping Robot for Exploring Enceladus” by Evan Ackerman was published on 03/03/2025 by spectrum.ieee.org