Microwave Beaming and the Cosmos Sail

We’re closing in on the launch of the Cosmos 1 solar sail, the first free-flying spacecraft to be powered by the momentum of solar photons. Yes, there have been deployment experiments before this, such as the Russian Znamya missions and the Japanese deployment of a thin film just last summer. But Cosmos 1 will be a functioning spacecraft capable of returning data, and its launch thus marks an important first in sail development. The launch window opens on June 21. The spacecraft is to be launched into a near polar and circular 800 kilometer orbit, with sail deployment four days later.

Cosmos 1 has been moved from the test facility in Moscow to Severomorsk; the plan calls for it to be launched by a converted Russian ICBM from a submerged submarine. If successful, the mission will be a landmark not only for sail propulsion but also for commercial space development. The vehicle was funded by Cosmos Studios, with donations from members of the Planetary Society, whose latest update on the mission can be read here.

Photon propulsion works because although photons have no mass, they do impart momentum. We’re talking about forces that are vanishingly small at Earth’s distance from the Sun, where the solar flux is roughly 1.4 kilowatts per square meter, some nine orders of magnitude weaker than the wind on Earth’s surface. But keep pushing on the sail in a zero-g environment and it will move, each push adding up, gradually gaining enough velocity to make missions within the inner Solar System practical. Missions to the nearby interstellar medium may also be in range, as witness the Oort Cloud Explorer we looked at in these pages in April.

Cosmos 1 will not, of course, attempt anything so dramatic. It’s an experiment in sail technology to gather information on how a sail performs under actual flight conditions. But particularly interesting for interstellar theorists will be its performance under microwave beam. James Benford of Microwave Sciences and his brother Greg (of the University of California-Irvine, and one of the most highly regarded of all practicing science fiction writers) will lead a team using NASA’s Goldstone antenna to send the beam to Cosmos 1. That will occur only when and if all other flight objectives have been accomplished.

But let’s hope that it happens, because laboratory data also compiled by Microwave Sciences indicates that beamed propulsion ought to work. The experiments used a 10-kilowatt microwave beam pushing a 10-square centimeter sail in a vacuum chamber. Exactly what effect a microwave beam from Earth will have is something Cosmos 1 should be able to measure with its onboard accelerometer and GPS equipment.

Ultimately, beamed technology will be necessary as we look toward missions to nearby stars, since the dropoff of solar radiation beyond Jupiter’s orbit makes solar sails less practical beyond that range (although ‘sundiver’ maneuvers close to the solar disc can push a sail-driven device to high enough velocities for missions to the nearby interstellar medium, furling the sail when it is no longer effective for propulsion).

James Benford’s paper on the microwave experiments is “Flight and Spin of Microwave-Driven Sails: First Experiments,” in Proceedings Pulsed Power Plasma Science 2001, IEEE 01CH37251, p. 548 (available at the Microwave Sciences site. Also available at the site: J. Benford, G. Benford et al., “Microwave Beam-Driven Sail Flight Experiments,” Space Technology and Applications International Forum, AIP Conference Proceedings 552, ISBN 1-56396-980-7STAIF, pg. 540 (2001).

How to Observe a Wormhole

If wormholes exist, is it possible to observe one? A fascinating, decade-old paper argues for the possibility, based on the observed phenomenon of mass curving space, which shows up in numerous instances of gravitational lensing. Just as the image of a background object like a distant galaxy can be bent by an intervening mass to produce a magnified image, so wormholes might be detected through their visual effects.

But wormholes, remember, are odd beasts. They should display negative mass. The upshot: instead of focusing light like a gravitational lens, a wormhole should diffuse it in all directions.

Artist\'s conception of a wormhole

I discussed the possibilities with Geoffrey Landis at Glenn Research Center a couple of years ago. Landis, who worked on the paper “Natural Wormholes as Gravitational Lenses” (Physical Review D, March 15, 1995: pp. 3124-27) with a remarkable team (John Cramer, Robert Forward, Gregory Benford et al.) pointed out that an actual wormhole would not be visible. But if a wormhole passed in front of an intervening star, a halo of light would form. And as the wormhole moved to one side of the background object, a spike of light would result. So the signature is: a spike of light, a halo, then another spike. And it might be that somebody searching for gravitational lenses will turn up such a signature one day, if wormholes do exist.

