April 16, 2008

Solar Wind Electric Sail Propulsion planning test mission


A simplified picture of the electric sail. An actual system would have 50 to 100 or more 20 kilometer wires. 100 kg spaceships could be accelerated to final speeds of 40-100 km/second. The electric sail is an extremely promising new propulsion technique which is nearly ready to be tested. If electron heating turns out to be successful performance may be increased even more. Costs for solar system missions will go down and new capabilities and performance will be possible.

The electric solar wind sail developed at the Finnish Meteorological Institute two years ago has moved rapidly from invention towards implementation. The main parts of the device are long metallic tethers and a solar-powered electron gun which keeps the tethers positively charged. The solar wind exerts a small but continuous thrust on the tethers and the spacecraft.

"We haven't encountered major problems in any of the technical fields thus far. This has already enabled us to start planning the first test mission,” says Dr. Pekka Janhunen. An important subgoal was reached when the Electronics Research Laboratory of the University of Helsinki managed to develop a method for constructing a multiline micrometeoroid-resistant tether out of very thin metal wires using ultrasonic welding. The newly developed technique allows the bonding together of thin metal wires in any geometry; thus, the method might also have spinoff applications outside the electric sail.

The electric sail could enable faster and cheaper solar system exploration. It might also enable economic utilisation of asteroid resources for, e.g. producing rocket fuel in orbit.


Deploying the wires

An ideal (i.e. fully reflecting) solar sail receives a radiation pressure force of 9μN/m2 at 1AU distance from the Sun. Let us calculate how thin a solar sail should be, to reach the same specific acceleration as an electric sail wire plus electron gun subsystems. Using an 82 km/s final speed, one obtains that the solar sail should have an areal density of 1.1 g/m**2, which translates to 200 nm thickness if the material is aluminium and 50% of the mass is assumed to go to support structures. This is 5–10 times thinner than present technology.

The electric sail resembles the solar sail in that it provides small but inexhaustible thrust which is directed outward from the Sun, with a modest control of the thrust direction allowed (probably by a few tens of degrees). Some possible missions:

1. Missions going outward in the solar system and aiming for >50 km/s final speed, such as missions going out of the heliosphere and fast and cheap flyby missions of any target in the outer solar system. 2-4 years to Pluto instead of 10 years with chemical rockets and gravity slingshots.
2. By inclining the sail to some angle it can also be used to spiral inward in the solar system to study e.g. Mercury and Sun. Also a nonzero inclination with respect to the ecliptic plane is possible to achieve which may be beneficial for observing the Sun. Also the return trip back to Earth from the inner solar system is possible, as is cruising back and forth in the inner solar system and visiting multiple targets such as asteroids.
3. the electric sail could be used to implement a solar wind monitoring spacecraft which is placed permanently between Earth and Sun at somewhere else than the Lagrange point, thus providing a space weather service with more than one hour of warning time. Propulsion and data taking phases probably must be interleaved because ion measurements are not possible when the platform is charged to high positive voltage, although the plasma density and dynamic pressure of the solar wind can probably be sensed by an electron detector and accelerometer even when the electric sail voltage is turned on.
4. Once accelerated to a high outward speed an electric sailing spacecraft cannot by itself stop to orbit a remote target because the radial component of the thrust is always positive. For stopping under those circumstances one has to use aerocapture or some other traditional technique. Although the electric sail does not provide a marked speed benefit for such missions, being propellantless it might still provide cost saving; this remains to be studied. In interstellar space the plasma flow is rather slow. Thus the electric sail cannot be used for acceleration, but it can instead be used for braking the spacecraft.
5. It might also provide cheap transportation of raw materials such as water mined from asteroids and used for in-situ fuel making at high Earth orbit.

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