by Paul Gilster | Nov 18, 2004 | Advanced Rocketry |
Can ion propulsion really lead the way to the outer planets? No one can know for sure, but recent advances in solar-electric propulsion surely make ion methods a prime candidate. Not only has SMART-1 conducted a thorough ion engine shakedown on its lengthy and circuitious route to the Moon, but a variety of new studies are showing the way to more powerful ion thrusters that will eventually lead to the nuclear-electric systems we’ll need for deep space missions.
Today’s standard ion engine is called NSTAR (it’s a short acronym for a long term: the NASA Solar Electric Propulsion Technology Application Readiness thruster). The agency used one of these on its highly successful Deep Space 1 mission. In tests at the Jet Propulsion Laboratory, an NSTAR thruster was operated for a continuous 30,352 hours. That’s almost five years of operation for an engine whose design life was only 8,000 hours. You can read more about that test in this NASA news release (PDF warning).
Image: This xenon ion engine prototype, photographed through a port of the vacuum chamber where it was being tested at NASA’s Jet Propulsion Laboratory, shows the faint blue glow of charged atoms being emitted from the engine. A similar engine powered the Deep Space 1 spacecraft. Credit: Jet Propulsion Laboratory.
But ion propulsion is a rapidly moving target, with numerous technologies under study at NASA and elsewhere. One effort to focus on is NEXT — the NASA Evolutionary Xenon Thruster — now being developed under the agency’s In-Space Propulsion effort. NEXT holds the promise of doubling current ion thruster power capacities while increasing specific impulse by 32 percent. Moreover, NEXT offers deep-throttling capability to manage changes in input power over the course of a mission. The system, wrote NASA’s Randy Baggett (in an abstract for his talk at the recent AAS meeting in Louisville) “…offers Discovery, New Frontiers, Mars Exploration and outer-planet missions a larger deliverable payload mass and a smaller launch vehicle size.”
As these studies coalesce, we’ll witness the advent of reliable and fuel efficient engines that will leave chemical methods in the dust. The high thrust of a chemical engine can get you into orbit, but it’s the slow and steady burn of an ion thruster that maximizes your chances in the outer Solar System. More on NEXT is available at Glenn Research Center’s Power & Propulsion Office and via MSFC. Also, be aware of Richard Hofer’s Weblog Electric Propulsion, which tracks new developments in the field. A precis of Baggett’s Louisville presentation is available here.
by Paul Gilster | Nov 17, 2004 | Culture and Society |
Although it originally ran in the January, 1944 issue of Astounding, I first ran into A.E. Van Vogt’s “Far Centaurus” in a collection of short stories called Destination: Universe (New York: Signet Books, 1952). It would be hard today to re-create the power of the story’s opening, so imbued have we become with reality-stretching concepts, but “Far Centaurus” remains the ultimate illustration of the starship paradox: why send a slow ship when a faster one will surely be built that will one day overtake it?
Van Vogt’s crew arrives in Alpha Centauri space only to find that there is an inhabited planetary system waiting for them, one settled long after their departure from Earth by the much faster ships that were built later. The dialogue is a bit bumpy and the science occasionally awry (van Vogt seems to think there are four, rather than three Centauri stars, for example), but the story has retained its power to this day.
Image: The first paperback edition of Destination: Universe. Although “Far Centaurus” leads off the collection, the book is filled with other worthwhile stories, most of them from Astounding. They include “The Enchanted Village,” “A Can of Paint,” “The Monster” and “Dormant.” The cover of this 1952 edition was by Stanley Meltztoff.
I remember talking to Gregory Matloff, author of the indispensable The Starflight Handbook (New York: John Wiley & Sons, 1989), about “Far Centaurus” when we met at Marshall Space Flight Center. Calling it a ‘terrific story,’ Matloff discussed it in terms of Robert Forward’s thinking:
“Bob had a couple of concepts of technological advancement,” Matloff said. “He had a famous plot of the velocity of human beings versus time. And he said if this is true, and you launch a thousand-year ship today, in a century somebody could fly the same mission in a hundred years. Theyre going to be passed and will probably have to go through customs when they get to Alpha Centauri A-2.”
