Just how we follow up on the investigations of New Horizons remains an open question. But we need to be thinking about how we can push past the outer planets to continue our study of the heliopause and the larger interstellar environment in which the Sun moves. I notice that Bruce Wiegmann, writing a precis of a mission concept called the Heliopause Electrostatic Rapid Transit System (HERTS) has drawn inspiration from the Heliophysics Decadal Survey, which cites the need for in situ measurements of the outer heliosphere and beyond.

It’s good to see a bit more momentum building for continuing the grand voyages of exploration exemplified by the Pioneers, the Voyagers and New Horizons. I often cite the Innovative Interstellar Explorer concept developed at Johns Hopkins (APL), which targets nearby interstellar space at a distance of over 200 AU, but whether we’re talking about IIE or Claudio Maccone’s FOCAL mission or any other design aimed at exiting the Solar System, the key problem is propulsion. Weigmann’s team at Marshall Space Flight Center has been awarded a Phase I grant from NASA’s Institute for Advanced Concepts to work on a dramatic solution.

The Heliopause Electrostatic Rapid Transit System involves a sail and thus propellant-less propulsion, but it’s not the conventional solar sail that uses the momentum provided by solar photons. The nomenclature is confusing, because the electric sail HERTS is designed around would interact with the solar ‘wind,’ which is not made up of photons at all but a stream of charged particles that flows constantly though erratically from the Sun at high velocity. A spacecraft riding the solar wind could, by some calculations, move between five and ten times faster than our best outer-system result so far, the 17.1 km/sec Voyager 1.

Wiegmann explains the principle at play in the precis:

The basic principle on which the HERTS operates is the exchange of momentum between an array of long electrically biased wires and the solar wind protons, which flow radially away from the sun at speeds ranging from 300 to 700 km/s. A high-voltage, positive bias on the wires, which are oriented normal to the solar wind flow, deflects the streaming protons, resulting in a reaction force on the wires—also directed radially away from the sun. Over periods of months, this small force can accelerate the spacecraft to enormous speeds—on the order of 100-150 km/s (~ 20 to 30 AU/year). The proposed HERTS can provide the unique ability to explore the Heliopause and the extreme outer solar system on timescales of less than a decade.

If you’re an old Centauri Dreams hand, you’ll recognize the HERTS sail as the offspring of Pekka Janhunen (Finnish Meteorological Institute), whose concept involves long tethers (perhaps reaching 20 kilometers in length) extended from the spacecraft, each maintaining a steady electric potential with the help of a solar-powered electron gun aboard the vehicle. As many as a hundred tethers — these are thinner than a human hair — could be deployed to achieve maximum effect. While the solar wind is far weaker than solar photon pressure, an electric sail with tethers in place is still efficient, according to Janhunen’s calculations, and can create an effective solar wind sail area of several square kilometers.

Image: A full-scale electric sail consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of metres into the surrounding solar wind plasma. Therefore the solar wind ions “see” the wires as rather thick, about 100 m wide obstacles. A technical concept exists for deploying (opening) the wires in a relatively simple way and guiding or “flying” the resulting spacecraft electrically. Credit: Artwork by Alexandre Szames. Caption via Pekka Janhunen/Kumpula Space Centre.

MSFC’s Advanced Concepts Office has been studying the feasibility of the Janhunen sail during the past year, finding that the electric sail is able to reach velocities three to four times greater than any realistic current technology including solar (photon) sails and solar electric propulsion systems. Because we are dealing with a stream of particles flowing outward from the Sun (and because the electric sail can, like a solar sail, be ‘tacked’ for maneuvering), we are looking at a fast interplanetary propulsion system that avoids the deployment issues faced by large solar sails using photon momentum for their push. Deploying reels of tethers is, by comparison, straightforward.

Both photon-pushed sails and those riding the solar wind are limited by distance from the Sun, but the electric sail may have applications in future interstellar missions nonetheless. If we accelerate a (non-electric) sail by the use of a laser or microwave beam up to a small percentage of the speed of light, we could slow it down upon arrival by using the solar wind from the destination star, interacting with a tether system deployed as the spacecraft enters the new system. Having decelerated, the spacecraft could then use electric sail technology for exploration. Janhunen has explored the concept for electric sails (though not yet in detail), but an idea like this was also broached by Robert Zubrin and Dana Andrews for magnetic sail deceleration in 1990.

A key paper on electric sails is Janhunen and Sandroos, “Simulation study of solar wind push on a charged wire: solar wind electric sail propulsion,” Annales Geophysicae 25, (2007), pp. 755-767. For background, see Electric Solar Wind Sail Spacecraft Propulsion, which provides diagrams, a FAQ and various links to published papers.