by Paul Gilster | May 11, 2005 | Advanced Rocketry, Breakthrough Propulsion |
A team of NASA and university-based investigators is studying the physics of magnetic nozzles, devices that could be used in plasma-based propulsion systems that would sharply reduce the length of journeys within the Solar System. The project began in April and is led by the University of Texas, with support from Marshall Space Flight Center in Huntsville (AL), along with the University of Alabama at Huntsville and NASA’s Johnson Space Center in Houston.
“The technology we’re pursuing could play an important role in NASA’s exploration of the Moon, Mars and the rest of the Solar System,” said Dr. Greg Chavers, a plasma physicist at Marshall and co-investigator for the new project. “Magnetic nozzles enable a new type of plasma-based propulsion system that could significantly reduce travel times to different planetary destinations, providing a new means of exploring space.”
Plasma forms when a hot gas is ionized, causing the atoms to lose their electrons and take on a positive charge. That charge is the key — it allows a magnetic nozzle to accelerate the plasma to far higher velocities than those available through chemical propulsion systems. In rocket terms, the nozzle allows much higher specific impulse (ISP), which tells us a great deal about the engine’s efficiency.
Image: In April, NASA scientists began generating plasma energy in a 9-inch vacuum chamber in NASA’s Propulsion Research Laboratory at the Marshall Space Flight Center in Huntsville, Ala. Credit: MSFC.
Specific impulse measures how much thrust is produced per unit of fuel in each second of a rocket engine’s burn. Put another way, it is a measure of how many seconds one pound of propellant can produce one pound of thrust (which is why ISP is measured in ‘seconds’).
We need breakthroughs in specific impulse to achieve manned missions to the outer planets, and even greater breakthroughs to enable interstellar missions. The Space Shuttle’s main engines deliver an ISP of roughly 465 seconds. Missions to the Kuiper Belt, from 100 to 1000 AU, require ISPs in the range of 10,000 seconds, while an Oort Cloud probe demands 100,000. For a workable mission to Alpha Centauri within the lifetime of the researchers who built it, an ISP of 3,000,000 seconds is needed. Thus finding ways to boost ISP is at the heart of the interstellar enterprise.
Magnetic nozzles can certainly channel plasma, but how to generate maximum thrust under the constraint of closed magnetic field lines is a tricky proposition — the magnetic energy could prevent the plasma from detaching from the spacecraft. A nine-inch chamber in MSFC’s Propulsion Research Laboratory has been the venue for plasma generation since April as the team works on these issues. You can read more about this work in an MSFC news release.
Centauri Dreams note: the figures cited above on ISP for various missions come from John L. Anderson, “Roadmap to a Star,” Acta Astronautica 44 (1999), pp. 91-97. Anderson believes we reach a limit on ISP with a fusion rocket using magnetic nozzles, a system no one today knows how to build. But he points out that the ISP of a laser beamed energy propulsion spacecraft is infinity “…by definition since it consumes no propellant at the spacecraft.” Moreover, the physics of momentum exchange that would drive such a craft is well known. Anderson thus uses beamed energy/momentum exchange as the basis for his interstellar roadmap.
by Paul Gilster | May 10, 2005 | Missions, Outer Solar System
The New Horizons mission to Pluto and Charon is on schedule. The spacecraft is now completely assembled and has undergone a comprehensive performance test of its own systems and its seven instruments, according to principal investigator Alan Stern. The first of the major flight mission simulations began at the end of April; this will be followed by another run of performance tests, with environmental testing beginning in mid-May. Eying the January 2006 launch window, NASA plans a readiness review at the end of May.
Image: Pluto and Charon are primary targets for this first targeted Kuiper Belt mission. Credit: Johns Hopkins University Applied Physics Laboratory.
Stern’s contributions to the New Horizons site at Johns Hopkins University’s Applied Physics Laboratory are a great way to keep up with the mission’s progress. In the latest, Stern recalls how New Horizons got its name:
…as I waited for a streetlight to change near the intersection of Foothills and Arapahoe and looked west to the Rocky Mountains on the horizon, it just hit me. We could call it “New Horizons”— for we were seeking new horizons to explore at Pluto and Charon and the Kuiper Belt, and we were pioneering new horizons programmatically too as the first-ever PI-led outer planets mission.
More New Horizons news at the site, where Stern’s updates can be followed monthly.
Assuming New Horizons passes its environmental and launch risk analysis and launches as planned on January 11, 2006, the next mission milestone will be a gravity assist near Jupiter in February of 2007. New Horizons is scheduled for a much closer approach than Cassini’s, coming within about 32 Jupiter radii of the giant planet at some 21 kilometers per second. The gravity assist will be followed by the eight-year cruise to encounter at Pluto/Charon, with further Kuiper Belt investigations to follow.
