Touring Asteroids with Electric Sails

A recently closed activity with TDE and the Finnish Meteorological Institute has made a detailed analysis of the most promising electric solar wind sails (E-sail), which use the solar wind to generate propulsion without using any propellant.

The activity analysed a variety of asteroid-related E-sail mission architectures, using single or multi-tethers, to find that an E-sail would allow a variety of advanced mission scenarios.

For asteroids, gravity assist manoeuvres are typically not available and so low-thrust propulsion methods such as electric propulsion and E-sail often have an advantage over chemical propulsion. The E-sail is the most efficient low-thrust method known, so it suits asteroid missions well. The main limitation of the E-sail is that it is not possible to land, or even get close to an asteroid, with an opened tether rig, and the tethers cannot be reliably retracted and re-opened so only flyby missions are possible. But a flyby mission could allow surveying of a large number of asteroids quickly, making the cost per asteroid extremely low. A fleet of 50 single-tether E-sails performing flybys of more than 300 main-belt asteroids was proposed under the name “Multi-Asteroid Touring” (MAT).

For MAT, the mission proposed that optical images and near-infrared spectra of the flown-by asteroids would be stored in memory throughout the nominally 3.2 year mission for each member of the fleet, and downlinked at a final Earth flyby to a 16 m ground antenna in a 3-hour telemetry session transferring 10 gigabytes of data from flash memory, for each 50 spacecraft. The activity later redesigned the mission to address near-Earth orbit (NEO) asteroids. Surveying NEOs instead of main-belt asteroids relaxes the characteristic acceleration requirement and removed the heavy mass constraints on the design, which enabled ordinary navigation methods and telemetry sessions to be used.

For rendezvous, orbiting, landing and sample return missions other solutions have to be found. Sample return is possible with moderate mass and asteroid mining can become economical because the propellantless nature and low mass of the E-sail enable a high mass ratio. The E-sail also dictates the platform’s orientation, so instruments and antennas cannot be pointed accurately.

The activity proposed a two-spacecraft mission architecture, where the mothership used E-sail technology, and was ‘parked’ at the edge of the asteroid belt, to prevent the tethers colliding with any, potentially unseen, mini-moons for the asteroid. A separate science spacecraft would land, take the sample and re-dock with this mothership. The science spacecraft can be turned freely because it has no tethers, so pointing antennas and science instruments would be possible, without moving parts.

While the activity focused on science missions, it also analysed economically profitable missions for asteroid mining. These missions would require a high mass ratio (a higher returning mass compared to the initial launch mass). The activity found that a small, 6U cubesat would be able to collect around 300kg of regolith and return it to Low Earth Orbit and still be economically viable.

T719-409MP closed in August 2020.