The first-ever interstellar probe may cruise through space like a boat through the ocean, propelled by super-focused light beamed onto a sail the size of Texas.
Solar sailing is perhaps humanity's best bet for reaching star systems beyond our own in the foreseeable future, some scientists say, though they caution that the first robotic interstellar flight is not exactly around the corner.
Solar sailing is perhaps humanity's best bet for reaching star systems beyond our own in the foreseeable future, some scientists say, though they caution that the first robotic interstellar flight is not exactly around the corner.
"I think it's 300 to 500 years [away], personally," said Les Johnson, deputy manager of the Advanced Concepts Office at NASA's Marshall Space Flight Center in Huntsville, Ala. "I think before we ever really undertake sending something to another star, we will probably have to be masters of our own solar system."
The vastness of space
Humanity will need to employ new propulsion technology if it hopes to launch spacecraft to other stars, because the distances involved are just too huge for traditional chemical rockets to handle.
For example, the closest star system to our own is the three-star Alpha Centuari, which lies 4.3 light-years away, or more than 25 trillion miles (40 trillion kilometers). It would take a traditionally powered spacecraft about 40,000 years to reach Alpha Centuari if it blasted off today, scientists say.
Engineers are currently researching a number of different alternative-propulsion technologies, including space-bending "warp drives" and engines that harness the power of matter-antimatter reactions. But Johnson thinks the most attractive options at the moment are nuclear fusion drives and gigantic solar sails.
A fusion-powered probe is a long way off, since researchers are still trying to figure out how to build fusion reactors here on Earth that produce more energy than they take in.
"Not only do we have to solve that problem, but we've got to get a lot more out than we put in, and we've got to miniaturize it all dramatically so you can even consider launching it on a spacecraft," Johnson told SPACE.com.
Solar sailing has its own mix of promise and problems, which researchers are still trying to work through.
A light breeze
Solar sails take advantage of the curious fact that light particles, called photons, have momentum even though they possess no rest mass.
When photons hit the sail's reflective surface, they impart their momentum to the sail and the spacecraft, providing a slight push. The effect builds up over time, potentially accelerating a sail-equipped probe to tremendous speeds without the need for propellant.
The technology has already been tested in space, with Japan's Ikaros probe deploying a 46-foot-wide (14 meters) sail in June 2010 and NASA launching an even smaller craft called NanoSail-D five months later.
These demonstrators may be first steps toward an interstellar mission, but they're halting and tiny ones. A solar sail would have to be far bigger to capture enough photons to reach another star system within a reasonable time frame — a few centuries, say.
"The physics tells us it's going to be the size of Texas," Johnson said.
The sail material would also need to be much thinner than a human hair, posing serious manufacturing, handling and deployment challenges, he added.
A spacecraft on an interstellar journey would ideally deploy its enormous sail relatively close to the sun — perhaps near the orbit of Mercury — to get the biggest photon push possible at the outset, Johnson said.
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| Intrstellar Probe Spacecraft- Voyager |
That push will drop off as the probe gets farther and farther from the sun, of course. So humanity will have to pick up the slack if the vehicle is to make good time, shining a space-based laser on the sail as it recedes into the dark depths. But that's easier said than done, to put it mildly. "You'd have to point [the laser] more accurately than we can point anything today to keep it focused on the sail," Johnson said. "And you'd have to put a lot of energy into it, so that the beam doesn't spread out and lose all that energy. The estimates I've seen are that you'd have lasers that have a power output basically comparable to the whole of humanity today."
The energy beam could be pretty much any type of electromagnetic radiation, including microwaves, said Richard Obousy, president of Icarus Interstellar, a nonprofit group devoted to pursuing interstellar spaceflight.
"You can actually reach reasonable fractions of the speed of light, a few percent of the speed of light," Obousy told.
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