Centauri Dreams note: in the eyes of many investigators, Pluto and Charon themselves are Kuiper Belt objects, probably the largest in this vast reserve of icy objects extending past the orbit of Pluto out to roughly 50 AU. While both Pluto and Charon have solid surfaces, they differ from the rocky terrestrial planets in the amount of icy material — carbon dioxide, frozen water, methane, molecular nitrogen and carbon monoxide — that makes up their mass. Getting to know these objects, and comparing them to what we know about cometary nuclei, will tell us much about the formation of the Solar System.
Note that a secondary launch window exists in the first two weeks of February, 2007. If used, the secondary window would see New Horizons put into a direct trajectory to Pluto (no Jupiter gravity assist), with arrival in late 2019 or early 2020, depending on the exact launch date.
by Paul Gilster | May 9, 2005 | Advanced Rocketry, Breakthrough Propulsion |
Sending a probe to another star would be NASA’s greatest adventure, but how do we lay the groundwork for such a mission? The agency likes ‘roadmaps,’ spelling out clear and specific objectives and beginning with missions not so far beyond those we could fly today. NASA’s Interstellar Probe Science and Technology Definition Team (IPSTDT) recently prepared studies on a solar sail mission into nearby interstellar space, reaching approximately 400 AU from the Sun in 20 years of flight time. Think of it as a logical follow-on to the Voyager probes.
But Ralph McNutt and colleagues at Johns Hopkins’ Applied Physics Laboratory have been defining a more ambitious mission. As worked out in several recent papers, McNutt’s probe would approach the Sun to within 4 solar radii before a fifteen minute engine burn would establish its high-speed escape trajectory from the Solar System. At this point all acceleration would end; unlike the IPSTDT design, no sail would be deployed.
The McNutt mission follows the NASA roadmap idea: “If we are to take seriously the notion of interstellar travel toward the middle of the next century, and try to make it happen, then the best we can do is to rely on currently known physics,” McNutt writes, “which can still push the limits of the doable even under the most optimistic of technological advances.”
Getting a probe this far from the Sun is tricky, but the first problem comes in approaching it. Heat constraints are only part of the story. The probe’s angular momentum around the Sun has to be removed to allow the close approach; the team envisions sending it to Jupiter for a gravity ‘slingshot’ maneuver to kill the momentum. It would then fall in toward the solar furnace for the engine burn at perihelion.
Image: Epsilon Eridani. At 10.7 light years, it provides a useful target for early interstellar probe concepts, including tests of optical communications against the background of a stellar spectrum. Credit: European Southern Observatory.
While the IPSTDT design calls for a workable solar sail as its enabling technology, McNutt’s probe relies upon a carbon-carbon thermal shield and a propulsion system capable of high specific impulse (ISP). The baseline mission objective is 1000 AU from the Sun within the lifetime of a researcher, or 50 years. And as McNutt writes, the key is to develop a spacecraft capable of operating in long-term cruise mode for a voyage of this immensity. “With an emphasis on long-lived, self-healing architectures and redundancies that will extend the probe lifetime to well over a century, a long-lived probe could be queried at random over decades of otherwise hands-off operations.”
For propulsion, a scaled-down Orion approach (using nuclear explosions behind the craft) has been considered, as has solar heating of a gas propellant. Intriguingly, the probe would be launched toward Epsilon Eridani, a K-type star some 10.7 light years from Earth. Although not designed as a true star probe (not at these speeds, some 20 AU per year!), the probe would be able to test data downlink capabilities against the background of a stellar spectrum, something that will have to be done to provide communications for the true interstellar probes that will follow. For reasonable bandwidth at distances of 1000 AU or more, an optical communication system would be used.
McNutt sees this probe concept as a 65-year development program costing roughly $1 billion. And it would itself be a precursor mission for a more advanced mission. From a paper in Acta Astronautica:
At 200 AU/yr, such a second generation probe could make the first targeted interstellar crossing in ~3500 years, the approximate duration of the Egyptian Empire. A more robust propulsion system that enabled a similar trajectory toward higher declination stars such as Alpha Centauri could make the corresponding shorter crossing in a correspondingly shorter time of ~1400 years, the time that some buildings have been maintained, e.g. Hagia Sophia in Istanbul (Constantinople) and the Pantheon in Rome. Though far from ideal, the stars would be within our reach.
Centauri Dreams‘ take: the mind-boggling trip times cited for McNutt’s second generation probe are a reminder of something Geoffrey Landis once told me, apropos of science fictional starships that make the journey look easy: “A trip to the stars is going to be hard, and it’s going to take a long time.” As a culture, we have to debate just where mission duration fits into our concept of scientific inquiry. If it is the lifetime of a researcher, then we will need to reach speeds of 30,000 kilometers per second to consider a star mission. But McNutt, in sketching out a hypothetical mission measured in thousands of years, sets out a useful counter-limit. While a thousand year mission may never be launched, the guess here is that if the technology emerges to make a crossing measured in the low hundreds of years, such a probe will be built, its data handed from generation to generation as a gift to the human future.
