Free Astronomy Magazine November-December 2015

ASTRONAUTICS with which it is transferred, the robot can perform more or less ample manoeuvres, from fine corrections to its position up to great leaps upwards. The eight spikes jut- ting from the cube’s corners serve as “pro- tective limbs”, preventing the Hedgehog from directly impacting its side faces against ground projections. The designers have also planned to incorporate in these spikes some scientific instruments, such as thermal probes capable of mea- suring the ground temperature. Although at first sight the im- pression may be that on a rough surface a Hedgehog can only tum- ble around in an haphazard man- ner, in reality this is only partly true, since the robot can point to a specific target and hop exactly in that di- rection, without then needing major correc- tive manoeuvres for reaching its final tar- get. For the mission controllers it will be a bit like playing golf in an alien environment under microgravity con- ditions, with the differ- ence that the flywheels will replace the clubs, while the balls will be much bigger in size. In fact, the sides of the two prototypes built so far (one by JPL and one by Stanford) mea- sure 30-40 cm, while the weight, depend- ing on the instruments carried (recording cameras, spectrometers, various sensors), can vary from a few kilograms up to ten ki- lograms. The glaring differences between the two versions are the smaller size and lighter weight of the Stanford’s prototype, and also the braking system: discs are used on the one built by JPL, while belts are used on the Stanford’s version. The two Hedgehog prototypes were tested last summer aboard the Zero-G NASA air- craft, a C-9 specially modified to perform dizzying dives during parabolic flights, which allow to almost completely cancel the gravitational pull of the Earth, thus creating a microgravity environment simi- lar to that found on the surfaces of aster- oids and comets. Over the course of 4 flights, involving more than 180 parabolas, the two O n this page, we can see the overall struc- ture and internal components of a Hedgehog proto- type. Initially de- signed in the shape of hedge- hog with several “spines”, the proj- ect has evolved into a cube with spikes on its cor- ners; an engi- neering solution most suited for exploring the sur- faces of asteroids and comets. [NASA/JPL-Cal- tech/Stanford Univ., MIT] Mobility components Three internal flywheels for mobility Motors and brakes generate controlled and abrupt torques on flywheels Spikes on each corner protect from terrain and act as feet for hopping Key features Mechanically and thermally sealed from environment Symmetric design allows mobility in any configuration Large internal volume for scientific payload Minimalistic Scalable