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Daedalus illustration
The 1970s-era Daedalus concept was one of a number of historic interstellar mission designs discussed during sessions of the recent 100-Year Starship Study Symposium. (credit: Adrian Mann/BIS)

The journey of 100 years begins with a single weekend

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This is a time of transition and upheaval for human spaceflight. Earlier this year the Space Shuttle program came to an end, a little over 30 years after its first flight. After an extended period of uncertainty prompted by the Obama Administration’s decision to cancel the Constellation program, NASA now has in place plans to build a new heavy-lift rocket, the Space Launch System, as well as the Orion spacecraft, but with no guarantee that the budgets will be there over the long term to support their development. Meanwhile, commercial ventures are making progress on both suborbital and orbital vehicles, the latter to support the International Space Station, but at slower paces than previously anticipated.

In other words, it’s the perfect time to start thinking about building starships.

“If we cannot do that,” said physicist James Benford, of whether interstellar propulsion was possible, “the other questions are moot.”

That might not seem like the obvious takeaway from the current state of affairs, but the prospect of at least laying the intellectual foundation needed for sending spacecraft beyond the solar system attracted 700 people to Orlando from September 30 to October 2. The 100 Year Starship Study Symposium, jointly organized by DARPA and NASA, was the most public part of the year-long effort by the agencies to study how to maintain a century-long focus to develop interstellar spacecraft, and spin off technologies along the way (see “It’s not (just) about the starship”, The Space Review, June 20, 2011). And that meant answering some critical questions about traveling to other stars—or, at least, figuring out the right questions to ask.

How to go?

One of the biggest challenges of interstellar spaceflight is transiting the enormous distances between our solar system and even the nearest stars. One speaker, in a room that could hold no more than a couple hundred people, put the problem in perspective this way: if the Earth-Moon system was scaled down to fit into that room, the Alpha Centauri system would be at the approximate distance of the actual Moon. Given the difficulty we have today traveling in the solar system—or even just getting into Earth orbit—what hope is there to going to going another star? “If we cannot do that,” said physicist James Benford, who chaired the propulsion track of the symposium, “the other questions are moot.”

Benford and other participants, though, were cautiously optimistic about the ability to develop propulsion systems for interstellar flight. “There are several credible approaches that have been proposed and researched,” he said. “This surprises people frequently, because this is scattered in a community that has been looking at it for decades.”

The propulsion systems considered fell into two broad categories. The first features a progression of nuclear systems, starting with nuclear thermal rockets and evolving into fission and fusion rockets and, eventually, antimatter. These have long been discussed as options for interstellar flight, but beyond work done decades ago on nuclear thermal rockets, there’s been little progress on making these systems possible. “So far, fusion has conquered us,” Benford said.

Nuclear thermal rockets are themselves insufficient for interstellar flight, but can help open up the solar system and its resources to enable interstellar missions. “If we are going to go to the stars, we are going to need to use the resources of the solar system,” said Geoffrey Landis, a scientist at NASA’s Glenn Research Center who is also a science fiction author. “The nuclear thermal rocket will probably not be the system that takes us all the way to Alpha Centauri, but it is going to be the pickup truck that can drive us around the solar system.”

The second category of propulsion starts with solar sails and advances to beamed propulsion, where extremely powerful lasers based in the solar system beam power to a spacecraft’s sail, pushing it forward. That approach, Benford said, has the advantage of leaving effectively all the propulsion system—including its mass and complexity—behind, making the interstellar ship simpler and lighter. Such systems still face unresolved questions of physics, engineering, and economics, he noted.

“Within 15 light-years we’d expect at least 20 planetary systems,” said Crawford, although only two have been found so far.

Benford and others expressed optimism that either or both of these approaches should eventually be feasible. Richard Obousy, president of Project Icarus, a group working on the design for an interstellar spacecraft, argued that the technology has been available for some time. The decades-old Orion concept would have used hundreds of thousands of “pulse units”—aka atomic bombs—to propel a 400,000-ton spacecraft to a few percent of the speed of light, allowing it to reach Alpha Centauri in 130 years “with 1950s technology, which I think is just incredible,” he said.

Beyond these relatively conventional approaches—if one can call beamed power and fusion “conventional”—there is so-called “exotic science” that seeks ways to drastically shorten travel times through approaches ranging from warp drives to wormholes, also discussed at the conference. “No real conclusions were reached” about the viability of various approaches, Benford said. “It is a contentious area. It is very difficult to do experiments… It really is a step beyond, or a bridge too far, at this point.”