The paper cited above is R.L. McNutt Jr., G.B. Andrews et al., “Low-cost interstellar probe,” Acta Astronautica 52 (2003), pp. 267-279. See also McNutt’s Phase I and II studies for NIAC, available here. And be aware of McNutt, Andrews, Gold et al., “A realistic interstellar explorer,” Advances in Space Research 34 (1): 192-197 2004. For more on the interstellar roadmap idea, see J.L. Anderson, “Roadmap to a star,” Acta Astronautica 44 (1999) 91-97.
by Paul Gilster | May 8, 2005 | Astrobiology and SETI, Culture and Society
“MacDonald paused outside the long, low concrete building which housed the offices and laboratories and computers. It was twilight. The sun had descended below the green hills, but orange and purpling wisps of cirrus trailed down the western sky.
“Between MacDonald and the sky was a giant dish held aloft by skeleton metal fingers — held high as if to catch the stardust that drifted down at night from the Milky Way.
Go and catch a falling star,
Get with child a mandrake root,
Tell me where all past years are,
Or who cleft the Devil’s foot;
Teach me to hear mermaids singing,
Or to keep off envy’s stinging,
And find
What wind
Serves to advance an honest mind.
“Then the dish began to turn, noiselessly, incredibly, and to tip. And it was not a dish any more but an ear, a listening ear cupped by the surrounding hills to overhear the whispering universe.
“Perhaps this was what kept them at their jobs, MacDonald thought. In spite of all disappointments, in spite of all vain efforts, perhaps it was this massive machinery, as sensitive as their fingertips, which kept them struggling with the unfathomable. When they grew weary at their electronic listening posts, when their eyes grew dim with looking at unrevealing dials and studying uneventful graphs, they could step outside their concrete cells and renew their dull spirits in communion with the giant mechanism they commanded, the silent, sensing instrument in which the smallest packets of energy, the smallest waves of matter, were detected in their headlong, eternal flight across the universe. It was the stethoscope with which they took the pulse of the all and noted the birth and death of stars, the probe with which, here on an insignificant planet of an undistinguished star on the edge of the galaxy, they explored the infinite.
“Or perhaps it was not just the reality but the imagery, like poetry, which soothed their doubting souls, the bowl help up to catch Donne’s falling star, the ear cocked to catch the suspected shout that faded to an indistinguishable murmur by the time it reached them…”
James Gunn, The Listeners (New York: Charles Scribner’s Sons, 1972), pp. 7-8
by Paul Gilster | May 7, 2005 | Outer Solar System
The Kuiper Belt yields up its secrets grudgingly, but sometimes we get help from objects much closer to the Sun. Cassini’s flyby of Saturn’s moon Phoebe on June 11, 2004 has provided all the information scientists needed to declare the ancient object a relic from the outer Solar System, much like Pluto and other Kuiper Belt members. Two papers in Nature provide our best look yet at Phoebe.
Determining a planetary object’s origin isn’t easy, but Phoebe can be analyzed in terms of its density, which is calculated on the basis of the moon’s mass studied in relation to volume estimates from the Cassini images. The resulting figure is 1.6 grams per cubic centimeter, lighter than most rocks but heavier than pure ice, and suggesting an ice/rock composition similar to Pluto and Neptune’s moon Triton.
“Cassini is showing us that Phoebe is quite different from Saturn’s other icy satellites, not just in its orbit but in the relative proportions of rock and ice. It resembles Pluto in this regard much more than it does the other Saturnian satellites,” said Dr. Jonathan Lunine, Cassini interdisciplinary scientist from the University of Arizona, Tucson.
A second paper analyzes Phoebe’s surface composition. Imaging spectroscopy has revealed Phoebe to be “…one of the most compositionally diverse objects yet observed in our Solar System. It is likely that Phoebe’s surface contains primitive materials from the outer Solar System, indicating a surface of cometary origin,” write Roger N. Clark and team.
Image: Phoebe’s true nature is revealed in startling clarity in this mosaic of two images taken during Cassini’s flyby on June 11, 2004. The image shows evidence for the emerging view that Phoebe may be an ice-rich body coated with a thin layer of dark material. Small bright craters in the image are probably fairly young features. Credit: NASA/JPL/Space Science Institute.
More on Phoebe in the May 5 issue of Nature. The papers are Torrence Johnson and Jonathan Lunine, “Saturn’s moon Phoebe as a captured body from the outer Solar System,” Nature 435, 69-71 (5 May 2005 — abstract available here); see also Roger Clark, Robert H. Brown et al., “Compositional maps of Saturn’s moon Phoebe from imaging spectroscopy,” Nature 435, 66-69.