Image: This artist’s rendition depicts a hypothetical spacecraft with a “negative energy” induction ring, inspired by recent theories describing how space could be warped with negative energy to produce hyperfast transport to reach distant star systems. Credit: Les Bossinas (Cortez III Service Corp.), 1998; NASA.

Then again, most wormholes might have closed immediately after the Big Bang, leaving us to build our own. Stabilizing a wormhole mouth would require bizarre forms of matter possessing negative mass, which is just one of a series of problems making wormhole production unlikely. In an interview in Universe Today, physicist Dr. Stephen Hsu of the University of Oregon discusses his recent work on the constraints the universe puts upon matter.

To get the very weird exotic matter that I mentioned before with very negative pressure, it turns out the equations show that when you force the pressure to be that negative, there always some unstable mode in the matter, which means that if you were to bump your apparatus, you might find the exotic matter – which is stabilizing the wormhole – just collapses into a bunch of photos or something.

And again:

I would say it’s theoretically impossible to build classical matter which is stable and can stabilize a wormhole. You might ask, well maybe I’ll just avoid bumping the thing, but if you were to send a person through the wormhole, that itself would provide a bump and would very likely cause the whole thing to fall apart.

The key to this work is trying to determine whether the problem in creating wormholes is purely technological or the result of fundamental limitations of physics. That question is clearly worth investigating, though we won’t know the answer for some time. But Hsu’s studies so far lead him to believe that science fictional futures in which the human race stays close to the Sun are more realizable than the Star Trek vision of easy interstellar travel. Better to work, perhaps, on bioengineering, or artificial intelligence than starflight.

Centauri Dreams leans to a third alternative: interstellar flight does not violate the laws of physics, but it will be long, slow and rare. In the probable absence of wormholes, the earliest flights will be accomplished by beamed propulsion using lasers or particle beams, or perhaps engines drive by some form of antimatter-initiated fusion. If we do get to the stage of manned starflight, it will involve not vehicles the size of ocean liners with hundreds of crew members, but small teams spending decades in cramped vehicles on one-way colonizing missions. That’s not a Star Trek scenario, but a human future in the stars looks feasible whether or not we ever find wormholes.

For more, see Roman Buniy and Stephen Hsu, “Semi-classical wormholes and time machines are unstable,” an abstract of which is available here. BBC News also did a recent story on Hsu’s work.

On Shielding a Starship

Just how empty is interstellar space? We know that atoms of hydrogen and helium are the primary elements found there, but widely scattered atoms of every other element also show up in greater or lesser densities, along with grains of dust that are pushed into deep space by the pressure of stellar winds. You can also figure on cosmic rays — ionized atoms accelerated to extremely high energy states. So energetic are galactic cosmic rays that they correspond to the energy of protons moving anywhere from 43 to 99.6 percent of the speed of light. And let’s not forget magnetic fields — a weak interstellar field aligns with our galaxy — and high-energy gamma rays that emerge from stellar events that are still poorly understood.

Granted, the density of material in the nearby interstellar medium is far lower than the best vacuums we can create on Earth. The average in the Sun’s vicinity seems to be .01 atoms of hydrogen for every cubic centimeter of space, a number that is lower than the galactic average. As for dust, expect one dust particle for every trillion atoms. Even so, probes moving at 20 to 30 percent of light speed have to reckon with the effect even a single particle could have on the spacecraft. Dana G. Andrews (Andrews Space, Seattle) has made a recent study of these issues as they apply to a manned interstellar mission in a paper called “Things to Do While Coasting Through Interstellar Space.”

Here is Andrews discussing the nature of the problem, as it presents itself to a hypothetical starhip carrying four passengers on a thirty year trip crossing ten light years:

For a spacecraft traveling at 0.3c…even small grains of dust behave like very energetic cosmic rays as we collide with them. For instance, a relatively abundant carbonaceous dust grain 1/10 of a micron in diameter will have a relative kinetic energy of 37,500,000 GeV, and our interstellar craft should impact just over 13 of this size dust particles per second over every square meter of frontal area. Obiously, stopping or redirecting these dust particles is a major design goal.

NGC 1999 nebulaHow to deal with cosmic rays and dust particles? They can either be absorbed or redirected. Protecting the vehicle through absorption might mean packaging it like an onion, with the crew in the center of a layered craft made up of concentric rings of supplies and equipment. Andrews sees the top level as primarily machinery and storage, the middle deck as living area, and the bottom level as the Closed Environment Life Support System, the entire structure connected to the stowed accelerating sail by long cables. The entire assembly would be rotated so as to provide 1 g of acceleration. He sees the outer shell of the craft as 25 cm of multiple layers of polyamides, metal foils and polyethylenes to provide a degree of radiation protection.

