Microwave Beaming: Groundwork for Sundiver

A ‘Sundiver’ mission may offer the best acceleration we can muster given the current state of our technology. New Horizons is currently moving toward Pluto/Charon at roughly 19 kilometers per second, but back of the envelope calculations can pull out 500 kilometers per second for a solar sail that makes the optimum close approach to our star and then unfurls to full diameter, riding the photon storm outward to the edge of the Solar System and beyond in record time.

But Sundivers are tricky missions even on paper (we have yet to attempt one). Gregory Benford (UC-Irvine), who coined the ‘Sundiver’ term, and brother James (Microwave Sciences) have studied the matter in depth, and bring a unique perspective. They’ve not only theorized about sails and acceleration, but have actually tested the concept in the laboratory. Specifically, they’ve used an intense beam of microwaves to lift a carbon sail vertically in a vacuum chamber, and have studied how to spin and control it.

A Sail Takes Flight

This work took place in 1999 and 2000, and while I wrote the experiments up in my Centauri Dreams book some years back, I hadn’t seen the final report on it (“Wireless Power Transmission for Science Applications” NAS8-99135) until a conversation with James Benford last fall resulted in his passing along a copy. It’s absorbing reading because it’s the kind of essential laboratory work that leads to new thinking in propulsion on a practical level. It’s startling that these significant experiments have received as comparatively little attention in the space community as they have to this point.


The sail material chosen was a carbon fiber ten times thinner than a human hair. A carbon sail — the Benfords used small sails just inches across — has ‘memory,’ able to regain its shape after being rolled or folded. That could make deployment easier in space. And carbon fiber has other benefits that make it stand out as opposed to aluminized mylar sails. A micro-fiber mat like this can handle high temperatures even though it’s lighter than tissue paper. Putting this material close to the Sun poses no risk to the sail’s survivability.

Image: Carbon disk sail lifting off of truncated rectangular waveguide under 10 kW microwave power (four frames, 30 ms interval, first at top). Credit: James and Gregory Benford.

But laboratory work on sail materials is tricky indeed. Trying to get a sail to lift off against the force of gravity poses serious temperature problems, for the beam intensity (the Benfords used a 10 kW, 7 GHz microwave beam in a vacuum chamber) would melt conventional materials. But the Benford’s carbon fiber microtruss reached temperatures above 2000 kelvin from microwave absorption without melting. The concept of microwave beaming to push a spacecraft has been initially validated, and the requisite material tested to ensure it could handle the temperatures involved in a close solar pass.

A Sundiver Concept Emerges

The general shape of the Sundiver mission begins to take on substance. Surely we could use microwave beam technologies to launch a sail into a trajectory that, over time and multiple orbits, would reach the vicinity of 0.1 AU, at which point the sail receives the mighty wallop of solar photons after rotating to face the Sun. It’s an idea with a pedigree in propulsion studies, but one to which the Benford’s laboratory work has added a significant new dimension.

For there seems to be a way to kick in a new form of propulsive ‘burn’ at perihelion to maximize the resultant acceleration. Remarkably, in their experiments both at the Jet Propulsion Laboratory and UC-Irvine, the Benfords found that when they turned their microwave beam on the test sail, it experienced accelerations well beyond what photon pressure alone could account for. Tomorrow I want to look at how this effect could be modified and enhanced, capable of being used both at the initiation of a Sundiver mission and at the critical moment of closest Solar approach.

Unusual Genesis of a Supernova

More on Saturday’s supernova story, which was truncated both because I was wrestling with a flu bug but also because I needed to verify that the supernova under study at the Weizmann Institute of Science (Israel) was the event — SN 2005gl — examined in Nature this past week. A quick response from the Institute’s Avishay Gal-Yam confirmed the identity, which means we have more to say about this unusual observation.


Located some 215 million light years from us, SN 2005gl is striking on several counts, not least of which is that the blast of a supernova generally covers up all evidence of what the star once was. What Gal-Yam and co-author Douglas Leonard (San Diego State) discovered is that the Hubble Space Telescope had an image of the galaxy containing the progenitor star as it appeared eight years before it exploded. Moreover, the star stood out, being one of the brightest and most massive in the host galaxy.

Image: Eight years later: A 2005 Keck Adaptive Optics Image of the event, SN 2005gl in NGC 266.

