New Sail Design to Reach 60 Kilometers Per Second

New Scientist is covering Gregory and James Benford’s intriguing sail concept that would get a spacecraft up to 60 kilometers per second. That’s faster than any spacecraft we’ve ever launched; by comparison, the fastest vehicle out there is Voyager 1, now pushing toward the heliopause at some 17.5 kilometers per second. The brothers Benford (Gregory from the University of California — Irvine and James of Microwave Sciences in Lafayette, CA), talk about beamed microwaves driving a sail design with a difference.

At play here is an effect James Benford discovered when testing a thin, carbon-mesh sail with beamed microwaves. The forces exerted on the sail turned out to be stronger than expected, because the heat from the microwave beam was causing outgassing from material in the sail itself. It was the push from these unexpected gas molecules that gave the sail the extra push. You can read the New Scientist story here.

I haven’t talked to James Benford since 2003, but even then he was zeroing in on this phenomenon, which is called ‘desorption’. It seemed clear that desorption could be used purposefully.

“You want to use desorption as a tool,” Benford said then. “You could paint on materials that will come off at the right temperature, and plan a mission to use that effect… The engine is basically the beam from earth. The fuel is the material that’s on the sail.

“All of this comes from our experiments, where we found materials coming out of the sail. We expected this from some materials; if you just have an ordinary surface and put it in vacuum, the surface will have weakly absorbed molecules of air and water, the sort of stuff that clings to any surface. Water is very clingy in vacuum, but if you heat it up to a few hundred degrees for a little while, you burn all that off.

“But when you go up to much higher temperatures, you start to bring out things that are actually embedded in the lattice of the carbon, things that have been in there like carbon dioxide. It takes high temperatures to get those out. We found we were getting a lot of gas out of the material, and that gas, being very hot when it comes out, gives you quite a bit of thrust.”

The Benfords’ concept will be described in an upcoming issue of Acta Astronautica. In it, they speculate that such a sail could be covered with a paint designed to emit gas to harness the desorption effect. They believe a one-hour microwave burst could bring the vehicle up to speeds that would take a spacecraft to Mars in a month’s time. Pushing the vehicle would be a 60-megawatt microwave beam, currently beyond our capabilities, but surely not for long.

Sources: Interview with James Benford (16 February 2003); Celeste Biever Boston, “Solar super-sail could reach Mars in a month,” New Scientist 29 January 2005. Details on the Acta Astronautica article when they become available.

A Quote for the Weekend

“It was only a few centuries ago that people began to realize that those points of light in the night sky were suns, like our Sun, and like our Sun, they might have planets around them. Many visionaries then dreamed and wrote of visiting those other planets in ships that traveled between the stars. Later, when astronomers were able to estimate the distance to the nearer stars, others concluded that, because interstellar distances were so immense and human life so short, interstellar travel was impossible.

“Travel to the stars will be difficult and expensive. It will take decades of time, gigawatts of power, kilograms of energy and trillions of dollars. Recently, however, some new technologies have emerged and are under development for other purposes, that show promise of providing propulsion systems that will make interstellar travel feasible within the forseeable future — if the world community decides to direct its energies and resources in that direction. Make no mistake — interstellar travel will always be difficult and expensive, but it can no longer be considered impossible.”

— Robert Forward, “Ad Astra!” Journal of the British Interplanetary Society Vol. 49, p. 23 (1996).

Centauri Dreams note: For just one example of Forward’s innovative work, see his “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets Vol. 21, Mar-Apr (1984), pp. 187-195. This seminal paper appeared two years after Forward’s novel Rocheworld, which illustrated the physics behind a manned, one-way mission to Barnard’s Star. Rocheworld was published in Analog Science Fiction/Science Fact in December of 1982, and was later released in a longer version as The Flight of the Dragonfly (New York: Timescape, 1984). A final, still longer version, once again titled Rocheworld, appeared in 1990 (New York: Baen Books).

Interstellar Boundary Explorer Chosen by NASA

Our first interstellar mission won’t be a long jump to Alpha Centauri or Barnard’s Star. In fact, we’ve already launched not one but several interstellar missions — the two Pioneer probes, and the two Voyagers that followed them, will all exit the Solar System; i.e., they will eventually cross the boundaries of the heliosphere to emerge into pure interstellar space. Some scientists believe that Voyager 1 is already pushing up against the so-called ‘termination shock,’ where the speed of the solar wind of gas and charged particles from the Sun drops to subsonic levels.

But we need far more information than the Voyagers, with their rapidly fading signals, can tell us. The next mission designed to explore the outer limits of the Sun’s influence will be the Interstellar Boundary Explorer (IBEX). Under development at Southwest Research Institute, IBEX is designed to explore how the solar wind interacts with the interstellar medium through which our entire Solar System moves. IBEX won’t be going to the outer Solar System itself. Instead, it will consist of a pair of neutral atom imagers that will be launched into an Earth orbit by a Pegasus rocket dropped from an airplane. The mission was selected by NASA as part of the agency’s Small Explorer program on January 26 of this year.

“In addition to revealing many of the interstellar boundary’s unknown properties, IBEX will explore how the solar wind regulates the radiation from the galaxy,” says Dr. David J. McComas, senior executive director of the SwRI Space Science and Engineering Division. “This radiation poses a major hazard to human space exploration and may have affected the formation and evolution of life on Earth. By examining the underlying physics of our solar system’s outer boundaries, IBEX will allow us to extrapolate the present day conditions to those of the past and the future, and offer insight into similar boundaries that surround other stars and stellar systems.”