Image: A Hubble image of NGC 1999, a nebula in the constellation Orion. Here a region of dusty gas surrounds a star and reflects its light. Depending on your destination, the interstellar medium is anything but empty. Hubble’s Wide Field Planetary Camera 2 (WFPC2) was used to obtain the color image. Credit: NASA and The Hubble Heritage Team (STScI).

Cosmic ray radiation, with its large kinetic energies, is quite penetrating. To protect his crew, Andrews uses plastics with high hydrogen content, saying “…since 99% of the Galactic Cosmic Radiation is ionized hydrogen or helium, it’s obvious why hydrogen makes the best shielding (because like masses scatter better).” Hull shielding is combined with internal shielding of the crew’s sleeping quarters with 30 gm/cm2 of water storage, which reduces the annual radiation dose to about three times the recommended yearly dose for radiation workers in the USA, within the overall guidelines for astronauts.

Two other options for shielding come into play: magnetic shielding would use large current loops of superconducting wire to create a magnetic dipole field around the habitat (the habitat, in turn, would be protected inside a Faraday Cage, which acts as an electromagnetic shield). Entering this dipole field, cosmic rays would be reflected or deflected around the habitat. Magnetic shielding is heavier than hull shielding but the difference in living space in Andrews’ design allows for a much larger crew (in fact, the habitable volume jumps to twenty times what was found in the first design). Finally, multiple layers of foil ‘bumpers’ can be used as debris shielding that would ensure that incoming dust particles can be broken down to atoms and ionized, thus making them capable of being deflected by the magnetic field.

Andrews believes that power can be extracted from the dust particle annihilation. But is it possible to harvest some of the interstellar material for use aboard the vessel? Some form of resource collection may be possible, but Andrews doubts its effectiveness:

It should…be possible to seal the cylinder gas tight, set the voltage to completely stop atoms with predetermined energies, so that they don’t penetrate the upper foil, but instead are neutralized, and collected. This is in situ resource collection. Unfortunately, the mass density in interstellar space is too low to make in situ resource scavenging worthwhile. The total amount of silicates dust impacting the debris bumper foils over the ten light-year journey amounts to 38 grams. Likewise the carbonaceous dust impacting over ten light-years totals 13 grams. Better we carry all our provisions with us.

As humanity moves out beyond the Solar System, then, enabling technologies for our first interstellar voyages will include magnetic shielding and the ability to ionize dust particles to ensure adequate deflection. All contingent, of course, upon steep reductions in the cost of power in deep space and, if we are hoping for human crews, advances in life support and long-term equipment maintenance. These seem achievable goals whose technologies can be developed right here in the Solar System, and Andrews is sanguine about their resolution, saying “…there appears to be nothing in physics to prevent interstellar travel by humans.”

The paper is D. Andrews, “Things To Do While Coasting Through Interstellar Space,” AIAA-2004-3706, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, July 11-14, 2004. On interstellar shielding, also be aware of the classic paper by A.R. Martin, “Bombardment by Interstellar Material and Its Effects on the Vehicle,” Project Daedalus Final Report (Journal of the British Interplanetary Society, 1978): S116-S121.

Voyager at the Edge

NASA is now confirming that Voyager 1 has entered the heliosheath, where the solar wind and interstellar materials begin to mix. The heliosheath is the outermost layer of the heliosphere, beyond which the spacecraft passes into interstellar space. Among the confirmatory data noted by the Voyager team: the magnetic field carried by the solar wind has increased by a factor of two and a half, which is the natural result of the solar wind slowing down. These readings have remained high ever since mid-December 2004, when the spacecraft crossed the termination shock at 94 AU. The issue, controversial ever since, now seems resolved.

Voyager spacecraft“The consensus of the team now is that Voyager 1, at 8.7 billion miles from the Sun, has at last entered the heliosheath, the region beyond the
termination shock,” said Dr. John Richardson from MIT, Principal Investigator of the Voyager plasma science investigation.

Analogies are always useful in explaining such matters, and NASA offers the following in relation to the termination shock, that region where the solar wind is slowed by pressure from interstellar gases. “Consider a highway with moderate traffic. If something makes the drivers slow down, say a puddle of water, the cars pile up – their density increases. In the same way, the density (intensity) of the magnetic field carried by the solar wind will increase if the solar wind slows down.” Hence the observed increase in magnetic field strength. For more, see this NASA story.