If you recall Saturday’s story, the star in question was thought to be some fifty to one hundred times as massive as the Sun, in the range where we expect an exploding star to produce a black hole. The ‘disappearance’ of the stellar remnant after the explosion, as confirmed with Keck and later Hubble imagery, gives the theory credence. But the star itself poses still more provocative questions. Eight years ago, its absolute visual magnitude (-10.3) flagged it as one of the class known as Luminous Blue Variables (LBVs).

We would expect an LBV-class star to lose a great deal of its mass through its solar wind, the assumption being that this precedes the formation of a large iron core and the ultimate core-collapse supernova. But in the case of SN 2005gl, that process seems to have been short-circuited, as Leonard points out:

“The progenitor identification shows that, at least in some cases, massive stars explode before losing most of their hydrogen envelope, suggesting that the evolution of the core and the evolution of the envelope are less coupled than previously thought, a finding which may require a revision of stellar evolution theory.”

All of that from the identification of a star getting ready to go supernova on an archival Hubble image. Sometimes we get reminders of how much good science can come from the massive archives we are building through computer technology, much of this material awaiting further productive study. In the case of supernova explosions, this latest finds says that we may have to continue looking for the causative mechanisms, at least in terms of stars of this size and their process of shedding mass.


Image (click to enlarge): [Top Center] This is a 2005 ground-based photograph of the supernova as seen in host galaxy NGC 266, located in the constellation Pisces. Credit: Puckett Observatory

[Bottom Left] This is a 1997 Hubble archival visible-light image of the region of the galaxy where the supernova exploded. The white circle marks a star that Hubble measured to have an absolute magnitude of -10.3. This corresponds to the brightness of 1 million suns (at the galaxy’s distance of 215 million light-years). Credit: NASA, ESA, and A. Gal-Yam (Weizmann Institute of Science, Israel)

[Bottom Center] This is a near-infrared-light photo of the supernova explosion taken on Nov. 11, 2005, with the Keck telescope, using adaptive optics. The blast is centered on the position of the progenitor. Credit: NASA, ESA, and A. Gal-Yam (Weizmann Institute of Science, Israel), D. Leonard (San Diego State University), and D. Fox (Penn State University)

[Bottom Right] This is a visible-light Hubble follow-up image taken on September 26, 2007. Note that a bright source near the site of the supernova can be seen in all three panels, but the progenitor star is gone. The Hubble pictures from both epochs were taken with the Wide Field Planetary Camera 2. Credit: NASA, ESA, and A. Gal-Yam (Weizmann Institute of Science, Israel)

Only a small part of SN 2005gl’s mass was blown away in the explosion, with most of it being drawn into the collapsing core. The black hole that resulted is thought to be between ten and fifteen solar masses. SN 2005gl becomes only the second supernova progenitor we’ve definitively identified, and bear in mind that the earlier one, SN 1987A, led to a re-thinking of supernova theory. This new supernova seems to put our theories once again to the test, with results that require yet more revision.

The paper is Gal-Yam and Leonard, “A massive hypergiant star as the progenitor of the supernova SN 2005gl,” Nature online publication 22 March 2009 (abstract). A Space Telescope Science Institute news release is also available.

Explosion of a Gigantic Star

A star on the verge of exploding is an exceedingly useful thing. Identify it through a telescope and you can examine its telltale behavior before and after the event, in the process learning whether our existing theories about neutron star and black hole formation are supported by observation. We’ve seen stars on the order of twenty solar masses go into supernova mode, their internal elements becoming heavier and heavier through the progress of nuclear fusion.

Iron is the result, but at stellar center the iron breaks down into protons and neutrons, causing an internal collapse and a supernova flash that causes the star’s outer layers to be blasted into space. The core, meanwhile, mutates into a neutron star, its radius reduced to a matter of ten or so kilometers. All of this occurs more or less as theory describes, but until recently, we hadn’t had the chance to study a larger 50 solar mass star in its supernova agonies. A black hole should result.

Avishay Gal-Yam (Weizmann Institute Faculty of Physics) and Douglas Leonard (San Diego State) located a star possibly twice that size, watching as its mass collapsed in on itself to form a black hole. This Weizman Institute news release reveals that only a small part of the star’s mass was blown into the stellar neighborhood. Most of it was drawn into the collapsing core.