Where to go?

If propulsion technologies make starships possible, what stars should those ships voyage to? The last 15 years have featured a bounty of extrasolar planet discoveries, from both groundbased telescopes as well as spacecraft like NASA’s Kepler mission, demonstrating that solar systems are not uncommon. However, those discoveries don’t make the selection of destinations any easier, in part because of the relative lack of planets found to date close to home.

“Assuming Kepler gets its extended mission, I think we will know what ‘eta Earth’ is: what the frequency of Earthlike planets really is,” said Jill Tarter of the SETI Institute, chairing a session about destinations at the symposium. “But those Kepler worlds are very far away.” What the Kepler data will do is help set the scale for follow-on missions to look for Earthlike exoplanets closer to home.

Within the immediate vicinity of the Sun—15 light-years—only two stars are known to have planets: Epsilon Eridani and GJ 674, said Ian Crawford, a planetary scientist at Birkbeck College, London. “I think we can be certain that there are many more planetary systems within 15 light-years than just these two, it’s just that we’ve not discovered them yet,” he said, a conclusion based on statistics of exoplanet discoveries elsewhere. “Within 15 light-years we’d expect at least 20 planetary systems.”

“The biggest thing the interstellar flight community has to do to advance interstellar flight is to get rid of the humans,” said Friedman, who argued for robotic probes, at least for initial missions.

That uncertainty makes it difficult to identify a clear target for an interstellar mission now—although, of course, there’s no rush. If there is a frontrunner, it’s likely Alpha Centauri, because it’s the closest star system to us, at 4.4 light-years, and also features three stars, making it more diverse and scientifically interesting. So far, though, no planets have been found around any of the stars there. “My view is that Alpha Centauri will only lose its place atop of the shortlist for exploring nearby stars if, within the next 100 years, we find there are no planets around any of these three stars” but there are planets found around other nearby stars, Crawford said.

Pete Worden, director of NASA's Ames Research Center, spoke to symposium attendees during its opening session September 30. (credit: J. Foust)

Who should go?

With a destination star system picked out and the technology available to make a journey there possible, the next question that comes up is, who gets to go? Sessions of the conference covered various human factors and environmental issues, but a more fundamental issue came up during the symposium: are starships meant for humans at all, at least for the foreseeable future?

Some argued that robotic interstellar probes make more sense for at least initial missions, since you’re relieved of the overhead of providing a habitat that may have to sustain a crew for generations in order to reach even the nearest stars. “The biggest thing the interstellar flight community has to do to advance interstellar flight is to get rid of the humans,” concluded Lou Friedman, the former executive director of The Planetary Society. Technologies such as beamed power “lightsails” and miniaturization can make interstellar spacecraft affordable, he said, but that means leaving humans behind, at least for now. “As long as continue to think of this as a human activity, sending heavy people in big spacecraft, it’s going to be a subject of science fiction.”

“The first starship doesn’t have to be manned,” said science fiction author Gregory Benford. Small beamed-power starships, like the ones Friedman advocated, could be the first spacecraft to go to another star, and in the relatively near future. “We could be launching those easily in less than a century because all the technologies needed are readily available now.”

Others, though, said human interstellar flight should remain under consideration. “The humans may actually be the point of the 100-year starship,” said Chris McKay of NASA’s Ames Research Center, the chair of the habitats and environmental science track of the conference. “The point may be to take humans to another place and start another branch of humanity, in which case humans are the payload and you can’t design a mission without them.” He added that besides being the point of a starship, humans “may be useful in maintenance and who knows what else” during the journey. “I, for one, opt for the human case.”

If humans do travel on interstellar spacecraft, it won’t be anything like the starship Enterprise from Star Trek or other science fiction, where humans flit from one star system to another and back again, of course. “One of the things that’s bugged me about this whole conference is the terminology of ‘starship,’” said science fiction author Charles Stross. The “ship” part of the term “comes with an awful lot of cultural baggage attached,” he said, including the need for a crew, a destination, and the fact that a ship generally returns from that destination.

“For me, it’s the story of life elsewhere in the cosmos that’s likely to drive our exploration beyond the solar system,” Tarter said.

