By John Croft
At the intersection of science fiction and engineering is an interstellar propulsion technology known as the beam-powered light sail. The brainchild of physicist and science fiction writer Robert L. Forward, who died at 70 in 2002, the sail might just be the best bet going for the first visit to the nearest stars.
Ten years after his death, Forward's idea is a top contender of potential designs being considered by the Dorothy Jemison Foundation for Excellence, a non-profit organisation that in May received $500,000 of seed money from the Defence Advanced Research Projects Agency (Darpa) to form "100 Year Starship", a project to "ensure that the capabilities for human interstellar flight exist as soon as possible, and definitely within the next 100 years".
The Jemison Foundation, run by former NASA astronaut Mae Jemison, beat the competition for the Darpa grant with a 30-page proposal for the 100 Year Starship (100YSS). Partners of the Foundation include Icarus Interstellar and the Foundation for Enterprise Development. Icarus Interstellar's "Project Forward" lists beamed energy and sails as candidates for interstellar propulsion.
Along with holding an annual symposium, the first of which is scheduled for September in Houston, the Jemison Foundation will launch a research arm called "The Way", for "non-traditional, speculative types of research", says a spokeswoman.
While liquid-fuelled rockets have been the staple of solar system travel since Robert Goddard first experimented with the propulsion technology in the 1920s, interstellar travel will require a more radical approach.
Physics supplies the reasons why. Apollo Moon missions launched at 3,110T and brought back only 5T to Earth at the end, a mass ratio of 622:1, driven largely by the need to carry their own fuel for the trip. "You can't do much better than that [with chemical propulsion]," Forward had said. A trip to the Moon represents a distance of about 0.0025AU, where 1 "AU", or astronomical unit, is the distance from the Earth to the Sun. The nearest stars, those in Alpha Centauri, are 272,000AU from Earth.
Four Voyager and Pioneer probes are the only NASA-made spacecraft travelling in interstellar space; all were boosted by chemical rockets that gave them travelling speeds between 2.2 and 3.5AU per year, according to Icarus Interstellar. Voyager 1, after travelling at 2.2AU per year for almost 35 years, is today only 116AU from Earth.
The company notes that a chemical propulsion rocket that could boost a spacecraft to 1/10th the speed of light, or 3 million meters per second, for a fly-by mission to our nearest stars in the Alpha Centauri system would need "more fuel than there exists mass in the known universe" in terms of the needed delta-V. Flying at 1/10 the speed of light allows a round trip in one lifetime, or more than 80 years.
Interstellar advocates are looking into several alternatives, including sails. Geoffrey Landis, a propulsion scientist at NASA's Glenn Research Center in Ohio, says beamed power systems, which would propel a sail-equipped spacecraft using energy from an Earthbound laser or microwave beam focused on the sail, may be practical for sending an interstellar probe on a mission, but a human mission may require "either fusion or something even more advanced beyond that".
In 1984, Forward had proposed such a system requiring no on-board propulsion as a "fly-by" mission to Alpha Centauri. The 1t spacecraft was made up of a 0.33t sail, a 0.33t structure and 0.33t payload that would consist of sensors, communications systems and other devices. The aluminium sail was to have a diameter of 3.6km, which, when propelled by a 65Gigawatt laser beam directed through a 1,000km diameter spider web-like Fresnel lens (about the size of Texas) in space, would allow for a maximum speed of 0.12c (where "c" is the speed of light) and a 50-year one-way transit time to the stars. "A laser beam going through this Fresnel lens will not diverge for 44 light years," Forward had said. "That is why you've got to think big."
For a rendezvous mission to Alpha Centauri (where the spacecraft must stop in the vicinity), Forward had proposed designing a magnetic sail (mag sail) built around the main light sail to decelerate the vehicle as it interacts with the solar plasma being pushed out in front of the destination star. To return a capsule to Earth, he suggested a detachable module at the hub of the spacecraft, which would have its own independent light sail and mag sail to capture light bouncing off the main vehicle's sail from the Earth-bound laser once the two separated.
In the late 1990s, Landis refined Forward's design for a fly-by mission, finding that by using dielectric material for the sail instead of aluminium, he could cut the power requirement by a factor of 145, or from 65GW to 448MW. He notes that 448MW is "about half the power output of a standard nuclear power plant", compared with 65GW, which would have to be generated by "nearly a hundred dedicated electrical powerplants".
While there are no missions planned to test a beam-powered sail, NASA and others are taking some baby steps in the learning process.
The Japanese space agency in May 2010 launched its Ikaros (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) satellite, along with the Venus Climate Orbiter. As well as getting its propulsion from the sun via a 200m2 sail, the first satellite to do so, the Ikaros also incorporated an attitude-control system for the square solar sail and solar cells in the sail for onboard power. The craft reached Venus six months after launch, coming within 80,000km of the planet during the fly-by. Japan had discussed a follow-on mission with a larger solar sail to travel to Jupiter and beyond later this decade.
In November 2010, NASA tested NanoSat-D2 with a 10m2 solar sail in a circular low-Earth orbit in a mission that lasted 240 days.
Landis says solar sails that could withstand high temperatures at close range to the Sun could generate accelerations that could propel a spacecraft to Pluto from Earth in just one month, or out as far as 1,000AU in two years, making the technology suitable for interplanetary probes. The Sun's rays, however, do not have the reach to send a sail into interstellar space at the desired speeds.
In terms of a technology plan, Landis says sail technology is "moving forward slowly", with several follow-on missions to the Ikaros and NanaSail-D2 missions being discussed. While NASA is funding some technology-development work for the sails, Landis says there are no missions planned. The next move in terms of a demonstration, he says, might be to "dive in close to the Sun" as part of a high-velocity, multi-year mission out toward the heliopause, a few hundred AU from the Earth. "That gets you out into interstellar space," he says, adding that future mission might come as the result of the private and public funding obtained by organisations such as 100YSS.
"The next thing would be to demonstrate that you can push one of these sails with a laser or microwave beam to accelerate it higher than you can get with sunlight alone," says Landis. "Some of that you can do in small scale, but eventually you'd want to push the sail with a beam as part of a mission."
As for sending humans to the stars, Landis thinks the 100-year window suggested by the Darpa project could be realisable, but says the mission would likely be a "colonisation" mission on the first try, thanks to the time involved in the journey.
"The laws of physics don't forbid it," he says of interstellar travel. "We don't have the technology yet, but there's no reason why we can't develop it."