Hitching a Ride on a Magnetic Bubble

NASA A luminous plasma plume inflates an invisible magnetic bubble inside the vacuum chamber at NASA's Marshall Space Flight Center.

The force exerted on a magnetic bubble depends on how big it is. Big bubbles intercept more solar wind than little ones do and thus lend greater thrust to the spacecraft inside. Bubbles that travel away from the Sun naturally expand as the solar wind pressure plummets. They grow for the same reason that a child's balloon inflated at sea level will expand in rarefied air at high-altitudes.

"The sizes of balloons and of magnetic bubbles are set by the same thing - a balance of internal and external pressures," says Gallagher. "For M2P2, the internal pressure comes from the plasma and the solenoidal magnetic field. The external pressure is the solar wind."

The solar wind's force per unit area decreases as the square of the distance from the Sun. Doubling the distance, for instance, decreases the solar wind pressure by a factor of four. "The solar wind is weaker far from the Sun, but the bubble is bigger, too (precisely because the solar wind pressure is lower)," he continued. "It so happens that the cross section of the bubble increases by the same factor that the solar wind pressure declines. The two effects completely cancel." It seems amazing, but the propulsive thrust of an M2P2-powered craft remains the same whether the spacecraft is near the Sun or in the outer reaches of the solar system.

For human travelers the greatest advantage of M2P2 might not be steady acceleration or fuel efficiency, but rather its impressive safety features. Just as the Earth's magnetosphere protects us from solar radiation, an M2P2 bubble could shield spacefarers from cosmic rays and solar flares.

NASA An artist impression of the solar winds.

"The magnetic shielding idea needs to be investigated more carefully," noted Gallagher, "but it looks promising. By chaining multiple M2P2 units together on the same spacecraft, we should be able to minimize plasma losses and end up with a better cosmic ray shield as a bonus."

"I like to think of M2P2 as the first externally-powered fusion engine," he concluded. "The engine is the Sun itself - M2P2 bubbles just ride along on the exhaust."

The next round of M2P2 tests is slated for 2001. Buoyed the success of the ongoing lab experiments (and, of course, by the solar wind) magnetic bubbles could well become the space carriage of choice for the next century.