“What we were actually talking about here seems to break down into two types of vehicles, neither of which is a ship,” he said. One kind of vessel is a robotic probe, while the other is a crewed vessel, but one that’s likely a generational ship that takes many decades or centuries to reach its destination, with no plans to return. “You have to be very careful about how the language you use biases your ideas about what we’re talking about.”

Why go?

Whether or not an interstellar mission is robotic or human may be closely tied to the reason for sending the mission at all. There are plenty of scientific justifications for a mission, including study of another star and its planets and the search for life there, as well as studies of the interstellar medium en route to that star. But is that alone enough to drive the considerable investment needed for such an expedition?

Tarter believes that life, both the search for it elsewhere and the preservation of our own, will motivate such missions. “For me, it’s the story of life elsewhere in the cosmos that’s likely to drive our exploration beyond the solar system,” she said. “Once we have established a survivability for humanity on multiple bodies in our solar system, I think the question that will be worth the effort and the pain and the investment in traveling to a distant star system is… what’s the biodiversity? How many different ways did chemistry turn into biology?”

Later, she summarized the destination for exploration as simply “off Earth.” “We’re going off Earth because Earth is a single-point failure for humanity, and a species that can contemplate extinction is probably a species that should do something about it.”

“The motivations will be many,” said former astronaut Mae Jemison, who chaired a track devoted to economic, legal, social, and educational issues. Some people in her track thought a starship was important for the preservation of the species from natural or human-made catastrophes, while others said “I just want to go because it’s really cool,” she explained.

What’s next?

The symposium was the public culmination of a year-long effort by DARPA and NASA on the concept of the “100-Year Starship”: what is needed from technical, economic, organizational, and other vantage points to create a long-term effort that would, in a century, permit the launch of the first starship. The conference, in addition to covering some of the technical and motivational issues discussed above, also tackled topics ranging from the financing of such an effort to the ethics of exploring other worlds that could host alien life.

The symposium itself was an odd mix of openness and stealth. The conference was open to the general public, with no registration fee. According to a DARPA spokesperson last week, about 1,100 people registered for the event, of which 700 actually attended. The programming was also eclectic, which two panels of science fiction authors opining on interstellar flight and a panel on “communicating the vision” at the end of the conference featuring space tourist Anousheh Ansari, Challenger Center founder June Scobee Rodgers, and director and special effects expert Douglas Trumbull. “This is one of the best allocations of federal money I’ve ever seen,” one audience member said during the summary panels at the end of the conference.

On the other hand, elements of the conference weren’t nearly as open. There was little information about the scheduled talks other than the title and the name and affiliation of the presenter: no abstracts or other descriptions of the talks in the program. While speakers submitted papers for their talks, only a subset of those papers will be published, in a future edition of the Journal of the British Interplanetary Society. A DARPA spokesperson confirmed last week that there are no plans by the agency to publish the other papers on the project web site or elsewhere. Similarly, while most of the tracks were recorded, DARPA has no plans to release the videos from those sessions.

DARPA and NASA also kept a curiously low profile during the event, even though they’re sponsoring the project. While officials from both agencies—David Neyland, director of DARPA’s Tactical Technology Office, and Pete Worden, director of NASA’s Ames Research Center—spoke at the event, there was no use of the agency’s logos and no mention of either agency in the official program, beyond the titles of various speakers or in the bios. The “background” section of the program, describing the genesis of the project and work leading up to the symposium, never mentions either organization, leading to heavy use of the passive voice (a workshop “was held”, a solicitation document “was released”, and so on.)

In his opening keynote, Neyland made it clear that DARPA’s interest in this project was not so much in developing a starship, but harnessing technologies needed for the project that could have spinoff applications, particularly for the military. “When we envisioned this, and created this study and symposium, it was with self-interest for the Department of Defense and for NASA,” he said.

At the end of the conference, Neyland declared success, at least for that near-term mission. “It is very, very clear from my perspective that in the long term there are definitely offshoots that will come out of the research opportunities presented here and talked about that are relevant to the Department of Defense,” he said, without elaborating. “We’ve achieved my objective 100 percent.”

The project, though, is not quite over. In August DARPA issued a request for proposals to establish an organization that would carry on the work of the 100-Year Starship Study, with the winning organization set to receive $500,000 of seed funding to bootstrap their initial efforts. Proposals are due next month (November 11, or 11/11/11). The recent symposium made it clear that the winning organization has in front of it a task that could easily take a century or more to complete, if it’s even feasible.