And from a University of Iowa press release:

“The solar wind creates a bubble (the heliosphere) around the sun, and near the edges of the bubble is a place where the solar wind piles up as it encounters the interstellar wind,” says Ed Stone, Voyager project chief scientist and professor of physics at the California Institute of Technology. “We think the sun is currently in a phase where the heliosphere is shrinking. If so, Voyager would continue to be in this thicker and hotter region until it reaches the heliopause, the outer edge of the bubble. This is a wonderful opportunity to reach interstellar space, and we hope we can keep the spacecraft operating through the year 2020.”

Also from the University of Iowa come thoughts on how far Voyager has to go before encountering true interstellar space. Iowa physics professor Don Gurnett, principal investigator for the plasma wave instrument on Voyager 1, estimates the craft to be 25 to 35 AU from interstellar space. That would require another ten years for the transit, a time frame still within range for Voyager’s instruments. Both vehicles are expected to keep sending good science until at least 2020, using onboard radioisotope thermoelectric generators for power. We have, in other words, fifteen years of data ahead of us, provided NASA can find the funds to keep receiving this precious information.

Bow shock seen by HubbleFascinating sounds of Voyager’s encounter with the termination shock can be heard at Gurnett’s Web site. And ponder the image at the left. This is a Hubble photograph showing a bow shock about half a light-year across. It was created by the solar wind from the star LL Orionis as it collided with the flow of gases from the Orion Nebula. Interstellar space is, in many ways, anything but empty, something we’ll have to bear in mind when we get to the point where we’re building probes that move at an appreciable fraction of light speed. More on this tomorrow, when we discuss interstellar shielding in this environment in relation to a recent paper by Dana G. Andrews.

Image credit: NASA, The Hubble Heritage Team (STScI/AURA).

Microlensing Finds Distant Planet

One of the most distant planets ever discovered has been found 15,000 light years from Earth by an international team of astronomers helped by two amateurs from New Zealand. The method of discovery was gravitational microlensing, which occurs when a massive object like a star crosses in front of a star shining in the background. Light from the more distant object is bent and magnified as if by a lens. From astronmers’ perspective here on Earth, the background star gets brighter as the lens crosses in front of it, and then fades as the lens moves away.

Which is what happened on March 17, 2005 when Andrzej Udalski, professor of astronomy at Warsaw University and leader of the Optical Gravitational Lensing Experiment (OGLE) realized that a star he was observing was moving in front of a much more distant star. The brightening of the distant star was significant — almost a hundred-fold — and it was then that OGLE astronomers (and a team from the Microlensing Follow Up Network (MicroFUN) found a new pattern: a distortion in the brightening of the star that indicated a planetary transit. The new world seems to weigh three Jupiter masses and orbits a star similar to the Sun at a distance of three AU.

Microlensing detects a planetAndrew Gould, professor of astronomy at Ohio State, has no doubt that a planet passed in front of the star, and even more significantly, he believes that the microlensing technique would have worked even on a much smaller world. “If an Earth-mass planet was in the same position, we would have been able to detect it,” Gould said. A paper on the find is available on the arXiv site.

Image (click to enlarge): Astronomers discovered a new planet when it and its host star crossed in front of a very faint background star (marked by crosshairs) and magnified the distant star’s light in a process called gravitational microlensing. Although the newfound planet is 3 times the size of Jupiter, microlensing offers the possibility of detecting earth-sized worlds using existing technologies. Credit: Optical Gravitational Lensing Experiment.

In late May, the team will study the new planetary system with the Hubble Space Telescope. With OGLE’s dedicated installation at Las Campanas Observatory in Chile teaming with the MicroFUN collaboration of astronomers from the US, Korea, New Zealand, and Israel, microlensing events can be tracked around the clock (OGLE finds more than 600 of these a year — not all, obviously, lead to planets). The two New Zealand amateurs — Grant Christie of Auckland (14-inch telescope) and Jennie McCormick of Pakuranga (10-inch telescope) share authorship on the arXiv paper, which has been submitted to Astrophysical Journal Letters. Both are part of the MicroFUN network.

This is the second discovery of a planet using microlensing, the first having occured in 2004 in a star system some 17,000 light years away in the constellation Sagittarius. For more on that discovery, see this JPL news release.