by Paul Gilster | Apr 7, 2009 | Sail Concepts |
We need to find a way to get NanoSail-D into space. You’ll recall that the original NanoSail-D perished in the explosion of a SpaceX Falcon rocket. But the opportunistic mission, a sail whose central components are three inexpensive Cubesats, two of which house a small, deployable sail, may yet get into the black. As we noted in this story from August of last year, a duplicate NanoSail-D is available. The trick is to find the funding and the booster.
A joint project featuring The Planetary Society, NASA and the Russian Space Research Institute is attempting to do just that, looking at a solar sail experiment that may or may not involve NanoSail-D. The question is whether the 7 by 7-meter sail is the payload the mission planners will choose, the other option being a Russian-designed sail experiment of equally small size. You can read more about the design choices at The Planetary Society site.
Image: The Huntsville-based NanoSail-D team stands with the fully deployed sail at ManTech SRS technologies on April 16, 2008, after the successful deployment test.
Remembering what happened to the earlier Cosmos 1 sail, destroyed when the Volna rocket that was to put it into orbit failed, it’s reassuring to hear that the Volna is no longer an option. Launching the new sail on a Soyuz-Fregat booster is one possibility, but NASA seems to be holding out the option of a piggyback launch aboard a US vehicle. I still wonder whether another SpaceX Falcon attempt could be made. After all, the cost is minimal. A Shuttle launch comes in at $10,000 per pound of payload, while SpaceX promises costs as little as a tenth of this.
Figuring that NanoSail-D weighs less than ten pounds, we could be looking at the launch of an operational solar sail experiment for well less than $15,000, including margins. Moreover, SpaceX is a player. The company has already announced it would donate experimental payload space aboard an upcoming flight of its Dragon spacecraft to support the Heinlein Trust Microgravity Research Competition, just the kind of partnership that could also now be applied to solar sail research.
SpaceX was recently awarded a NASA contract to provide cargo delivery flights to the International Space Station. Thus the synergy between the NASA sail researchers and commercial space interests grows, a fact that has to be going through the minds of the solar sail team at Marshall Space Flight Center, as well as The Planetary Society planners and their Russian colleagues as they work toward concluding their nanosail studies this summer.
Ponder where we are today. Budget constraints have put an end to NASA’s solar sail work just at the point when we were closing on the vital next step, the deployment and operation of a test sail in space. Now we have a basic sail that can be augmented and enhanced to fly that mission, a potential spacecraft that sits in storage. Surely the lessons learned from building the Cosmos 1 sail can be applied in modifying NanoSail-D to gather data about controlling a solar sail in the environment it was designed for.
by Paul Gilster | Apr 6, 2009 | Exotic Physics |
How can the space between the stars be so full of stuff? So commented a friend who chanced upon this site, reading our discussion of interstellar gas and dust and the troubling fact that moving through it at high speeds bathes a spacecraft in radiation. Not an issue for our current generation of spacecraft, dust and gas rise in significance as we reach velocities that are an appreciable fraction of the speed of light, creating the need for various kinds of shielding. So what exactly is that stuff in outer space?
Break down the interstellar medium and you get almost 90 percent hydrogen, with ten percent or so helium and trace elements like carbon, oxygen, silicon and iron accreted in dust particles. Oleg Semyonov, in his recent Acta Astronautica paper, examines all this, noting that the concentration of interstellar gas varies greatly between 104 cm-3 in galactic clouds to less than 1 cm-3 in the regions between the clouds. Our own Solar System lies in a cavity of low-density gas, with the nearest ‘wall’ located about 170 light years out in the direction of the galactic core.
From Relativistic Dust to FTL
So much for empty space, in regions which, if we produced them in a laboratory here on Earth, we would consider the hardest of vacuums. Writing about anything is always serendipitous, but never more so than when looking at interstellar matters. Had it not been for the Semyonov paper, I wouldn’t have gotten to Stefano Finazzi’s study of warp drive theory as quickly as I did. Both Larry Klaes and Adam Crowl discussed the Finazzi paper in comments to the interstellar dust story. Adam wrote the matter up on his Crowlspace site and Larry pointed to this squib in Technology Review.
