TL;DR – Very, very gently.
Exploiting the resources of the rock-strewn expanse of space between Mars and the outer planets has been the stuff of science fiction for ages. There’s gold in them ‘thar space rocks, or diamonds, or platinum, or something that makes them attractive targets for capitalists and scientists alike. But before actually extracting the riches of the asteroid belt, stuck here as we are at the bottom of a very deep gravity well that’s very expensive to climb out of, we have to answer a few questions. Like, how does one rendezvous with an asteroid? What’s involved with maneuvering near a comparatively tiny celestial body? And most importantly, how exactly does one land on an asteroid and do any useful work?
Back in June, a spacecraft launched by the Japanese Aerospace Exploration Agency (JAXA) finally caught up to an asteroid named Ryugu after having chased it for the better part of four years. The Hayabusa2 was equipped to answer all those questions and more, and as it settled in close to the asteroid with a small fleet of robotic rovers on board, it was about to make history. Here’s how they managed to not only land on an asteroid, but how the rovers move around on the surface, and how they’ll return samples of the asteroid to Earth for study.
These Are Not the Rovers You’re Looking For
Rendering of MINERVA-II-1 rovers on Ryugu. Source: JAXA, University of Tokyo & collaborators
As interplanetary spacecraft go, Hayabusa2 looks pretty much like what you’d expect, with two long rectangular solar panels flanking the cube-shaped main part of the craft. It is similar in design to its predecessor Hayabusa — the name means “Peregrine falcon” in Japanese — which explored the asteroid Itokawa in the mid-2000s and returned samples of the asteroid to Earth. Hayabusa was equipped with a robotic rover, dubbed MINERVA; sadly, a communication error caused the rover to be released while Hayabusa was ascending from the asteroid and the rover was not captured by its minuscule gravitation. Hayabusa2 was to give the landing attempt another go, and this time the mothership carried not one rover but four! Three of them were improved MINERVA-II rovers, with a German-built vehicle called MASCOT as the fourth.
These are not the type of “rover” you are picturing. We’re all familiar with the Apollo-era lunar rovers, or even the Mars Curiosity rovers, which are wheeled vehicles that can be driven around by remote control. But driving around on a tiny fleck of space rock with 1/10,000th the acceleration due to gravity that Earth has is a problem. With so little force pulling a rover down, the wheels would lack enough friction to get any traction, So rather than wheels, the rovers all hop around the surface of Ryugu instead.
Lose the Wheels; Go with Torquers
Prototype torquers used for testing hopping in a microgravity environment. Source: JAXA, University of Tokyo & collaborators
There are plenty of ways to make a rover hop, including small jets of compressed gas, arms that deploy to push off against the asteroid’s surface, or even springs that propel the rover upwards. But the MINERVA-II rovers went with a different solution: small, motor-driven flywheels called “torquers.” Torquers work by modulating their angular velocity very carefully, producing a moment of torque that can flip the rover up into a ballistic orbit above the surface of the asteroid. With two torquers at a 90° angle to each other, the rover can control not only its speed but its altitude as well. This lets the rovers set down anywhere else on the asteroid.
The surface of Ryugu from Rover 1B’s camera. Source: JAXA, University of Tokyo & collaborators
The first two MINERVA-II rovers were released from Hayabusa2 on September 21st, and both successfully landed on the surface of Ryugu. Both rovers have successfully hopped around the asteroid since then. The rovers, tiny flattened cylinders 18 cm across and about 1.1-kg each, are both solar powered and are designed for long-term operation on the surface. Their instrument packages, which include cameras, send data back to the mothership whenever it’s in view.
The MASCOT rover, on the other hand, is a battery-powered, 10-kg package that’s designed to conduct spectroscopic and magnetic studies on Ryugu’s surface. Designed for a limited mission and with enough power for only one hop, MASCOT was sent down to the surface on October 3rd, and managed to send back data for 17 hours before shutting down. The one remaining MINERVA-II rover, a larger model with more sensors and three other types of mobility systems in addition to the torquers, is not scheduled for deployment until July of 2019.
Shots in the Dark
While the rovers are getting a lot of attention with all their hopping about, the mothership has a few tricks of her own to perform. If all goes to plan, sometime in January of 2019 Hayabusa2 will maneuver slowly and carefully down to the surface of Ryugu and extend a sampling probe to the asteroid’s surface. The probe is designed to collect anywhere from a few milligrams up to 10 grams of sample. To accomplish this, the sampler will fire a 5-g slug of tantalum down the barrel of the sampler and into Ryugu’s surface. Ejecta will collect in the sampler and be sealed inside a reentry vehicle for return to Earth.
Hayabusa2 will perform a similar collection later in the year, and shortly thereafter put on a really big show. After maneuvering to about 200 meters above the surface, it’ll use a 4.7-kg shaped-charge to blast a 2-kg copper disk down at the surface. The projectile will impact at about 2 km/s and blast a new crater into the asteroid to expose material that has been sequestered from the ravages of space. Hayabusa2 will scamper off to the other side of the asteroid to avoid the debris cloud, but not before deploying a remote-controlled camera to monitor the scene of the crime. Once the dust settles, Hayabusa2 will make one last sample pickup from the crater before getting ready for the long trip home in late 2019.
All things considered, the mission to Ryugu is incredibly ambitious, and should yield a wealth of data on asteroids. The mission has many moving parts, both literally and figuratively, and there’s a significant chance that something will fail before the mission wraps up. But the fact that everything has gone so well thus far is encouraging.
If all goes well, the first few grams of what is currently estimated to be $83 billion worth of iron, cobalt, nickel and other substances locked up in the asteroid will float back to Earth over Australia sometime in 2020. And if that marks the point at which asteroid mining becomes something more than science fiction, it’ll be because of the Hayabusa missions and what their hopping rovers teach us.
Read more: hackaday.com