For years now Pekka Janhunen has been working on his concept of an electric sail with the same intensity that Claudio Maccone has brought to the gravitational focus mission called FOCAL. Both men are engaging advocates of their ideas, and having just had a good conversation with Dr. Maccone (by phone, unfortunately, as I’ve been down with the flu), I was pleased to see Dr. Janhunen’s electric sail pop up again in online discussions. It turns out that the physicist has been envisioning a sail mission to an unusual target.
Let’s talk a bit about the mission an electric sail enables. This is a solar wind-rider, taking advantage not of the momentum imparted by photons from the Sun but the stream of charged particles pushing from the Sun out to the heliopause (thereby blowing out the bubble’ in the interstellar medium we call the heliosphere). As Janhunen (Finnish Meteorological Institute) has designed it, the electric sail taps the Coulomb interaction in which particles are attracted or repulsed by an electric charge. The rotational motion of the spacecraft would allow the deployment of perhaps 100 tethers, thin wires that would be subsequently charged by an electron gun with the beam sent out along the spin axis.
Image: The electric sail is a space propulsion concept that uses the momentum of the solar wind to produce thrust. Credit: Alexandre Szames.
The electron gun keeps the spacecraft and tethers charged, with the electric field of the tethers extending tens of meters into the surrounding solar wind plasma — as the solar wind ‘blows,’ it pushes up against thin tethers that act, because of their charge, as wide surfaces against which the wind can push. The sail uses the attraction or repulsion of particles caused by the electric charge to ride the wind, the positively charged solar wind protons repelled by the positive voltage they meet in the charged tethers.
One disadvantage that electric sails bring to the mix, as opposed to solar sails like IKAROS, is that the solar wind is much weaker — Janhunen’s figures have it 5000 times weaker — than solar photon pressure at Earth’s distance from the Sun. This has come up before in comments here and it’s worth quoting Janhunen on the matter, from a site he maintains on electric sails:
The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun… Due to the very large effective area and very low weight per unit length of a thin metal wire, the electric sail is still efficient, however. A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but when opened in space and connected to the spacecraft’s electron gun, it can produce several square kilometre effective solar wind sail area which is capable of extracting about 10 millinewton force from the solar wind.
Computer simulations using tethers up to 20 kilometers in length have yielded speeds of 100 kilometers per second, a nice step up from the 17 kps of Voyager 1, and enough to get a payload into the nearby interstellar medium in fifteen years. Or, as Janhunen describes in the recent paper on a Uranus atmospheric probe, an electric sail could reach the 7th planet in six years. Janhunen sees such a probe as equally applicable for a Titan mission and, indeed, missions to Neptune and Saturn itself, but notice that none of these are conceived as orbiter missions. A significant amount of chemical propellant is needed for orbital insertion unless we were to try aerocapture, but the problem with the latter is that it is at a much lower technical readiness level.
A demonstrator electric sail mission, then, is designed to keep costs down and reach its destination as fast as possible, with the interesting spin that, because we’re in need of no gravitational assists, the Uranus probe will have no launch window constraints. As defined in the paper on this work, the probe would consist of three modules stacked together: The electric sail module, a carrier module and an entry module. The entry module would be composed of the atmospheric probe and a heat-shield.
At approximately Saturn’s distance from the Sun, the electric sail module would be jettisoned and the carrier module used to adjust the trajectory as needed with small chemical thrusters (50 kg of propellant budgeted for here). And then the fun begins:
About 13 million km (8 days) before Uranus, the carrier module detaches itself from the entry module and makes a ~ 0.15 km/s transverse burn so that it passes by the planet at ~ 105 km distance, safely outside the ring system. Also a slowing down burn of the carrier module may be needed to optimise the link geometry during flyby.
Now events happen quickly. The entry module, protected by its heat shield, enters the atmosphere. A parachute is deployed and the heat shield drops away, with the probe now drifting down through the atmosphere of Uranus (think Huygens descending through Titan’s clouds), making measurements and transmitting data to the high gain antenna on the carrier module.
Thus we get atmospheric measurements of Uranus similar to what the Galileo probe was able to deliver at Jupiter, measuring the chemical and isotopic composition of the atmosphere. A successful mission builds the case for a series of such probes to Neptune, Saturn and Titan. Thus far Jupiter is the only giant planet whose atmosphere has been probed directly, and a second Jupiter probe using a similar instrument package would allow further useful comparisons. Our planet formation models, which predict chemical and isotope composition of the giant planet atmospheres, can thus be supplemented by in situ data.
Not to mention that we would learn much about flying and navigating an electric sail during the testing and implementation of the Uranus mission. The paper is Janhunen et al., “Fast E-sail Uranus entry probe mission,” submitted to the Meudon Uranus workshop (Sept 16-18, 2013) special issue of Planetary and Space Science (preprint).