The fascinating question becomes, just when do you launch? A 1,000-year ‘ark’ mission, a so-called ‘generation ship,’ would have to move at 260 AU per year to make the crossing, and according to Matloff, is forseeable using sail technologies and materials that are being developed now. Would we launch such a mission, or would we demand a different standard, such as a mission that could be completed in a single human lifetime? And the suspicion arises that both types of mission might be tried, the “Far Centaurus” paradox notwithstanding.
I’m reminded of what NASA sail expert Sandy Montgomery said about manned missions at the same meeting at MSFC: “The whole human race won’t go; some subgroup will. I think of the 14-year old boys that signed on as cabin boys in the clipper ships. They had to go from New York to San Francisco, around Cape Horn and then back; it took years and years. They left everything that was their life behind, and as they sailed they became adults. They spent most of their lives on that trip, and gave up everything that this crew would have to. So this is not a new issue. It’s a problem some people have already faced into.”
The broader question remains: how do we as a culture view multi-generational projects, and do we have the will and capability to build for futures we ourselves will never see? The question is hardly idle in a time that seems paralyzed by short-term thinking, and the answer may determine our legacy.
by Paul Gilster | Nov 16, 2004 | Advanced Rocketry
It used to be said that the Sun never set on the British Empire. Those days may be long gone, but there is still a place where the Sun forever shines, and it’s on the Moon. The Peak of Eternal Light is a mountain at the lunar south pole that is always in view of the Sun. Its year-round temperature is a comparatively mild (by lunar standards) -20C, making it possibly useful as a site for a future lunar base. The possibility of water ice in nearby craters, though not proven, could be an attractive bonus.
No wonder the European Space Agency is fascinated with the Peak of Eternal Light. Fascinated enough to make it a prime survey target for SMART-1, the ion-powered spacecraft that entered lunar orbit on Monday. SMART-1’s studies of the Moon’s south pole will surely be fascinating, as will its look at the South Pole-Aitken Basin, a huge impact crater that punches deep into the Moon’s mantle. At stake may be new theories about the Moon’s formation.
But for deep-space enthusiasts, the SMART-1 mission is equally valuable as a shakeout of ion engine technologies. To adjust the spacecraft’s orbit for the long haul, its engine will burn almost continuously for the next four days before a set of shorter burns achieves the final orbit some time in January. SMART-1’s engine uses electricity from solar panels to heat xenon gas, creating ions that are accelerated by electrical forces to emerge as thrust. So far the engine has performed flawlessly. SMART-1 saved fuel at the expense of speed and time, and its 82 liters of xenon (the stuff that used to be found in flash bulbs) proved more than adequate for a spiralling, 84-million kilometer cruise to the Moon.
From an article on SMART-1 by David McAlary for the Voice of America, quoting Marc Rayman, deputy mission manager for NASA’s Deep Space 1:
“Ion propulsion is applicable to any place we want to get to in the solar system that previously has been unaffordable or physically impossible to get to with what we had,” said Marc Rayman. “The set of technologies we have, by allowing us to make the spacecraft less expensive, launch on more affordable launch vehicles, means that now we can be more rapid in responding to scientific questions that we devise and sending spacecraft to more places in the solar system and conducting more detailed studies.”
Deep Space 1’s mission was the first time an ion engine had been used for primary propulsion. What Rayman is talking about are trips to the outer Solar System and perhaps the Kuiper Belt. Early NASA concept studies for missions to nearby interstellar space, one called the Interstellar Precursor, the other the Thousand Astronomical Unit mission, were designed around nuclear-electric ion engines. These would use nuclear power to ionize the xenon fuel because sunlight at those distances would no longer be adequate for the job. The ion option won’t get us to Centauri, but it’s a key player in the expansion to the outer planets that will one day spread to nearby stars.
by Paul Gilster | Nov 15, 2004 | Sail Concepts
The Planetary Society has announced that its Cosmos 1 solar sail is to be launched on March 1, 2005. A letter to members from executive director Louis Friedman, who worked on NASA sail designs for an aborted Halley’s comet mission in the 1970s, called Cosmos 1 ‘the world’s first solar sail spacecraft.’ And indeed it is, if by ‘spacecraft’ we mean ‘free-flying vehicle.’ The first Russian Znamya experiments with sail deployment are over a decade old, and involved a 20-meter spinning sail-mirror.
