A lightness of being

AFTER hurtling more than 6 billion kilometres through space for over a decade, the European Space Agency’s (ESA) probe Rosetta began orbiting comet 67P/Churyumov–Gerasimenko last year. In November the mother ship released its lander, Philae, which appeared to descend to the surface successfully. But elation at the European mission-control centres soon turned to concern. Philae had bounced back up again due to a failure of the explosives-powered harpoons that were supposed to anchor it to the surface. The harpoons were necessary because a small body like a comet generates little gravity. So little, in fact, that if Philae bounced faster than 44cm per second it was in danger of exceeding the comet’s escape velocity, the speed that an object needs to be travelling to break free of a body’s gravity.

As luck would have it, Philae fell back to the surface and eventually came to a stop where insufficient sunlight could reach its solar panels. The craft managed to deliver some data until its batteries ran out of power 64 hours later. One day Philae might be revived if 67P happens to move into more sunlight. Even so, the difficulties the mission encountered help to explain why space agencies are putting so much effort into designing machines which are capable of not only landing on bodies with microgravity but also travelling around them without flying off in all directions.

Wheeled rovers have long trundled across the Moon and Mars, but their gravities are merely low—a sixth and a third, respectively, of that on Earth, which has an escape velocity of 11km per second. Wheeled and tracked rovers could probably be made to work in gravity as low as a hundredth of that on Earth, says Issa Nesnas, head of the Robotic Mobility Group at NASA’s Jet Propulsion Laboratory in Pasadena, California. But in the far weaker microgravity of small bodies like asteroids and comets, they would fail to get a grip in fine regolith. Wheels might also hover above the ground, spinning hopelessly and using up power. So an entirely different system of locomotion is needed for rovers operating in a microgravity.

Surprising as it may seem, one promising form of transportation in microgravity is a space hopper. These machines are nothing like the bouncy toys made popular in the 1970s-1980s. But they share the same idea, because bouncing from one place to another has its advantages.

The first hop

No one has yet demonstrated if a space hopper will work in space. But in a few years that opportunity will arise. A spacecraft loaded with four robotic hopping rovers blasted off from Japan’s Tanegashima Space Centre on December 3rd. The mission, called Hayabusa 2, is being run by the Japan Aerospace Exploration Agency (JAXA). The aim is to collect samples from an asteroid called 1999 JU3 and return them to Earth. The spacecraft will arrive at the asteroid in the summer of 2018 and spend about a year surveying it. It will then move in extremely close to fire projectiles into the asteroid’s surface. This will throw up material which the spacecraft will suck in with a suction nozzle.

With a diameter of only about 1km, 1999 JU3 has an escape velocity of just 32cm per second. To hop across its surface the rovers will use a moving internal mass. The largest rover on board the Hayabusa 2 spacecraft is a 10kg cube-shaped machine called MASCOT (for Mobile Asteroid Surface Scout, and illustrated on the previous page with its mother ship). MASCOT employs a weighted internal swing-arm, a bit like a pendulum. An electric motor swings the arm around and then suddenly brakes the movement. This jolt transfers inertia to the body of the rover, pushing it down into the surface, which results in the machine bouncing up. To ensure that the rover drops back down again and does not drift off into space, its hopping speed will be capped at about two-third’s of the asteroid’s escape velocity.

MASCOT was built by DLR, Germany’s aerospace centre. Besides hopping it can use its swing-arm to tumble over if it lands the wrong way up. This is to ensure that its instruments—a camera, magnetometer (to measure magnetic fields), radiometer (to measure temperature and radiation), and an infra-red microscope (to study minerals)—are all pointed in the right direction.

Until they are tested in a real microgravity no one can be sure these rovers will work

Hopping mechanisms such as these are lighter and less intricate than wheeled and tracked systems. And by hopping the rovers do not require detailed information about the terrain to ensure safe routes. Even if a space hopper lands on a sharp rock it is unlikely to damage itself, because in microgravity objects are a fraction of their weight on Earth. Hopping also requires less energy than turning wheels. The equivalent amount of power required to run an iPad for not much more than 30 seconds will toss MASCOT 70 metres or so, reckons Tra-Mi Ho, who leads the project at DLR.

To keep the €28m ($32m) rover small and light enough to be carried by the mother ship MASCOT does not have solar panels to recharge its batteries. These will last for just 16 hours, the equivalent of two of the asteroid’s days and nights. So the rover has to pack in a lot of work between its hops.

CNES, the French space agency, is analysing data on Philae’s ill-fated bounces to better calibrate the hops which MASCOT will undertake. CNES will use information from the Hayabusa 2 survey of 1999 JU3’s gravity and surface composition to calculate the swing-arm velocities needed for the most efficient hops, says Pierre Bousquet, head of microgravity projects.

In free fall

The biggest challenge will be getting the four rovers onto the asteroid, says DLR’s Dr Ho. They must be ejected from the Hayabusa 2 mother ship at precise velocities and locations to free fall to the surface from about 100 metres, she adds. Such separations are tricky, as the first Hayabusa mission showed. In 2005 its mother ship released a space hopper named MINERVA 200 metres above an asteroid called Itokawa. That was 130 metres too far. MINERVA was not captured by the asteroid’s gravity and floated off into space. The three MINERVA-II Japanese space hoppers on the current Hayabusa 2 mission are improved variations of the lost original.

