A Solar Sail Manifesto

I was startled to see the Breakthrough Propulsion Physics project make the pages of The Atlantic in its current issue. Novelist Thomas Mallon, in an essay largely devoted to solar sailing and The Planetary Society’s efforts in that direction, gives vent to some of the frustration, if not exasperation, many of us feel as we see basic research losing out to short-term missions whose purpose is by no means clear. “American politicians now mostly avoid the old conditional trope ‘If we can put a man on the moon’ — because we can’t, not anymore,” writes Mallon, who goes on to lament the passage of the BPP project and, five years later, NASA’s Institute for Advanced Concepts.

Questioning Why We Explore

In Mallon’s view, the sense of exploration is itself under attack:

Even the most spectacular unmanned successes of the American space program — from the Voyager probes of the ’70s to the Galileo and Cassini missions of the ’90s — seem to belong to a fading worldview. A desire to explore, even by mechanical proxy, is now a self-indulgence to be resisted, since the end result would only be the imperial spreading of that pollutant known as humankind. In the new view, people have made such a mess of things here that we have no right to any more of the universe, and certainly not to a “backup planet,” which some space enthusiasts have suggested as the ultimate hedge against environmental catastrophe on Earth.

This would once have struck me as a Jeremiah-like screed, but that was before I began routinely writing of these matters and hearing comments to much the same effect. The worst example of this misanthropic worldview I’ve encountered occurred at a dinner party where the subject of space exploration came up. I was defending the idea of expanding into the Solar System as a necessity in terms of acquiring the tools of asteroid deflection, at which point my host said that an incoming asteroid would do the universe a favor if it destroyed our planet, and that we shouldn’t try to stop it.

Statements like that are easy to make and seem to confer a kind of cynical sophistication on those who speak them, but of course the response is this: If you choose to commit suicide, that’s your decision, but do you really think you have the right to make it for your children and grandchildren? The answer being no, then developing at least enough smarts about space travel to reach and change the trajectory of incoming objects should be a high priority for our species.


Hope for a Second Cosmos

Mallon’s point in The Atlantic, however, is not to explore our modern fatalism but to showcase solar sailing, and in particular, the experiment called Cosmos 1, a solar sail whose Volna booster failed to deliver it to orbit. A Cosmos 2 is in the cards, or so believes Louis Friedman, whose continuing labors for The Planetary Society keep him on the plane to Moscow as he tries to keep his Russian/American team intact. It’s reassuring to read that any Cosmos 2 attempt will not fly on a Volna.

Image: What might have been — Cosmos 1 in space. Credit: Rick Sternbach/The Planetary Society.

And how invigorating to see that the fundamentals of solar sailing, which many of us believe will be the workhorse of our early space infrastructure, are laid out for an audience not generally given to these matters. Thus Mallon’s conversation in Pasadena with Friedman, Bud Schurmeier (retired Voyager project manager) and astronautics veteran Viktor Kerzhanovich:

“Light has energy,” said Friedman. “That you can’t argue with.”

“More important,” said Kerzhanovich, “it has momentum.”

“Therefore it has a force,” added Friedman. “You’re using the energy of light, and the force derived thereof, to transfer momentum of light energy to your vehicle, in order to propel the spacecraft. Basically your spacecraft, your solar sail, looks like a sail, but it really is a mirror. And so it’s reflecting the light, and that reflection is where the momentum transfer occurs.” If the mirror were fixed to a wall, there would be no transfer. But in free space, with no gravity and no air pressure? You’re off to the cosmic races.

“It’s not the solar wind,” Friedman reminded me.

“Things got named wrong,” said Schurmeier. However pretty it sounds, “sailing” is really a metaphor. There is such a thing as solar wind, but as Friedman explained, “Solar wind is electrons and protons that come from the sun, and they have mass, but they go very much slower than light.”

It’s photons, not protons, that we’re talking about?

“Right,” said Friedman. “Photons have no mass, they’re all energy. You do get a force from the solar wind, but it’s about a thousand times less than the force you get from this reflection. You turn your mirror in different directions, you can point the force in any direction you want!”

Getting a Sail into Space


And so on. It’s been ninety years since Friedrich Tsander and Konstantin Tsiolkovsky wrote about practical solar sailing and high time we got a sail into Earth orbit to test what can and cannot be done with sunlight. NASA has cut solar sail funding just at the point when we were closing on having the technology operational, as least for test purposes, and the European Space Agency is likewise turning its attentions elsewhere. The Marshall Space Flight Center team has a second NanoSail-D ready for deployment, but who will launch it, who will pay, and when will the event occur? Best to keep The Planetary Society’s efforts in full gear, because someone has to do this.

But in matters like these, everything depends upon funding, and at the moment the money is tight, despite the efforts of Ann Druyan at Cosmos Studios, who is devoting as much personal energy into Cosmos 2 as she did with Cosmos 1. Mallon calls Cosmos 2 “a Hail Mary pass, an audacious leap,” but in reality it’s a simple first step, and a tentative one at that, in pushing solar sail technology to a higher level of readiness. The fact that it has to be done by private initiative instead of government, for the price (as Druyan notes) of “a nice New York apartment,” could actually become its salvation, but only if the right philanthropist decides to dig deep into his pockets.