Subsequent telescope images show that the star has, for all intents and purposes, vanished from view, a black hole that light cannot escape. The notion that stars with masses from tens to hundreds of times larger than our Sun all end up as black holes thus receives dramatic confirmation.

Saturn’s G Ring Gets a Moonlet

by Larry Klaes

Our fascination with ringed worlds continues to grow as we learn more about what circles the worlds of the outer system. If you’re looking for what may be the most spectacular ring system imagined — two ringed exoplanets locked in a tight gravitational embrace — be sure to read Jack McDevitt’s novel Chindi (Ace, 2003), and spend some time with his crew on the surface of the moon that orbits their center of mass. Meanwhile, join Tau Zero journalist Larry Klaes as he focuses on continuing revelations from Cassini about Saturn’s rings and the moons that feed them. And join us in our celebration of the extended Cassini mission. Who knows what discoveries await?

In the 1968 novel version of 2001: A Space Odyssey, author Arthur C. Clarke said that the magnificent rings of the gas giant planet Saturn were made by visiting advanced extraterrestrial intelligences who tore up some moons in the Saturn system in the process of making their incredible Star Gate. This artificial cosmic wormhole would ages later transport fictional astronaut David Bowman to another part of the Universe in order to transform him into “something wonderful.”


While most scientists are not inclined to believe or suggest that the rings of the second largest planet in the Solar System were made by some aliens on a construction project, they now think that Saturn’s rings were formed with the planet at the beginning of the Solar System roughly five billion years ago. Over the eons, moons would both smash into each other or be torn apart by Saturn’s massive bulk, creating the extensive rings we now see girdling the planet.

Image (click to enlarge): ”It’s full of stars!” David Bowman proclaimed in 2001: A Space Odyssey. This Cassini spacecraft view evokes the exclamation, ”It’s full of moons!” Credit: NASA/JPL/Space Science Institute.

This process continues today. As the first space probe missions to Saturn discovered several decades ago, the rings are still being formed by a number of moons and even smaller moonlets, which contribute debris to the countless number of icy boulders and dust that make up the ring system.

There was one ring section that did not seem to have a moon or moonlet involved in its continued existence, the outer dusty ring labeled simply G. There was an extra collection of material at a certain point in the thin ring, but scientists operating the Cassini probe that is now entering its fifth year in orbit about Saturn had been unable to discover if the G ring had a companion world.

Now the Cassini imaging team, which include members of the Astronomy Department at Cornell University, have at last found the moonlet which supplies the debris for the G ring. The discovery was reported in the March 3 International Astronomical Union (IAU) Circular # 2093.

The moonlet, which has been given the provisional label of S/2008 S1 until an appropriate name from ancient mythology can be chosen, is the 61st known natural satellite for Saturn. This is just two less than the Solar System moon record holder, Jupiter, the largest planet in our celestial neighborhood.


Image: This sequence of three images, obtained by NASA’s Cassini spacecraft over the course of about 10 minutes, shows the path of a newly found moonlet in a bright arc of Saturn’s faint G ring. In each image, a small streak of light within the ring is visible. Unlike the streaks in the background, which are distant stars smeared by the camera’s long exposure time of 46 seconds, this streak is aligned with the G ring and moves along the ring as expected for an object embedded in the ring. Credit: NASA/JPL/Space Science Institute.

S/2008 S1 is deeply embedded in the main arc of the G ring. The moonlet’s very small size, only 600 yards across (Earth’s Moon is 2,160 miles in diameter, by comparison), explains why it eluded Cassini’s electronic eyes and those of its imaging team until now.

“Before Cassini, the G ring was the only dusty ring that was not clearly associated with a known moon, which made it odd,” said Matthew Hedman, a Cornell Cassini imaging team associate. “The discovery of this moonlet, together with other Cassini data, should help us make sense of this previously mysterious ring.”

Hedman and colleagues suspected that debris from collisions between micrometeorites and objects within the arc are the likely source for the material that makes up the G ring.

“We had evidence that there were big particles in the arc and thought that collisions between these objects and various micrometeoroids could release the dust that formed the ring,” Hedman said. “[This moon is likely] one of the objects that dust is knocked off of to form the ring.”

There may be other moonlets within the G ring arc, but S/2008 S1 is likely the biggest member of that group. Cassini will be able to photograph this region of Saturn again in 2010 and 2015 to get actual images of the moonlet and any companions in the arc. The discovery images of S/2008 S1 and others taken since then only show a faint smear of light, which is understandable considering the moonlet’s small size.

