Rocket Ride Spaceships

The lowest-mass propulsion systems, however, may be those that carry no on-board propellant at all. In fact, they're not even rockets. Instead, in true pioneer style, they "live off the land" - relying for energy on natural resources abundant in space, much as pioneers of yore relied for food on trapping animals and finding roots and berries on the frontier.

The two leading candidates are solar sails and plasma sails. Although the effect is similar, the operating mechanisms are very different.

A solar sail consists of an enormous area of gossamer, highly reflective material that is unfurled in deep space to capture light from the sun (or from a microwave or laser beam from Earth). For very ambitious missions, sails could range up to many square kilometers in area.

Solar sails take advantage of the fact that solar photons, although having no mass, do have momentum - several micronewtons (about the weight of a coin) per square meter at the distance of Earth. This gentle radiation pressure will slowly but surely accelerate the sail and its payload away from the sun, reaching speeds of up to 150,000 miles per hour, or more than 40 miles per second.

A common misconception is that solar sails catch the solar wind, a stream of energetic electrons and protons that boil away from the Sun's outer atmosphere. Not so. Solar sails get their momentum from sunlight itself. It is possible, however, to tap the momentum of the solar wind using so-called "plasma sails."

Plasma sails are modelled on Earth's own magnetic field. Powerful on-board electromagnets would surround a spacecraft with a magnetic bubble 15 or 20 kilometers across. High-speed charged particles in the solar wind would push the magnetic bubble, just as they do Earth's magnetic field. Earth doesn't move when it's pushed in this way - our planet is too massive. But a spacecraft would be gradually shoved away from the Sun. (An added bonus: just as Earth's magnetic field shields our planet from solar explosions and radiation storms, so would a magnetic plasma sail protect the occupants of a spacecraft.)

An artist's concept of a space probe inside a magnetic bubble (or "plasma sail"). Charged particles in the solar wind hit the bubble, apply pressure, and propel the spacecraft.

Of course, the original, tried-and-true propellant-free technology is gravity assist. When a spacecraft swings by a planet, it can steal some of the planet's orbital momentum. This hardly makes a difference to a massive planet, but it can impressively boost the velocity of a spacecraft. For example, when Galileo swung by Earth in 1990, the speed of the spacecraft increased by 11,620 mph; meanwhile Earth slowed down in its orbit by an amount less than 5 billionths of an inch per year. Such gravity assists are valuable in supplementing any form of propulsion system.