Asteroids have a lot to teach us about the early days of the Solar System, but their nomadic nature makes them hard to study. To observe a wide range of space rocks, a European team is proposing a mission called the Asteroid Touring Nanosat Fleet, which would involve sending 50 tiny spacecraft to visit hundreds of asteroids, before returning to Earth with their data payloads.
Astronomers take the opportunity to point their instruments at asteroids whenever they buzz Earth, but there are a few more active missions in the works. The Hayabusa2 probe is currently en-route to rendezvous with asteroid Ryugu, where it will collect samples from the surface and bring them back for examination. NASA meanwhile has plans for an ambitious mission to snag a space rock and drag it into orbit around the Moon, where it can be studied at our leisure.
But as insightful as these missions may be, we're only studying two asteroids there. That's not a very big sample size to draw from, so increasing that snapshot to 300 different asteroids is the goal of the new mission, led by the Finnish Meteorological Institute.
"Asteroids are very diverse and, to date, we've only seen a small number at close range," says Pekka Janhunen, lead researcher on the project. "To understand them better, we need to study a large number in situ. The only way to do this affordably is by using small spacecraft."
The plan is to send a fleet of 50 small spacecraft, each one weighing about 5 kg (11 lb), on a 3.2 year-journey into the asteroid belt beyond Mars and back again. Each individual craft would perform a flyby of six or seven asteroids from a distance of about 1,000 km (621 mi), using a 4-cm (1.6-in) telescope to image the rock at a resolution of less than 100 m (328 ft) per pixel. A near-infrared spectrometer would also analyze what the asteroid might be made of.
"The nanosats could gather a great deal of information about the asteroids they encounter during their tour, including the overall size and shape, whether there are craters on the surface or dust, whether there are any moons, and whether the asteroids are primitive bodies or a rubble pile," says Janhunen. "They would also gather data on the chemical composition of surface features, such as whether the spectral signature of water is present."
Since room aboard the spacecraft is scarce, and keeping costs down is a key goal of the mission, the fleet wouldn't be fitted with high-gain antennas. Instead, the data they gather would be stored internally on flash memory, and delivered back home when the craft perform a flyby of Earth at the end of their journey. Each would collect about 10 GB of data, which should take around three hours for the ground team to download.
The Nanosat Fleet will be powered by electric solar wind sails (E-sails), which make use of the constant stream of charged particles blasted into space by the Sun. Each of the small spacecraft would be attached to an E-sail by a 20-km (12-mi) tether, which spins to control the orientation the craft.
While it is fairly slow-going – acceleration would be about 1 mm per second – it doesn't need any other propellant, and when combined with the initial launch speed, it should be enough to make the round trip in just over three years.
Ambitious as it sounds, the mission should also be far cheaper than other options. The team estimate the bill to ring up for around €60 million (US$72 million), and when spread across 300 targets, that brings the cost per asteroid studied down to about €200,000 ($240,000).
"The cost of a conventional, state-of-the-art mission to visit this number of asteroids could run into billions," says Janhunen. "This mission architecture, using a fleet of nanosats and innovative propulsion, would reduce the cost to just a few hundred thousand Euros per asteroid. Yet the value of the science gathered would be immense."
Janhunen presented the concept at the European Planetary Science Congress this week, and a paper outlining the proposal has been published online.