HOPPING may be the best way for robotic probes to explore the surface of
comets and asteroids. Japanese engineers have built a cylindrical prototype that
they say could take 9-metre hops in a low-gravity environment. They propose
adding a more advanced version of the probe to MUSES-C, a Japanese mission to
return an asteroid sample to Earth in June 2006.
Wheeled robots work well on moons and planets, but the very low gravity of
asteroids and comets poses problems. The traction needed for horizontal motion
comes from the vehicle鈥檚 weight pressing down on the surface, but on an asteroid
only 2 kilometres across the force of gravity is about 100 000 times weaker than
on Earth. That leaves so little traction that the robot鈥檚 wheels will slip
unless they move very slowly.
Researchers at the Jet Propulsion Laboratory in Pasadena, California, have
already developed a wheeled rover for MUSES-C, to be launched in July 2002.
Planetary scientist Don Yeomans says the 1-kilogram rover will crawl at one or
two millimetres per second across the surface of asteroid 1989 ML. The wheels
are mounted on pivoting struts, which can be pulled together to push the little
craft upwards and make it hop, but JPL won鈥檛 try that until the main part of the
mission is complete. 鈥淵ou can hop tens of metres or even a hundred metres at a
clip,鈥� Yeomans says.
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The Japanese hopping robot has a significantly different design, as Takushi
Kubota of the Institute of Space and Astronautical Science in Kanagawa made
clear in Baltimore last week at the fourth International Conference on Low-cost
Planetary Missions. It is a cylinder measuring 10 by 12 centimetres, and
weighing 550 grams. Switching on an electric motor makes a disc within the robot
rotate. Its body reacts by turning in the opposite direction, flinging it up
into the air like a spring-loaded toy. Stopping the motor changes the robot鈥檚
spin, helping to control its motion. And spinning a second disc inside the
sealed robot can steer it in a different direction. Kubota鈥檚 group calculates
that the hopper could leap 9 metres in a jump that would take 15 minutes and
bring it in for a soft landing.
His group would like to test their robot on MUSES-C. But Yeomans thinks
that鈥檚 unlikely to be possible because the JPL rover carries more instruments
and the spacecraft is already overweight.