Although Znamya was intended to demonstrate the practicality of beaming solar energy to polar and subarctic settlements, the design pointed to a larger concept. When I talked to him last year at JPL, NASA sail expert ‘Hoppy’ Price showed me a photograph of the deployed sail-mirror, which had problems. “…there are these wrinkles in the sail, so it didn’t really work quite the way it was supposed to work,” Price said. “And it was a lot heavier than what we’d like to build. But the more we study this, the basic configuration—for a large spin-stabilized sail that would be very low in mass—is a promising one.”
Image: The Znamya sail-mirror, the first space-based deployment of sail technologies. Earlier missions, like Mariner 10, had verified the effect of photon propulsion, but were not specifically designed to do so.
Znamya involved a sail that remained anchored to a Progress supply ship (a second Znamya deployment six years later failed when the sail became entangled with a spacecraft antenna). Cosmos 1 will indeed be an independent spacecraft, one that will test the basics of photon propulsion by maneuvering to a higher orbit. Each of its eight triangular blades can be pitched, varying the angle to change the pressure of reflected sunlight. If all goes well, the team will also try to illuminate the sail with a microwave beam to measure the acceleration on the spacecraft.
Centauri Dreams‘ take: Cosmos 1 is a proof of concept mission that stands out in two areas. First, it should awaken public attention to solar sail designs at a time when agencies like NASA and ESA are actively working on sail dynamics. Second, Cosmos 1 was built and will be launched without government funding, ‘the first space mission of a public interest organization,’ as The Planetary Society’s Cosmos 1 page puts it. On both counts the mission is fascinating; anyone interested in advanced propulsion concepts must wish it well.
by Paul Gilster | Nov 13, 2004 | Exoplanetary Science
Terrestrial Planet Finder will one day help us detect Earth-like worlds around other stars, no matter which technologies are deployed (Centauri Dreams remains an advocate of Webster Cash’s New Worlds Imager). But once we start finding such worlds, what sort of data signatures should we look for to help us identify habitable surface environments?
That question has been addressed in a new way by Penn State Erie assistant professor of physics and astronomy Darren M. Williams. Working with the University of Hawaii’s Eric Gaidos, Williams outlined a theory that planets with abundant water should show strong scattering of starlight from ocean surfaces and discussed ways of examining such data.
From a summary of the presentation:
“Here we simulate the specular reflection of starlight off the surface of Earth-like planets to calculate visible light curves for different viewing geometries, obliquities, and land-sea fractions. The amplitude and polarization of the reflected signal is found to be strongly dependent on the waviness and expanse of the ocean. Planets are naturally brightest in crescent phase when the oceans most-effectively scatter starlight in the direction of Earth.”
Centauri Dreams‘ take: It seems remarkable to be considering life signatures on Earth-like worlds, when at present the smallest planets we can detect (and non-visually at that) are at least the size of Neptune. But Kepler (proposed launch: 2007), the Space Interferometry Mission (launch: 2009) and the Terrestrial Planet Finder to follow could lead us to such worlds within a decade. If we continue to push the technology hard, we may one day see reflections off an alien ocean directly rather than just inferring them from raw data.
“Specular Reflection of Starlight off Distant Planetary Oceans” was presented on Friday at the AAS Division for Planetary Sciences meeting in Louisville. On other forms of biosignatures, see “Biosignatures and Planetary Properties to be Investigated by the TPF Mission,” (JPL Publication 01-008), available online.