If Hayabusa 2’s space hoppers work well such rovers would help to broaden extraterrestrial exploration, particularly on asteroids and comets. Scientists are interested in these bodies because they are the purest remnants of the early solar system, unadulterated by many of the chemical and geological transformations that have taken place on planets. Some may contain matter that predates the formation of stars. Many appear rich in complex organic molecules containing carbon, hydrogen, oxygen and nitrogen—elements that were needed for life to begin on Earth.

Asteroids that orbit near Earth can be easier and cheaper to reach than many planets or moons. And because the escape velocities of small bodies are so slow, only a little fuel needs to be carried for a space vehicle to take off from one. Asteroids could therefore serve as stepping stones to get astronauts into deep space, says Marco Pavone, a Stanford University roboticist who is designing a microgravity space hopper for NASA. The rocks could also be mined for elements such as oxygen and hydrogen to replenish supplies of water, breathable air and fuel.

The space hopper which Dr Pavone and his colleagues are working on (pictured) sports three internal flywheels, one for each axis of motion. Each flywheel is powered with an electric motor, so they can reach different speeds. In order to hop, the inertial energy from each flywheel must be transferred to the robot’s frame simultaneously.

Some microgravity space hoppers using a similar system are known as “hedgehogs” because of their protective spikes. Once such prototype built at the Jet Propulsion Laboratory (pictured) uses brakes to stop the flywheels. The Stanford team are experimenting with an alternative method that delivers momentum more suddenly and with less energy lost as braking heat. It uses a small metal part to snag each flywheel to an immediate halt. Having multiple flywheels allows hops to be more steerable and precise. Benjamin Hockman, a mechanical engineer working on the project, says hedgehogs could also be used to explore moons, such as Phobos, a Martian moon with a tiny microgravity.

A team at the University of Tokyo has gone about things in a different way. They have built a spherical space hopper that generates motion with electromagnets. Four electromagnets are fixed to the sphere’s inner wall and a small iron ball is suspended in the centre. Using battery power to activate one or more electromagnets results in the ball being pulled across to the side of the sphere. This imparts momentum to the robot’s frame and thus initiates a hop. If more precision in hopping is required then two additional electromagnets could be used. Such a set-up would also allow the rover to roll along, says its designer, Yoshihiko Nakamura.

Rolling is another option for a rover operating in low gravity. One type, known as “structurally compliant” rovers, are designed specifically to roll along. These are constructed from a latticework of rigid rods connected with elastic cables. Mechanical actuators are used to shorten and lengthen the cables, so that the rovers change shape as they repeatedly tip over in the direction they want to go. Although more jerky than graceful, little traction is needed resulting in a “punctuated rolling motion”, says Alice Agogino, a NASA-funded researcher working on such a project at the University of California, Berkeley. The rovers’ instruments and power supply would be suspended in the centre of the structure.

A partner team at NASA’s Ames Research Centre is developing structurally compliant rovers they call Super Ball Bots (one of which is pictured). The researchers hope their robots could be used on Phobos or Titan, one of Saturn’s moons. The two moons differ greatly. With a seventh of Earth’s gravity, Titan could be traversed with a conventional wheeled rover. A Super Ball Bot, however, makes sense for such a place because it could double as both the locomotion system and a landing mechanism, says Vytas SunSpiral, an Ames roboticist. The structures, lacking rigid joints, are able to absorb large shocks without damage. Conventional rovers dropped on Mars are cushioned with expensive, elaborate and heavy airbag systems. A Super Ball Bot could fall from orbit or roll off a cliff and become its own airbag, says Dr SunSpiral.

Yet mobility in a microgravity will only take a rover so far. Sometimes they must stop and analyse samples. The reason the Hayabusa 2 spacecraft will fire projectiles into the surface of 1999 JU3 to kick up samples is that drilling is not much of an option. No robotic microgravity anchoring system has yet been successfully used, and without one it is the spacecraft or the rover, rather than the drill bit, that would spin. Giving rovers claws might be a solution. Aaron Parness, who works in the Jet Propulsion Laboratory’s “extreme environment” robotics lab has developed a machine which uses hundreds of tiny claws to grip the rough surfaces often found on bodies like asteroids. The machine (pictured) is still under development but it has the potential to climb vertical rock faces and even creep along upside down on overhead formations.

Until they are tested in a real microgravity no one can be sure these rovers will work. There is no practical way to fully replicate a mission in a simulated microgravity on Earth. Some components of the MASCOT system have been tested in a 146-metre drop tower in Bremen, Germany, which uses a catapult to produce 9.3 seconds of near weightlessness. The Draper Laboratory, an independent research centre in Cambridge, Massachusetts, tested the guidance and control systems on a space hopper which it has developed during a reduced-gravity flight on board a NASA aeroplane known as a “vomit comet”. But it will be one of Hayabusa 2’s space hoppers that may be the first to complete such a mission, although which rover that will be has yet to be decided.

Saving the planet

The importance of a successful deployment is higher than you might imagine. For although they are designed for exploration, microgravity rovers might one day save Earth from a catastrophic collision with an asteroid. Many asteroids are composed of loosely coalesced rocks and would be hard to push or pull into a safe orbit. A paint job, however, might do the trick, reckons CNES’s Mr Bousquet. Just as space hoppers rely on every action having an equal and opposite reaction, light and heat reflected off an asteroid’s surface exerts a tiny pressure. So increasing the reflectivity of the rocks would alter this gentle pushback and, over time, the asteroid’s trajectory. However they move, rovers that can operate in extremely low gravity may one day have a very important job to do.