A steam-powered robotic probe designed to hop around the icy moons of Europa and Enceladus could get approval to enter the next phase of study as part of NASA’s Innovative Advanced Concepts (NIAC) program. Final selections for Phase II NIAC funding won’t be announced until next Spring. But to date, NASA seems pleased with the potential that the SPARROW (Steam Propelled Autonomous Retrieval Robot for Ocean Worlds) concept has shown.

The idea is that SPARROW would use steam propulsion to one day make giant leaps over some of the most hazardous terrains known (and unknown) in the solar system, reports NASA’s Jet Propulsion Laboratory (JPL).  Envisioned as a soccer ball-sized payload to a primary lander mission, SPARROW would have a protective shell and be able to freely rotate, self-right, and tumble over chaotic terrains, notes a 2019 NIAC report. 

The probe would use thrusters, avionics, and instruments encased in a protective spherical cage to hop from one terrain to the next using very short thrusts, says NASA.

The first logical targets to which SPARROW might actually be deployed are Jupiter’s moon of Europa and Saturn’s moon of Enceladus. 

Little is known about Europa’s actual terrain. But its surface is thought to be complex and porous, maybe even riddled with crevasses and/or meters-high penitentes —- long blades of ice known to form at high latitudes on Earth — that would stop most robots in their tracks, says JPL. But SPARROW would be ready for anything and thus eliminates the need for pre-launch information regarding these weird moons’ surface topography.

How SPARROW would work.

SPARROW would collect and heat ice inside its engines, creating bursts of steam to give a boost off the surface, says NASA. When low on fuel, the hopping bot would return to the lander for more, also dropping off any scientific samples for further analysis, says the agency.  Because both moons are thought to host vast subsurface oceans of salty water under a thick ice crust, the idea is to use abundant surface ice as in situ propellant.

How far would SPARROW be able to hop? 

If SPARROW remains a soccer ball-sized payload, it’s likely that hops could be performed on the one to five km scale, Gareth Meirion-Griffith, lead researcher of the SPARROW project, at NASA’s Jet Propulsion Laboratory (JPL), told me.  That’s still huge when you think about how many different regions you could explore with a hopper versus a static lander, he says.

Is it better suited for Europa or Enceladus?

The system could work equally well on either Europa or Enceladus, says Meirion-Griffith. But given the reduced gravity of Enceladus (just one percent of Earth’s), SPARROW’s hop distances on Enceladus could be greatly increased, he says.  Meirion-Griffith says this technology could potentially even be used at the lunar South Pole.

Whether there’s enough readily available lunar ice to make it worthwhile using steam as the propellant, I don’t know, but there’s nothing to stop SPARROW from using a more traditional propellant, says Meirion-Griffith.   

As for how it would be maneuvered?

If it were used on Earth’s Moon, it would most likely be controlled by Earth-based teams, says Meirion-Griffith. If deployed into the outer solar system, he says it would have to do its own onboard localization and make decisions as to where it is, how it is oriented, and how to get back to the lander, if required.

Is there potential for catastrophic failure if it hops into a nearby crevasse?

Even if SPARROW rolls or hops into a crevasse, as long as that feature isn’t too deep, it should be able to hop back out, says Meirion-Griffith. But he says that once the SPARROW is deployed on the surface of one of these icy moons, it’s likely only to last for a matter of days.  

SPARROW remains an advanced-stage technology concept for now; and is not attached to any other NASA mission and thus there’s no planned launch. But if it makes it to the second phase of study by spring of next year, the team hopes that SPARROW will begin hopping sooner rather than later.