Then a note from Kelvin Long pointed to discussion of warp drive instability at the recent interstellar session at the Charterhouse conference in the UK, and suddenly problems at superluminal speed were filling my hard disk. And rightly so, for it turns out that dangerous radiation isn’t simply an external issue, a matter of shielding a spacecraft from matter it encounters while at relativistic speeds. If warp drive is carried through to its logical conclusions, we may well run into something even worse when going superluminal — a devastating radiation that would destroy our fragile human payload.
Alcubierre’s Theory, and a Problem
Let’s look more closely. Warp drive as envisioned by Miguel Alcubierre relied on the concept that although nothing could move faster than the speed of light through spacetime, spacetime itself is not so restricted. We do not, in fact, have any notion of a limit to the ‘stretching’ of spacetime, a fact brought home by inflation theory, which posits an immense expansion of the early universe in a mere flicker of time. Contract the spacetime in front of a vehicle while expanding it behind and the spacecraft itself never exceeds the speed of light even though the ‘warped’ spacetime delivers it to its destination faster than would otherwise be possible.
All of which calls for immense supplies of energy, and negative energy at that, so that recent work has been (more or less unsuccessfully) devoted to understanding how to reduce those requirements to something remotely manageable. Now Finazzi and team have folded quantum mechanics into their consideration of warp drive theory, with the result that warp drive is shown to be untenable for the hapless crew. The inside of the ‘bubble’ housing the spacecraft, in fact, becomes filled with Hawking radiation, emitted by black holes due to these very quantum effects.
The Ultimate Show-Stopper?
Thus we’ve gone from shielding the vehicle from external radiation to coping with a radiation phenomenon arising from the drive itself. And in this context it’s pretty much of a show-stopper. Let me quote Adam Crowl’s treatment of this in Crowlspace, since it’s so much clearer than what I was able to come up with:
…a somewhat more serious difficulty arises because of the horizons that the warp-metric creates. These horizons act just like black-hole Event Horizons and so they produce Hawking radiation. Hawking radiation is normally quite benign as most horizons are pretty large – for example, a collapsed star’s horizon is typically 10 km across. But in a warp-metric there are two horizons – forward and aft, contracting and expanding – and to produce them very thin shells of “negative energy” are needed. And because they’re so very thin (~10-35 metres) the Hawking temperature is very, very high – i.e. a large fraction of the Planck Temperature (1032 K.) Ouch!
Ouch indeed. Add to this the other problem noted by the Finazzi team, that the bubble of spacetime we are manipulating seems itself to be unstable, too much so for our spacecraft, sitting in its zone of flat spacetime, to take advantage of the warp drive effect. Possible use of warp drive design at subluminal speeds does not seem to be ruled out, but is a definite come-down when compared to the ease with which we hoped warp drive might span the stars.
Is this the end of Alcubierre-style warp drive theory? Surely not, but it’s now up to the next round of investigators to look hard at the Finazzi results to see whether there are ways around the quantum challenge. The paper is Finazzi et al., “Semiclassical instability of dynamical warp drives,” available online.
by Paul Gilster | Apr 3, 2009 | Interstellar Medium |
“Interstellar travel may still be in its infancy,” write Gregory Matloff and Eugene Mallove in The Starflight Handbook (Wiley, 1989), “but adulthood is fast approaching, and our descendants will someday see childhood’s end.” The echo of Arthur C. Clarke is surely deliberate, a sign that one or both authors are familiar with Clarke’s 1953 novel about the end of human ‘childhood’ as we learn about the true destiny of our species in the universe. But becoming a mature species isn’t easy, nor is figuring out interstellar flight.
