Plasma Sails

Around 2000, the space propulsion community was abuzz with excitement about a newly proposed propulsion system that offered the potential of travel to the outer edges of the solar system riding the solar wind—and consuming very little propellant — called Minimagnetospheric Plasma Propulsion, or M2P2. The brainchild of Dr Robert Winglee of The University of Washington, the M2P2 would take advantage of the fact that the Sun is constantly shedding high-speed charged particles (called the solar wind) which race outward from it at speeds near 400 kilometers per second.

The M2P2 would work by creating around a spacecraft a miniature version of the Earth's ionosphere, taking advantage of the fact that like-charge electrically charged particles in a plasma repel each other. The ionosphere is the part of Earth's atmosphere that is ionized by solar radiation, producing a relatively dense plasma environment. A plasma is nothing more than a mixture of positively and negatively charged particles that are typically too energetic to recombine and form neutral molecules. The plasma is mostly "trapped" on the Earth's magnetic field lines due to electromagnetic interactions. As a general rule, charged particles moving in a magnetic field will experience a force due to their motion through the field. The forces acting on the ionospheric plasma tend to send it spiraling along the field lines, moving between the Earth's north and south magnetic poles. The solar wind, which is also a plasma, often interacts with the Earth's ionosphere, causing it to be depressed on the sunward side and extended on the opposite side. The depression on the sunward side is actually exerting a propulsive force on the Earth, though at a scale so small compared with the mass of the Earth as to be insignificant.

The M2P2 concept takes advantage of the forces acting on the Earth, in miniature, to propel a spacecraft sporting its own ionosphere. An M2P2 propulsion system would have on board a very strong magnet and a plasma source. The field generated by the magnet would trap the plasma and create a multi-kilometer-wide plasma bubble around the spacecraft. The solar wind would then hit the bubble at enormous speeds, with like-charged particles repelling each other, theoretically causing the bubble-encased spacecraft to be blown outward like a balloon tossed into the air on a windy March day. Since the solar wind does not decrease in intensity nearly as rapidly as sunlight, the forces acting on the plasma sail should theoretically be able to accelerate it for much longer periods, allowing the craft to attain very high speeds. Dr Winglee calculated that a plasma sail craft might be able to send a probe to Jupiter in only 1.5 years compared to the 5 years required for a conventional, chemically powered spacecraft.

As with any new technology, there remain many unanswered questions about the ultimate viability of the M2P2 as a spacecraft propulsion system.

• Will the plasma bubble remain intact after encountering the much more energetic solar wind? Or will it be ripped away from the spacecraft, leaving it motionless as the wind blows by?

• Can the very powerful magnets required to make it work actually be built and launched affordably? (Magnets can be very heavy and therefore difficult to launch.)

• What is the plasma leakage rate from the bubble? If too much plasma leaks away, then the overall efficiency of the system drops and might soon make it no more attractive than state-of-the-art electric propulsion systems.

More research (and time) are needed to answer these questions.

Magnetic Propulsion

Every child has played with magnets and learned about their attraction and repulsion. Since the Earth, jupiter, and the Sun are enormous cosmic magnets, it is not surprising that mature researchers have devoted serious effort to investigating the feasibility of magnetic space propulsion.

Much of the modern work in this field germinated from a paper by Gregory Matloff and Alphonsus j. Fennelly in the September 1974 issue of JBIS. This paper addressed the problem of collecting interstellar ions for the interstellar ramjet (see Chapter 6). A superconducting magnetic solenoid was proposed as an interstellar ion-collection device.

In research by Robert Zubrin and Dana Andrews, reviewed in Gregory Matloffs Deep-Space Probes, plasma-physics computer codes were employed to examine the trajectories of interstellar ions encountering the solenoid's magnetic field. It was found, however, that most proposed magnetic interstellar-ramjet ion-collection devices actually function as ion reflectors, not ion collectors.

Although bad for ramjet fans, these superconducting ion reflectors (dubbed "magsails") turned out to have interstellar applications. They can theoretically function as very effective drag brakes, to slow a speeding starship down to planetary velocities by reflecting sufficiently large quantities of ionized gas in the local space, wherever that might be. Large magsail field radii (measured in hundreds or thousands of kilometers) and long deceleration durations (measured in decades) are necessary to accomplish this task for a large, fast starship.

Magsail derivatives were considered that could theoretically reflect interplanetary ions and operate as a propellantless-propulsion device between the planets. It appeared that direct interaction with Earth's magnetic field, allowing for magnetic-assisted liftoff from Earth's surface, might be possible.

But alas, these proposals proved to be a little overoptimistic. In research reviewed in Gregory Matloffs Deep-Space Probes, Giovanni Vulpetti and Mauro Pecchioli demonstrated that thermal effects in the inner solar system would limit the operation of superconducting magsails.

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