Project Daedalus, discussed frequently in these pages, was the first in-depth design study of an interstellar probe. Its projected fifty-year flyby mission to Barnard’s Star at 12 percent of the speed of light was beyond contemporary technology (and certainly engineering!), but not so far beyond as to render the design purely an intellectual exercise. I bring up Daedalus again because I keep getting asked about Project Longshot, which some have mistakenly seen as a successor to Daedalus with a NASA pedigree. And wasn’t Longshot a far more advanced design?
Actually, no. But the other day I again ran into Longshot in the form of an online post describing it as a hundred-year mission to Alpha Centauri (true enough), evidence that NASA had the technology right now (not true) to get us to the nearest stellar system in a century, which would be faster by far than the thousand years I’ve always used as an absolute minimum for getting there with the technology we have today. Even that 1000 years is deeply problematic. I mentioned it several years back to Les Johnson at NASA’s Marshall Space Flight Center and he said, “If we can get to Alpha Centauri in a thousand years, I’ll take it!” Meaning we were, in his view, not even that far along.
So what is this Project Longshot? The first thing to do is to untangle its origins. This design for an unmanned interstellar probe grew out of the NASA/USRA University Advanced Design Program, which ran between 1984 and 1994. The idea of the program was to help integrate future NASA design projects into university curricula. Engineers from the agency would work with students and faculty from US engineering schools, thus fostering engineering design education and adding synergies to NASA’s own efforts in the area of advanced space design. Project Longshot was a concept that grew out of this program’s involvement with the U.S. Naval Academy, including seven first class midshipmen, faculty advisors and two visiting professors, one of whom was NASA representative Stephen Paddack, a visiting professor based at Goddard Space Flight Center.
As for using current technologies, the Longshot team made no such claim. This is what they had to say:
Our probe will be a completely autonomous design based upon a combination of current technology and technological advances which can be reasonably expected to be developed over the next 20 to 30 years. The expected launch date is in the beginning of the next century with a transit time of 100 years.
The expected launch date, in other words, would have been about now, but the technologies anticipated for it to occur still have a long way to go. Longshot was conceived as being built with modular components on the ground and then launched to low-Earth orbit for assembly at the space station presumed to be operational there. The enabling technologies included a “pulsed fusion micro-explosion drive” (I’m quoting from the Project Longshot report) with a specific impulse of 1 million seconds, along with a long-life fission reactor with 300 kilowatts power output.
The differences between this concept and Project Daedalus are profound in both emphasis and execution. Daedalus was to be a fast flyby of Barnard’s Star, scattering smaller probes as it entered the system to explore any planets found there. Longshot, audaciously enough, was intended to carry enough fuel to actually brake as it entered the Centauri system and go into orbit around Centauri B, which the report erroneously calls Beta Centauri (Beta Centauri is another star altogether, the components of Alpha Centauri being Centauri A, B and Proxima Centauri). That last just reminds us that the pooled light of the three Alpha Centauri stars makes it appear to be a single star, so that the second brightest object in Centaurus came to be known as Beta Centauri.
Needless to say, including enough fusion fuel to slow an object traveling at these speeds to brake into orbit around Centauri B would require an engine far more efficient and powerful than anything envisioned for Daedalus. That’s because you’re carrying, when you begin the journey, not just the fuel to get you up to cruising speed, but all the fuel needed for the deceleration. The numbers quickly start running away with you here — while Daedalus offered a first-step flyby that strained every technological resource we would possess in the near future (including the need to mine for helium-3 in places like the atmosphere of Jupiter), Longshot pushed credibility to the max by insisting that a similar design could stay in the Centauri system and do useful science, reporting the results via a laser beam communications system that seems workable.
Where Longshot was perhaps closest to technological realization was in the area of autonomy. Here’s what the report says on this subject:
Due to the great distance at which the probe will operate, positive control from earth will be impossible due to the great time delays involved. This fact necessitates that the probe be able to think for itself. In order to accomplish this, advances will be required in two related but separate fields, artificial intelligence and computer hardware. AI research is advancing at a tremendous rate. Progress during the last decade has been phenomenal and there is no reason to expect it to slow any time soon. Therefore, it should possible to design a system with the required intelligence by the time that this mission is expected to be 1aunched.
All of which seems reasonable enough. The Longshot report was compiled between 1987 and 1988, and we have certainly seen our share of computer advances in the time since. Indeed, I am now and again told by its partisans that the ‘Singularity’ event could happen any time now, but certainly within the next few decades, in which case AI systems to run such a probe would be plentiful, one assumes, although whether intelligent hardware would not want to re-design the whole spacecraft remains an unanswered question.
I, for one, appreciate the report’s attention to long-term thinking. In discussing the “human side of the infrastructure” supporting Longshot, the authors note that given the time for design, procurement, in-orbit assembly and transit, the likely time before return of data would be more on the order of two centuries than one. And they go on to say this:
…the greatest challenge comes with the caretaker portion of the mission — the century of travel time when very little data will be transmitted. The problem here is not the number of people required, since it will be small, but rather the time involved. There has never been a similar project in modern history carried out over such a long time scale. However, there have been organizations which have lasted for such a time. In fact, some have lasted longer! Some examples include: the militaries of nations such as the U.S. and the U.K., various research institutions like the National Geographic Society and the Smithsonian Institution, and private organizations such as the Red Cross and the Explorer’s Club.
Robert Forward used to worry about precisely this point. In considering one of his mind-boggling lightsail designs, he wondered what political will might be needed to keep the power supplied to the huge lasers that drove the lightsail over spans of a century or more. You can see the subject entertainingly explored in his novel Rocheworld (1990), expanded from his 1984 work The Flight of the Dragonfly. We’ve clearly got the patience to work with probes that are thirty years old and more, as witness our Voyagers and Pioneers, but a century or longer imposes more challenges, especially given the political changes that might take place in the interim.
The Longshot team pondered the possibility of laser lightsails for its work as well, but ended up with pulsed fusion. And again, the report points out that such a drive “…is not a current, but rather an enabling technology.” The concept is to fire high energy particle beams at small, fusion-able pellets whose implosion and subsequent channeling out the nozzle would drive the vehicle. Helium-3 is deemed necessary, as with Daedalus, with atmospheric mining of Jupiter being just one of the methods discussed for gathering sufficient quantities. “…[T]he collection of fuel will be the most difficult and time consuming portion of the building,” says the report, and that’s something of an understatement.
Project Longshot, then, should be seen as a gutsy academic exercise that never proceeded to the intricate analysis given to Daedalus, lacking the resources of time and expertise that the British Interplanetary Society was able to deliver to the latter. Even so, the Longshot report is a fun read that places many of our current interstellar concepts in context. The rough sketch of an interstellar probe called “Project Longshot: An Unmanned Probe to Alpha Centauri” can be downloaded here.