Awash in Hard Radiation
Consider just one layer of complexity. Suppose we somehow discover a propulsion system that gets us to relativistic speeds in the range of 0.3 c. That seems a minimum for regular manned starflight given the times and distances involved, but suddenly attaining it doesn’t end our problems. Interstellar space isn’t empty, and when we accelerate to cruising speed at a substantial percentage of the speed of light, our encounter with interstellar gas becomes a nightmare. Indeed, this haze of gas between the stars acts as a flow of nucleonic radiation bombarding the starship as we push ever higher into relativistic realms.
Image: As a playful example of science fiction mixing with science, this photo shows the luminosity from hot gas used in a hypersonic “super-orbital expansion tube X2” test rig at the University of Queensland, Australia. A toy model of the fictional starship Enterprise is subjected to a Mach 5 flows. (Credit: Tim McIntry, Queensland Physics Department/Marc Millis).
And let’s not forget high-energy cosmic rays and dust, all of which demand protection. Because sensitive electronics are susceptible to damage as well as humans, we have to work out the hazards whether our mission is manned or not. A non-relativistic capsule moving in interstellar space would, according to Oleg Semyonov (State University of New York at Stony Brook), experience a radiation dose of 70 rems per year, while the safety level for people is considered to be between 5 and 10 rems in the same period. Going relativistic drives the dosage level to far greater extremes.
Did I say ‘greater’? Try this much greater: Thousands or hundreds of thousands of rems per second, comparable to conditions in the core of a nuclear reactor. All this from a ship moving at high relativistic speeds through interstellar gas. But even slower velocities are a problem as we move through this medium.
Puzzling Out Shielding Options
Semyonov plots the radiation involved from encountering interstellar gas versus velocity and finds that at speeds much above a comparatively sedate 0.1 c, an astronaut could not be outside the hull without layers of shielding. Shielding the entire ship is problematic. A radiation-absorbing windscreen installed in front of the vehicle is possible, a titanium shield of 1-2 cm workable up to 0.3 c but becoming ‘dramatically thicker with acceleration.’
Water? It’s not a bad idea because the crew needs water anyway:
Placing a water tank (or an ice bulge) in front of a ship is advantageous in comparison with a shield made of metal or another solid material because it eliminates the damaging embrittlement of solids under intense nucleonic radiation; for a given cruising speed, the penetration depth of monoenergetic nucleons will be the same and a layer located near the penetration depth inside a solid shield will be largely damaged because all the nucleons deposit the bulk of their kinetic energy at the end of their penetration depth dislocating atoms from the lattice, weakening the material, and causing peeling or flaking.
On the other hand, our water shield adds significant mass to the vessel, and at speeds close to that of light, it would need to be tens of meters thick to form an effective barrier. So we can contemplate titanium or aluminum hull shielding up to about 0.3 c, but 0.8 c demands several meters of titanium or the water barrier.
The Cosmic Ray Hazard
Cosmic rays are a hazard to any interstellar mission, relativistic or not. Water is again an option, but Semyonov notes that a ship would require a ’round shell of water of 5 m in thickness,’ a huge increase in mass, and still insufficient for absorbing the highly penetrating secondary gamma and muonic radiation that will bathe the ship, demanding an additional shield of its own. Usefully, cosmic rays become increasingly beamed as we increase velocity, so that a frontal shield for interstellar gas can also absorb them.
More on this:
Isotropic cosmic rays are subjected to relativistic beaming when a starship is moving with a relativistic speed. For the ship’s velocities closer to the speed of light, most of cosmic rays form into a beam directed toward the front of the spaceship. While they do present a hazard, they can be easily absorbed or deflected by a frontal shielding system that is required anyway protecting the crew and electronics against the hard radiation of the oncoming flow of interstellar gas. Cosmic dust will also contribute to the radiation hazard, because the dust particles are actually lumps of high-energy nucleons at relativistic velocities. A serious problem will be the sputtering of a ship’s bow or a radiation shield by the relativistic dust particles. Nevertheless, the shielding of relativistic starships from hard ionizing radiation produced by interstellar gas and cosmic rays does not seem to be far beyond existing technology.