Renewed Challenge to the Dinosaur Killer

Some scientific hypotheses seem too perfect to be anything but true. Long before we understood the processes behind plate tectonics, the natural fit between the coasts of Africa and South America made the notion of their original linkage seem obvious. Although dismissed in many quarters as mere coincidence, the piecing together of earlier continents would follow. The hypothesis of continental movement, whatever the cause, was almost too obvious not to be true.

But does science really work so neatly? Writing about his work on the evident ‘dinosaur killer’ event at the Cretaceous/Tertiary boundary, Walter Alvarez once said:

“Much of the work we do as scientists involves filling in the details about matters that are basically understood already, or applying standard techniques to new specific cases. But occasionally there is a question that offers an opportunity for a really major discovery.”

And the K/T impact seemed, like the continental coastlines, to be an obvious fit, a breakthrough that changed everything.

But the theory of the 65 million year old killer impactor at Chicxulub is again being challenged, at least in terms of its effect on the dinosaurs and the destruction of countless species. Gerta Keller (Princeton University) and Thierry Adatte (University of Lausanne) believe the Chicxulub crater in northern Yucatan was formed some 300,000 years before the mass extinction occurred. Moreover, Keller argues in the upcoming Journal of the Geological Society that not a single species went extinct as a result of the impact.


The debate over Chicxulub is not new. In fact, The Geological Society held an online forum called the Great Chicxulub Debate back in 2004, allowing Keller to argue the case against a single impactor as dinosaur killer:

The Chicxulub impact was not the only thing making life horrible at the end of the Cretaceous. The mass extinction coincides in time with yet another – probably larger – impact right at the Cretaceous-Tertiary (K/T) boundary, which is well documented by virtue of its global iridium distribution. Moreover, massive volcanism that would one day create India’s Deccan traps began during the late Maastrichtian and continued into the early Tertiary 4.5, causing major climate changes and long-term biotic stress. This stress, we believe, culminated in the K/T mass extinction – which we think was the combined effect of both volcanism and impacts.

Image: Sediments show that the Chicxulub meteorite predates a mass extinction 65 million years ago. Credit: Gerta Keller (Princeton University).

The online debate is worth reading, as it lays out the basics of the case, which in its latest findings focuses on sediments deposited during the tens of thousands of years after the impact. Between four and nine meters of sediments, in Keller’s words, “…deposited at about two to three centimeters per thousand years after the impact. The mass extinction level can be seen in the sediments above this interval.”

This sandstone deposition accumulated over a long period and shows all the characteristics of normal sedimentation. That, Keller argues, makes it unlikely that disturbances caused by the impact could have made these deposits unreliable evidence, as some supporters of the Chicxulub impact theory have argued.

If we begin to see the K/T event as the result of more than a single impact at Chicxulub — and perhaps not related to Chicxulub at all — then we are tracing climatic and geological changes that worked in conjunction with a devastating impact elsewhere. In either case, the tale of mass extinctions the Earth tells reminds us that we live in an environment that can turn on its inhabitants. A space-based species backup plan — beginning with asteroid deflection technologies — continues to make long-term sense.

Addendum: A paper by James Fassett in Palaeontologia Electronica, just published today, argues that dinosaurs may have survived in a remote area of what is now New Mexico and Colorado for up to half a million years after the extinction event.

Eccentric Orbits and Bold Predictions

The 100th edition of the Carnival of Space is now up at the One-Minute Astronomer site, where I learned of the existence of Christopher Crockett’s Innumerable Worlds blog. Christopher’s story on two gas giants around subgiant stars is well worth reading. He’s a UCLA graduate student now working at Lowell Observatory who offers a good deal of background material in his posts, as in this comment on the new planets’ unusually eccentric orbits:

How planets end up on such crazy orbits is a matter that is currently being researched. These two worlds aren’t alone; many of the new worlds we’re finding sit on highly eccentric orbits. The leading hypothesis is that interactions between closely spaced planets might affect their orbits. If two planets get too close, the lighter one can get ejected from the planetary system entirely while the remaining, more massive, world is left behind on a very elliptical orbit. This is the same principle we use to slingshot probes out into deep space by stealing momentum from the planets. We may be seeing the remnants of long-past interplanetary bumper cars!

Interesting stuff, and it raises the issue of how many star-less planets might be wandering interstellar space. Not enough, according to recent studies, to explain the evident dark matter halo around the average galaxy, but enough to offer up a great deal of interesting real estate if we find that planetary ejection really is common.

Lots of good material on the latest Carnival besides this, including Brian Wang’s Projecting 250 years to a Star Trek Timeframe, which is packed with intriguing calls on population (89 billion by 2260) to the economy, with Brian’s usual set of useful links. One interesting possibility: A civilization reaching 100-200 light years in radius within 250 years. Also be sure to check astroENGINE‘s speculative piece on METI (transmitting to the stars) and Orbital Hub‘s Q&A session with alien hunter Seth Shostak, author of the recently released Confessions of an Alien Hunter.