In addition to providing material to its ring, the little moonlet shares another feature with the other satellites that are involved with their rings: It and the arc are in resonance with a larger outer moon, in this case Mimas, which is best known for having one large crater that makes it look like the Death Star from the film Star Wars. The G ring is the second farthest ring system from Saturn at a distance of 100,000 miles.


Image: Two other moons that have profound impacts on the rings, Mimas and Prometheus, are seen here with the F ring. Mimas (396 kilometers, or 246 miles across), the larger and much more distant of the moons, creates the Cassini division between the A and B rings. Prometheus (86 kilometers, or 53 miles across), although much smaller than Mimas, is half of a duo responsible for maintaining the narrow F ring. Credit: NASA/JPL/Space Science Institute.

Last July Cassini was given a two-year extension on its mission and a resulting new designation: The Cassini Equinox Mission. The probe will do sixty more orbits of Saturn during that time, flying by the planet’s largest moon, Titan, 21 times to conduct further radar mapping of the bizarre and fascinating surface otherwise hidden beneath thick orange clouds. Cassini will also make seven close passes of Enceladus, the moon which the machine found to have active geysers spewing ice crystals into space. Both satellites are future targets for deeper exploration, in no small part to see if they are abodes for some kind of life.

Surprise Find: Fragments of Fallen Asteroid

Asteroid 2008 TC3 is surely a sign of progress. The eighty ton asteroid, which made a spectacle of itself upon entry into Earth’s atmosphere on the morning of October 7, 2008, was the first space rock to have been observed before it collided with our planet. What we’re hoping, of course, is that any future objects headed our way will be spotted early enough that, if their size warrants, they can be diverted or destroyed.

It was thought that 2008 TC3 did a good job of destroying itself when it exploded some 37 kilometers above the Nubian desert, but two researchers recently traveled to the Sudan and, with help from students at the University of Khartoum, collected 280 pieces of asteroid over a 29-kilometer field. Peter Jenniskens (SETI Institute) calls the event “…an extraordinary opportunity, for the first time, to bring into the lab actual pieces of an asteroid we had seen in space.” Jenniskens is lead author on the paper that now appears as the cover on the latest Nature.

I mention the significance of detecting objects early, but this one is a reminder of how tough the challenge is, being detected by the Catalina Sky Survey a mere twenty hours before its explosive arrival. We’re going to need longer lead times than that in the event of future emergency, but of course 2008 TC3 was also a tiny object, roughly the size of a truck. It’s also interesting in its own right, as an analysis of its debris shows. So-called F-class asteroids like this one have never before yielded a sample that could be studied in the laboratory. Now it’s giving out potentially useful secrets.

The meteorites it produced — called polymict ureilites — are porous, dark and rich in carbon. Nature is running a gripping account of the fall of 2008 TC3 by Roberta Kwok in the same issue as the Jenniskens paper, from which this snip about the composition of the meteorite:

Jenniskens couriered a sample to Mike Zolensky, a cosmic mineralogist at the NASA Johnson Space Center in Houston, Texas. Examining the rock, Zolensky discovered that it contained large chunks of carbon and glassy mineral grains resembling sugar crystals. Tests at other labs confirmed that the sample was a ureilite, a type of meteorite thought to come from asteroids that have melted during their time in space. Only 0.5% of objects that hit Earth yield fragments in this category. But 2008 TC3’s pieces are strange even for ureilites: they are riddled with an unusually large number of holes, says Zolensky. “It boggles the mind that something that porous could survive as a solid object,” he says.

Mind boggling indeed. And the behavior of 2008 TC3 is also a potent indicator that these meteorites are the fragments of a fragile parent. 2008 TC3’s high-altitude explosion shows that any future F-class asteroid on an Earth-crossing trajectory may be likewise fragile, and thus likely to disintegrate into a lethal rain of debris if simply blown up. Alternative strategies for dealing with such asteroids are under consideration, but knowing which to use on what target is key. That also makes sharpening our asteroid identification skills from afar a continuing priority.

The paper is Jenniskens et al., “The impact and recovery of asteroid 2008 TC3,” Nature 458 (26 March 2009), pp. 485-488 (abstract).