In other words, if we can figure out the key question of propulsion, we should be able to overcome the shielding issue. Semyonov considers a range of options including combinations of material and magnetic shielding in arriving at this conclusion. His discussion is a wide-ranging and sobering reminder of how many barriers interstellar flight presents beyond tuning up the right kind of engine. Childhood may end, but we biological life-forms remain fragile creatures indeed when flung into the interstellar deep.
The paper is Semyonov, “Radiation hazard of relativistic interstellar flight,” Acta Astronautica 64 (2009), pp. 644-653.
by Paul Gilster | Apr 2, 2009 | Exoplanetary Science |
Depressing economic times inevitably cast a pall over our space plans. That makes it important to keep our eyes on the big picture — what we hope to accomplish — rather than succumbing to the fatigue induced by seeing good science pushed back on the calendar year after year. Will we get a terrestrial planet finder off in the next fifteen years? Will we get back to the Jupiter system some time before 2030? I don’t know, but times like these require persistence, patience, and continued hard thinking.
I was musing about this while looking through a paper Dave Moore passed along recently. It’s a discussion of where we need to go now that we’ve got missions like CoRoT and Kepler in space and the James Webb Space Telescope in the picture for 2014. Tom Greene (NASA Ames) and colleagues from various institutions are looking at a space telescope with relatively modest aperture in the 1.4-meter range, one that would use a coronagraph to block the light of central stars to allow direct imaging of planets all the way down to the habitable zones.
Why do we need such a mission? The JWST, able to take high quality spectra on transiting gas giants, is going to have trouble with low mass dust disks near habitable zones, and the same problem extends to small planets in those zones and planets that do not transit. An instrument like the one detailed here would be able to detect planets down to one to two Earth radii in the habitable zones of about two dozen of the nearest stars, looking for spectral features like H20, O2 or other molecules we believe necessary for life.
From the paper:
Our simulations show that any small planets in or near habitable zones of 20 of the nearest stars would have a 20% chance of detection in 6 – 12 hours of integration time with a moderate aperture space coronagraph. Thus there would be a 90% chance of detecting each one in a total of 10 uncorrelated visits. Therefore a complete survey and repeat followup characterization could be completed in about a year of real time. These observations will likely include short term monitoring for variation with rotation and longer term monitoring for seasonal effects (perhaps snow), phase effects in atmospheric scattering, and constraining orbits.
Add in what would be learned about giant planets and circumstellar debris disks over a wider range of target stars and the mission stacks up as a prudent one for our times, relatively modest in scope but capable of extending our knowledge significantly while affecting the design of future, more expansive projects. Kepler’s field of view doesn’t include the nearby F, G and K-class stars envisioned as targets for this hunt and ground-based radial velocity studies won’t be able to produce the detail of this mission, which would be capable of directly imaging many planets that are so far only inferred from our data.
The paper is Greene et al., “Discovering and Characterizing the Planetary Systems of Nearby Stars,” prepared for the Planetary Systems and Star Formation science frontier panel of the Astro2010 Subcommittee on Science (abstract).
by Paul Gilster | Apr 1, 2009 | Sail Concepts |
New propulsion technologies are under study in the laboratory, even if finding the funding for such work is always a problem. James and Gregory Benford have demonstrated that a powerful microwave beam can push an ultra-light carbon sail even to the point of liftoff under lab conditions at 1 gravity. That’s useful information, for if we can leave the propellant at home, we can contemplate deep space missions driven by beamed microwaves, a technology that not only can pack a wallop, but is also less destructive to sail materials than a laser, meaning the sail can be brought to high temperatures more efficiently.
