Rocket Ride Spaceships

Okay, now that you've zipping through interplanetary space, how do you slow down at your destination enough to go into a parking orbit and prepare for landing? With chemical propulsion, the usual technique is to fire retrorockets - once again, requiring large masses of onboard fuel.

A far more economical option is promised by aerocapture - braking the spacecraft by friction with the destination planet's own atmosphere. The trick, of course, is not to let a high-speed interplanetary spacecraft burn up. But NASA scientists feel that, with an appropriately designed heat shield, it would be possible for many missions to be captured into orbit around a destination planet with just one pass through its upper atmosphere.

"No single propulsion technology will do everything for everybody," Johnson cautioned. Indeed, solar sails and plasma sails would likely be useful primarily for propelling cargo rather than humans from Earth to Mars, because "it takes too long for those technologies to get up to escape velocity," Drake added.

Image credit: John Frassanito & Associates, Inc. A hybrid-technology Mars mission begins with chemical propulsion to low-Earth orbit.

Nonetheless, a hybrid of several technologies could prove to be very economical indeed in getting a manned mission to Mars. In fact, a combination of chemical propulsion, ion propulsion, and aerocapture could reduce the launch mass of a 6-person Mars mission to below 450 metric tons (requiring only six launches) - less than half that attainable with chemical propulsion alone.

Such a hybrid mission might go like this: Chemical rockets, as usual, would get the spacecraft off the ground. Once in low-Earth orbit, ion drive modules would ignite, or ground controllers might deploy a solar or plasma sail. For 6 to 12 months, the spaceship - temporarily unmanned to avoid exposing the crew to large doses of radiation in Earth's Van Allen radiation belts - would spiral away, gradually accelerating up to a final high Earth-departure orbit. The crew would then be ferried out to the Mars vehicle in a high-speed taxi; a small chemical stage would then kick the vehicle up to escape velocity, and it would head onward to Mars.

As Earth and Mars revolve in their respective orbits, the relative geometry between the two planets is constantly changing. Although launch opportunities to Mars occur every 26 months, the optimal alignments for the cheapest, fastest possible trips happen every 15 years - the next one coming in 2018.

Perhaps by then we'll have a different answer to the question, "Houston, are we there yet?"