Asteroids are material left behind from the formation of the Solar System.They range in size from the 946-kilometre (587-mile) Ceres down to pebbles, and most of them are irregular objects collected in the main asteroid belt between Mars and Jupiter. It is estimated that there are around 1.5 million asteroids larger than a kilometre in the main belt. Fortunately for our future exploration plans they don’t form a rocky cloud like in the movies, being on average two-million-kilometres apart. Despite their ubiquity asteroids’ small size means we have not been able to study many in great detail, and with nearly 1,500 out of around 10,000 known Earth crossing asteroids considered an impact risk for us, we really need to know more.
“Asteroids are very diverse and, to date, we’ve only seen a small number at close range. 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,” says Dr Pekka Janhunen of the Finnish Meteorological Institute in Helsinki. Dr Janhunen is the lead scientist on a new proposal to explore up to 300 asteroids with a fleet of ingenious nanosatellites. The enabling technology for this concept is a new propulsion technique called an E-sail.
Any spacecraft looking to do more than a oneway flyby needs some kind of propulsion, and this generally involves a finite amount of propellants, held in pressurised tanks, that supply an engine or thruster. This gets more difficult to accomplish the smaller a spacecraft is, and inevitably limits the lifetime of the mission. An alternative approach is to use the output of the Sun. Solar sails – huge lightweight mirrors pushed by the force of reflecting light – were first proposed in the 1920s, and recently tested for the first time on JAXA’s IKAROS mission, but Dr Janhunen has invented another option, the E-sail.
Where solar sails are pushed by light, the E-sail works by interacting with the solar wind, the stream of charged particles emitted by the Sun. A network of fine wires is stretched out from a spacecraft and electrified, solar particles passing by are deflected by the electric field around the wires, pushing the spacecraft along. Like a sailing ship, the E-sail could make manoeuvres by changing the angle of the sail. E-sails are potentially much easier to make than the huge areas of mirror needed for solar sails, while still retaining their advantages.
For Dr Janhunen’s asteroid explorers, a nanosatellite would be fitted with a four-centimetre (1.5-inch) telescope, capable of imaging asteroids down to 100 metre (300 feet) details on the surface and determining material composition by analysing the spectrum of the reflected light. To minimise the E-sail for a small five kilogram probe it will be a single 20 kilometre (12 mile) wire, stretched out between the main spacecraft and a counterweight. This E-sail can accelerate the nanosatellite at 0.1 miliG which, along with the initial launch boost, is enough for a 3.2-year tour of six to seven asteroids in the main belt. An Indian PSLV could launch 50 of these nanosats for a total mission cost of €60 million, visiting over 300 asteroids at a cost of only €200,000 per asteroid – much cheaper than traditional missions. Because the spacecraft are too small to carry a main antenna they would store up all their data over the three years and download it to Earth only on return to our vicinity.
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