Unusual Acceleration
Yesterday we talked about a possible ‘Sundiver’ mission built around the microwave beaming idea. The Benfords’ version of this mission depends upon a second effect they observed in the lab. The photon pressure applied to the small sail they used could not account for the observed acceleration. Something was clearly coming out of the carbon lattice, but what was it? The sail material had been heated before the test to drive out any contaminants and the sail had been placed in a hard vacuum. James Benford told me about the feeling he got upon analyzing the sail’s performance under the beam:
I calculated and got two numbers for the total acceleration on our sail — somewhere between 9.8 and 13.5 g‘s. And I sat there and thought boy, what is this? How could we maintain such acceleration? The photons couldn’t do it, so something else was going on.
That something else turned out to be absorbed molecules — CO2, hydrocarbons and hydrogen — that become incorporated in the lattice when the material is made. Only at the highest temperatures do these residual molecules emerge. Mass ejected from the material under high temperature by this desorption process becomes another form of acceleration, observed in the laboratory as it forced the sail upward. Meanwhile, the original sail material remains unharmed by the desorption effect — the team’s final report is clear on the point, and I’ve seen Benford’s images of the intact sail.
The Uses of Desorption
Think, then, of a variety of compounds that can be ‘painted’ onto a sail, perhaps in multiple layers. Use the desorption process carefully and you have created a propulsive layer that can be triggered by microwave beam or the Sun itself. The Sundiver mission in this new guise now emerges: Launch the carbon sail via microwave so as to make layer #1 of this propulsive ‘paint’ desorb. Enhanced thrust through this combined beam/desorption method gives the sail 15 kilometers per second velocity, canceling most of its solar orbital velocity, and allowing a quick fall toward perihelion (using solar pressure alone would take years to spiral down).
At perihelion, the spacecraft rotates to face the Sun where, under intense sunlight, sail layer #2 desorbs, producing a 50 kilometer per second boost. The sail then moves away from the Sun, now acting as a reflective solar sail, its aluminum layer revealed. The Benfords have calculated that a Pluto mission could be accomplished in five years with these techniques, doubling the pace of our New Horizons mission, while paving the way for faster missions as the technology is tested and improved.
Image: The Benford Sundiver mission, aided by the propulsive effects of desorption.
What an intriguing concept desorption turns out to be, an effect discovered (but not anticipated) in laboratory work. Ponder what might have happened with the ill-fated Cosmos-1 mission if, upon successful deployment, the flight team had discovered the effects of desorption for the first time in space, with the sail showing propulsive effects no one had reckoned with. I asked James Benford about this in our conversation:
We had a design review at the Planetary Society with the Russian team on Cosmos and I raised the question, saying that we know that desorption takes place when you put any kind of a beam, any kind of heat source, on a material. And shouldn’t we take that into account in the experiment? The Russians said that effect was not important. Nevertheless, I got it onto the list of action items that it would be looked into.
The Russian team came back at the next design review and said actually it is important, but that it goes away in a couple of days. But Cosmos 1 as built had an asymmetric surface — the front and back were not the same. They were going to get more outgassing out the back surface than the front surface. So therefore when the sunlight hits it, the acceleration from desorption will in fact produce a negative acceleration. So the sail would actually be propelled backward for a little while.
Stabilizing the Beam Rider
We’re used to surprises on our space missions, but this one would have been a whopper. Now we can ponder the possible uses of this helpful effect in getting an extra boost to a mission that effectively combines a solar sail with beamed propulsion and, in a sense, rocket technology. But what about beamed propulsion over longer periods for moving beyond the Solar System? Could a sail riding a microwave beam for an extended period be stable? The answer is yes, not only through stable ‘beam-riding’ effects but because a microwave beam can communicate angular momentum to a sail to provide additional control. All of which is why microwave beaming is emerging as a credible deep space technology, and why we’ll be discussing it again in coming posts.
A good place to start digging on microwave propulsion is James Benford’s “Space Applications of High-Power Microwaves,” IEEE Transactions on Plasma Science, Vol. 36, NO. 3 (June, 2008). Zeroing in on desorption and its uses is the Benford’s “Acceleration of sails by thermal desorption of coatings,” Acta Astronautica 56 (2005), p. 593 ff.
