Representative John Culberson (R–TX), a self-professed space fan who chairs the House of Representatives appropriations subpanel that oversees NASA, included the call for the ambitious voyage in a committee report released today. The report accompanies a bill setting NASA’s budget for the 2017 fiscal year, which begins 1 October; the full House appropriations panel is set to consider the bill on Tuesday.
In the report, Culberson’s panel “encourages NASA to study and develop propulsion concepts that could enable an interstellar scientific probe with the capability of achieving a cruise velocity of 0.1c [10% of the speed of light].” The report language doesn’t mandate any additional funding, but calls on NASA to draw up a technology assessment report and conceptual road map within 1 year.
Many scientists consider the idea of interstellar travel to be still firmly in the domain of science fiction. That’s principally because of the vast distances involved. Alpha Centauri is 4.4 light-years away, or nearly 40 trillion kilometers. The fastest spacecraft so far launched into space, the NASA-Germany Helios probes, traveled at 250,000 kilometers per hour. At that speed, it would take the probes 18,000 years to reach the nearest star to the sun. To get there in anywhere close to a human lifetime, spacecraft will need to travel a substantial fraction of light-speed—10% would get a craft to Alpha Centauri in 44 years.
Achieving such phenomenal speeds would require equally eye-watering amounts of energy, even for a relatively lightweight probe. It is this challenge that led the Starshot project, bankrolled by the Russian internet billionaire Yuri Milner, to propose developing a spacecraft-on-a-chip weighing less than 1 gram. And to avoid having to accelerate hefty engines and fuel, the craft will have large, featherweight light-sails and will be boosted to more than 20% of light-speed by an enormous array of high-powered lasers on Earth.
The House report acknowledges that conventional forms of propulsion, including chemical, solar-electric, and nuclear-thermal technologies, will never be able to achieve such interstellar speeds. It suggests a number of avenues that NASA might want to pursue, including propulsion based on nuclear fusion. This technology has yet to be mastered on Earth for conventional electricity supply, so developing a propulsion system for use in space is still a long way off. One variant mentioned in the report is antimatter-catalyzed fusion, which drives a spacecraft with a series of thermonuclear explosions. On Earth, such H-bombs are triggered by a conventional atomic bomb fueled with uranium or plutonium; in the antimatter triggered version, a small ball of antimatter meets normal matter and their annihilation provides the explosion, which sparks the much larger thermonuclear blast. How to protect a spacecraft and its crew from such forces, however, remains an unanswered question.
Another futuristic technology the House report mentions is the Bussard ramjet, which uses electromagnetic fields to scoop up hydrogen from the interstellar medium as it travels along, compressing it sufficiently for nuclear fusion to occur, providing thrust to drive the craft forward. Bussard ramjets are popular with science fiction authors because they effectively never run out of fuel, harvesting it as they move.
Starshot’s NASA roots
Although such approaches may seem impossibly sci-fi for many space scientists, the report also refers to “beam energy approaches.” The report mentions that the NASA Innovative Advanced Concepts (NIAC) program is already funding a study of “directed energy propulsion for wafer-sized spacecraft that in principle could achieve velocities exceeding 0.1c.” It’s no accident that this sounds familiar. The study in question, Directed Energy Propulsion for Interstellar Exploration (DEEP IN), was also the inspiration for Yuri Milner’s Starshot.
DEEP IN is the brainchild of Philip Lubin of the University of California, Santa Barbara. He says he had been developing various space applications for high-powered lasers and made several proposals to NIAC in 2014. The program jumped on the interstellar propulsion idea. “They just loved it,” Lubin says. NIAC began funding DEEP IN with $100,000 in early 2015 and recently awarded Lubin another $500,000 for further studies.
In the meantime, Lubin bumped into Pete Worden, former director of the NASA Ames Research Center in Moffett Federal Airfield in California, who had left to work with Milner on his Breakthrough Initiatives. Worden was enthused with Lubin’s idea of tiny, laser-driven interstellar spacecraft, and in January this year Lubin met with Milner, Worden, other Starshot staff, and Avi Loeb, a Harvard University astrophysicist who was advising Milner.
Last year, Loeb had studied a large number of proposals for interstellar travel on behalf of Milner. With the help of his Harvard students they had scoured the literature and studied other ideas that were brought to them. “None looked feasible, all except one,” says Loeb, and that was tiny spacecraft propelled by beams. According to Lubin, a Starshot meeting in March involving more than 20 people scrutinized the proposal in great detail looking for any flaws. Lubin’s original idea had been for the lasers to be stationed in space, but Milner insisted they had to be ground-based because otherwise the project would take too long and would be too expensive. Lubin says the meeting concluded that approach was hard, but not impossible and soon afterward the Starshot announcement was set for 12 April.
Loeb, for one, is excited to be starting on the project and says he already has two papers in the works. “This is more challenging than putting a man on the moon,” he says, because it involves a greater extrapolation from current technology. “It should be an exciting few years,” he says. “Our reputations now depend on this project.”
NIAC would be the natural home for any interstellar projects at NASA. NIAC Program Executive Jason Derleth in Washington, D.C., says it funds research that is “as close to the edge of science fiction without going over.” He welcomed the new interest from Congress. “To have anyone interested in what we’re doing is a good thing for the agency.”
But will it be NASA or Yuri Milner leading this Starshot? Loeb says he can see the two working together on it. “We need to keep our eye on the ball rather than who is holding it,” he says.