Robot hedgehogs on the moons of Mars may sound like the title of a B-grade sci-fi movie, but that is what Stanford University is working on. Marco Pavone, an assistant professor in the Department of Aeronautics and Astronautics, and his team are developing spherical robots called “hedgehogs” that are about half a meter (1.6 ft) wide and covered in spikes to better cope with rolling and hopping across the surface of the Martian moon Phobos with its very low gravity.
Phobos is small as moons go with a diameter of approximately 22.2 kilometers (13.8 mi). Since its discovery and that of its sister moon Deimos in 1877, very little has been learned about the nature of Phobos. It may be a captured asteroid or a chunk of Mars knocked off by an ancient impact. If it’s the latter, then Phobos could tell us a lot about Mars without the expense and danger of a landing on the planet.
It’s also a very good place to test technologies needed to explore Mars. For this reason, the United States, the ESA, Canada and Russia have all planned or attempted missions to explore Phobos. The problem is that Phobos’ gravity is only 1/1000th that of Earth. This makes getting around on wheels, treads or legs extremely difficult because the low gravity means little or no traction, so building a rover for Phobos presents a challenge.
Working in collaboration with NASA's Jet Propulsion Laboratory and MIT, Pavone’s team designed the robot hedgehog specially for Phobos. The hedgehog is a sort of “hybrid” machine that is part flying spacecraft and part rover. It’s roughly spherical, made up of many facets covered in solar panels and spikes. Inside the shell of the hedgehog are three rotating discs set at right angles to one another. As these spin, the spikes dig in and the hedgehog rolls, hops, tumbles and bounds in ten-meter (33 ft) arcs over the Phoboian surface.
However, the hedgehog is only half of the plan. The other is the Phobos Surveyor orbiting spacecraft. About the size of a coffee table and powered by two solar panels, it will act as the mother ship for up to six hedgehogs. Both the Phobos Surveyor and the hedgehogs are designed to be largely autonomous due to being hundreds of millions of miles from mission control.
The Phobos exploration mission will take up to three years including the two-year journey from Earth. Once at Phobos, Surveyor will conduct scans from orbit, mapping the topography. It will then release one hedgehog at a time several days apart for close up surveys. Surveyor and the hedgehogs work together to determine the spherical robots' locations and orientations and steer them toward targets. The hedgehogs then beam data to Surveyor for relay to Earth.
The focus of the mission will be on Stickney Crater because over it is a Lagrange point where the gravitational forces of Mars and Phobos balance out. This will allow Surveyor to hover over the crater. Another reason for choosing Stickney is that the crater is very deep, so the moon’s interior structure is already exposed there.
A prototype of the Phobos Surveyor has already been constructed and two generations of hedgehog prototypes have been built, with a third under development. Pavone’s team plans to test this at the Durand Building at Stanford using an overhead crane and a simulated landscape complete with flour filling in for Phoboian dust. This will be followed by testing aboard an aircraft flying parabolic trajectories to produce momentary weightless conditions.
Though no official go ahead has been given for the mission, Pavone hopes to see the hedgehogs deployed to Phobos within the next twenty years. The proposed mission’s plan will be presented at the Institute of Electrical and Electronics Engineers Aerospace Conference in March.
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