Auger Observatory detector

Image: The heliosphere is a bubble in space produced by the solar wind. Although electrically neutral atoms from interstellar space can penetrate this bubble, virtually all the material in the heliosphere emanates from the Sun itself. The solar wind streams off of the Sun in all directions at speeds of several hundred kilometers per second (about 1,000,000 mph in the Earth’s vicinity). At some distance from the Sun, well beyond the orbit of Pluto, this supersonic wind must slow down to meet the gases in the interstellar medium. It must first pass through a shock, the termination shock, to become subsonic. It then slows down and gets turned in the direction of the ambient flow of the interstellar medium to form a comet-like tail behind the Sun. This subsonic flow region is called the heliosheath. The outer surface of the heliosheath, where the heliosphere meets the interstellar medium, is called the heliopause. Credit: Lunar and Planetary Institute

IBEX will be able to make its observations by traveling in a highly elliptical, high-altitude orbit outside the Earth’s magnetosphere. Its large aperture, single-pixel cameras will look perpendicular to the Earth’s spin axis, generating global maps by using the motion of the spacecraft. If all goes well, IBEX should be the first spacecraft to detect the outer edge of the Solar System. The mission is to be launched in 2008.

You can read more about IBEX at SwRI’s Interstellar Boundary Explorer page. A NASA news release on the mission can be found here.

Did a Collision Create Pluto’s Moon Charon?

One way to explain the existence of the Moon is through a giant collision, one that tore off enough material to build a satellite in a planetary orbit. Can Pluto and its moon Charon be explained the same way? Robin Canup thinks so. Canup is assistant director of Southwest Research Institute’s Department of Space Studies; she argues the case in the January 28 issue of Science.

The Moon may seem large in our skies, but it makes up only about 1 percent of Earth’s mass. Charon, on the other hand, is 10 to 15 percent the mass of Pluto, which suggests to Canup that the corresponding collision must have been with an object almost as large as Pluto itself. She also believes that Charon probably formed intact as a result of the collision.

“This work suggests that despite their many differences, our Earth and the tiny, distant Pluto may share a key element in their formation histories. This provides further support for the emerging view that stochastic impact events may have played an important role in shaping final planetary properties in the early solar system,” said Canup.

Pluto and Charon

While the collision theory for both the Moon and the Pluto/Charon pair is not new, Canup is the first scientist to model the scenario successfully. Her work on Earth’s Moon indicated that an impact from a Mars-sized object could have produced it. This work accounted for both the iron depletion of our satellite as well as its mass and angular momentum.

Image: Pluto and Charon as viewed by the Hubble Space Telescope. Note the darker tint of Charon, indicating differences in surface composition. Also, note what may be a surface feature at the center of the Pluto image. Credit: Space Telescope Science Institute.

You can see an animation of the proposed Pluto/Charon collision here.

Sources: Canup’s work on the Earth’s Moon and its formation can be found in Canup, Robin M. and Erik Asphaug, “Origin of the Moon in a giant impact near the end of the Earth’s formation,” Nature 412, 708-712 (16 August 2001). Her most recent article on the Pluto/Charon simulations will appear in the January 28 issue of Science. It follows a series of earlier studies building the case for the collision hypothesis.

Meteorite Lends Credence to Supernova Hypothesis

How do solar systems form? The traditional model has been a slowly condensing cloud of matter within which planetary objects eventually emerge. But that view has been challenged sharply by Yunbin Guan and Laurie Leshin, from Arizona State University. Last year Leshin argued that our own system formed from the violent processes of star-birth within a dense nebula, one filled with supernova activity. Now a new meteorite find has provided solid backing for the idea.

The Triffid NebulaWorking with a team from the Chinese Academy of Sciences, Leshin and Guan have found evidence of chlorine-36 in a meteorite that was formed shortly after the solar system appeared. Although chlorine-36 has a short half-life, it decays into sulphur-36, providing strong evidence for the past presence of the earlier form of chlorine, which would have been formed in the explosion of a supernova. The team found sulphur-36 in association with sodalite, a mineral rich in chlorine.

“There is no ancient live chlorine-36 in the solar system now,” said Leshin, who is director of ASU’s Center for Meteorite Studies. “But this is direct evidence that it was here in the early solar system.”

“We have now discovered the first solid evidence for two different short-lived radionuclides in the GeoSIMS Lab at ASU — iron-60 and chlorine-36 — and both of them provide strong evidence for where the solar system’s short-lived radionuclides came from. It’s producing a really strong argument that these radionuclides were produced in a supernova that exploded near the forming solar system and seeded the solar system with these isotopes.”

Image: The Triffid Nebula, perhaps the birthplace of solar systems much like our own. Credit: NASA/HST/Jeff Hester.

For Leshin’s earlier work on iron-60 and the supernova hypothesis, see Hester, Desch, Leshin, “The Cradle of the Solar System,” Science 21 May 2004; 304: 1116-1117. The chlorine-36 work will be published in Proceedings of the National Academy of Sciences in February; there is an online abstract here, and you can get further details from this Arizona State press release.