Pushing Past Redshift Eight

No one has ever seen an object further away than the one at the center of the image below. It’s a gamma-ray burst known as GRB 090423, spotted by the Swift satellite on April 23rd and quickly observed by the Gemini Observatory and United Kingdom Infrared Telescope, both on Mauna Kea (Hawaii). The source is visible at longer wavelengths but disappears at the 1 micron level, all of which corresponds to a distance of about thirteen billion light years.


Image: The fading infrared afterglow of GRB 090423 appears in the center of this false-color image taken with the Gemini North Telescope in Hawaii. The burst is the farthest cosmic explosion yet seen. Credit: Gemini Observatory/NSF/AURA, D. Fox and A. Cucchiara (Penn State Univ.) and E. Berger (Harvard Univ.)

Spectacular, no? Numerous telescopes around the planet went on to observe the GRB’s afterglow, allowing the infrared light’s spectrum to confirm the highest redshift ever measured: z = 8.2.

The object in question was probably a massive star reaching the end of its life, its core collapsing into a black hole or neutron star. In the process, jets of gas punch out of the star and encounter gases previously shed during the star’s senescence, heating them up to produce the short afterglows that can be seen in various wavelengths. The visible light of the event was absorbed by hydrogen gas in the early universe, but the infrared glow was bright indeed, given that the light we see in the image has been traveling for most of the 13.7 billion year age of the universe.

Edo Berger (Harvard Smithsonian Center for Astrophysics) notes the significance of the find:

“I have been chasing gamma-ray bursts for a decade, trying to find such a spectacular event. We now have the first direct proof that the young universe was teeming with exploding stars and newly-born black holes only a few hundred million years after the Big Bang.”

Take a look at this second image, which places the GRB in perspective.


Image: Distribution of redshifts and corresponding age of the Universe for gamma-ray bursts detected by NASA’s Swift satellite. The new GRB 090423 at a redshift of z = 8.2 easily broke the previous record for gamma-ray bursts, and also exceeds the highest redshift galaxy and quasar discovered to date, making it the most distant known object in the Universe. GRB 090423 exploded on the scene when the Universe was only 630 million years old, and its light has been travelling to us for over 13 billion years. Credit: Edo Berger (Harvard/CfA).

Getting Closer to an Exomoon Detection

Finding moons around extrasolar planets is an invigorating quest. After all, at least three moons around gas giants right here in our own system — Europa, Enceladus and Titan — are considered of high astrobiological interest. What about gas giants in the habitable zone of some distant star? The image below shows what a moon of such a planet one might look like, as imagined by astronomer Dan Durda (Southwest Research Institute). Could such worlds be?


As we learn more, bear in mind that the hunt for ‘exomoons’ has already begun. The CoRoT spacecraft is searching for transit timing variation signals (TTV) — variations in the time it takes a planet to transit its star — as described by Sartoretti and Schneider in a 1999 paper. David Kipping (University College London) has been developing a second method called transit duration variation (TDV), which works in conjunction with the first. The TDV signal is brought about by velocity changes as the planet/moon ‘system’ is observed over time, the result of both planet and moon orbiting a common center of mass.

Dr. Kipping now offers a further take on these two effects, which should be able to detect and characterize an exomoon when used in tandem. In the new paper, the astronomer breaks transit duration variation itself into two parts, one based on velocity (V), the other on what he calls the transit impact parameter (TIP).

…an exomoon around a transiting exoplanet should induce a transit duration variation effect with two dominant components. One of these components is due to the moon altering the velocity of the host planet, which we label as the V-component. The second constituent is due to the impact parameter of the transiting planet varying as a result of the moon’s presence, which we label as the TIP-component.

The analysis of these combined effects should allow astronomers to tell the difference between moons in a prograde or retrograde orbit, because the TIP component acts constructively with the V-component in prograde situations and destructively with it in retrograde orbits. The effect is to boost the exomoon detection method via TDV by about ten percent in magnitude. All this helps us understand more about how the moon might have been formed and tells us something about the stability of its orbit.

The key point is that we have signals thrown by these effects that should be observable today, and will certainly be so as our instrumentation improves:

We therefore propose that it should be possible for future observations to not only detect an exomoon and determine its mass, but also provide a confident deduction of the sense of its orbital motion. Although this determination will likely require photometry at the limit of planned missions, it seems likely that once an exomoon is detected a more in-depth investigation would be able to answer the question of orbital sense of motion conclusively.

There are caveats here, including the fact that the calculations are effective for a planet-moon system in which the plane of the two objects’ orbits is aligned with the star-planet orbital plane — large exomoon inclinations would be disruptive. But it’s interesting to note that in Kipping’s view, exomoons at low inclination angles should be observable in the lightcurve during any planet-moon eclipse. That would be an exciting confirmation of the first detection of a moon around a distant extrasolar planet.

The paper is Kipping, “Transit Timing Effects due to an Exomoon II,” accepted for publication in Monthly Notices of the Royal Astronomical Society and available online.