A solar sail could use light to nudge an earthbound rock into an orbit we could live with
Sixty-five million years ago, a Manhattan-size meteorite traveling through space at about 11 kilometers per second punched through the sky before hitting the ground near what is now Mexico’s Yucatán Peninsula. The energy released by the impact poured into the atmosphere, heating Earth’s surface. Then the dust lofted by this impact blocked out the sun, bringing years of wintry conditions everywhere, wiping out many terrestrial species, including the nonfeathered dinosaurs. Birds and mammals thus owe their ascendancy to the intersection of two orbits: that of Earth and that of a devastating visitor from deep space.
We humans need not wait, like dinosaurs, for the next big rock to drop. We have an advanced understanding of the heavens and a spacefaring technology that could soon enable us to alter the orbits of any celestial object on a collision path with us. That capability just might come in handy.
We got a taste of the challenge in December 2004, when scientists at NASA and the Jet Propulsion Laboratory (JPL), in Pasadena, Calif., estimated there was a nearly 3 percent chance that a 30-billion-kilogram rock called 99942 Apophis would slam into Earth in 2029, releasing the energy equivalent of 500 million tons of TNT. That’s enough to level small countries or raise tsunamis [PDF] that could wash away coastal cities on several continents. More recent calculations have lowered the odds of a 2029 impact to about 1 in 250 000. This time around, Apophis will probably miss us—but only by 30 000 km, less than one-tenth of the distance to the moon.
But let’s not rejoice too quickly. We know next to nothing about that asteroid’s porosity, composition, and tensile strength. It’s possible that tidal stresses during its 2029 approach could cause it to break apart, adding to the odds of an Earth impact during another rendezvous further down the line.
There is some disagreement about the best course of action. In the United States, experts tend to want to experiment with various deflection techniques by first sending robots or even astronauts to asteroids that do not threaten Earth. But in Russia, many asteroid watchers believe the risk of a collision between Apophis and Earth has been underestimated. These analysts contend that we should therefore concentrate our experiments on this particular asteroid.
To be sure of diverting any interplanetary intruder, we would need several strings to our bow. A method that could swiftly deflect a hunk of iron might blow an icy rock into several parts, each of which could then become a danger. And the gentler method now being discussed—to vaporize part of the surface of the asteroid, creating an outpouring of gas that would generate a propulsive force—would do no more than warm a meteorite made of iron. So we’ll doubtless need to devise several strategies for dealing with threatening asteroids.
So I have proposed a new tool, one that would use the pressure of light to nudge threatening objects into safe trajectories. That I’ve been asked to explain it at all in a magazine article shows that there’s indeed one thing we can rejoice in: the enhanced awareness of the problem. The mention of killer asteroids no longer raises jeering comparisons to the cries of Chicken Little, now that we know celestial impacts are far more common than once thought.
The largest and most famous Earth impact in historical times occurred in Tunguska, Siberia, in 1908, when an object perhaps 30 meters wide entered the atmosphere and exploded aboveground, with the strength of several megatons of high explosive. It leveled forests and dispersed reindeer herds, and the dust it kicked up produced colorful sunrises and sunsets throughout Europe. Fortunately, the devastated area was sparsely populated, so few people were hurt.
Astronomers now know a good deal about the nature and location of objects posing threats of both the Yucatán and Tunguska kinds. Some of these objects are comets—celestial icebergs that spend most of their time in the depths of space far from the planets. At intervals of 100 000 years or more, stars may approach our solar system closely enough to disrupt the solar orbits of some of these comets, pushing them sunward. They would then swoop through the inner solar system at great speed. It is not impossible that such a comet is what destroyed the dinosaurs.
The main threat comes from what are known as near-Earth objects. They usually reside between the orbits of Venus and Mars, although their orbits aren’t very stable. Most are eventually flung out of the solar system, but replacements wandering in from the main asteroid belt maintain their population. Some 7000 near-Earth objects have been identified so far. As many as 100 000 more, all larger than the Tunguska object, may await discovery. This guesstimate, by analysts at JPL, is based on the assumption that astronomers are far better at spotting mountains than molehills.
NASA, now joined by the European Space Agency and other space agencies, has been conducting systematic searches for these objects. The agencies hope that by 2020 they will be able to discover 90 percent of those near-Earth objects wider than 140 meters. Both terrestrial and space telescopes are involved in the effort, and amateur astronomers equipped with small, dedicated telescopes also contribute.
Most of our information on the physical properties of these objects comes from low-resolution radar images created using such devices as the 300-meter William E. Gordon Telescope at the Arecibo Observatory, in Puerto Rico. This radio telescope can reveal rotation rates and shapes, at least for the nearer objects, although there is much we must still learn. What we do know, from meteorite samples in museum collections, is that some of the interplanetary objects that strike Earth are metallic, consisting largely of iron, and that some are rocky, consisting largely of silicates. Then there are extinct comet nuclei, in which rocky layers are interleaved with volatile material—ice made of water or methane, for example.
An additional asteroid category has recently been added to the list. The Japanese space probe Hayabusa (formerly called Muse-C) arrived at asteroid Itokawa in September 2005. In a cliffhanger of a mission, the probe landed on the asteroid and retrieved samples of the surface, returning with them to Earth in June 2010.
In early 2007, I took part in a NASA Marshall Space Flight Center study of proposed deflection techniques that could be ready for use by the end of 2020. My colleagues and I assumed that by that point we’d have a heavy-lift booster capable of sending 50 000 kg or more on an Earth-escape trajectory.
We considered several strategies. The most dramatic—and the favorite of Hollywood special-effects experts—is the nuclear option. Just load up the rocket with a bunch of thermonuclear bombs, aim carefully, and light the fuse when the spacecraft approaches the target. What could be simpler? The blast would blow off enough material to alter the trajectory of the body, nudging it into an orbit that wouldn’t intersect Earth.
But what if the target is brittle? The object might then fragment, and instead of one large body targeting Earth, there could be several rocks—now highly radioactive—headed our way. Also, a lot of people might object to even the mere testing of any plan that involved lobbing 100-megaton bombs into space. The nuclear option might then be limited to a last-ditch defense of Earth, should we get little warning of an impending impact.
Another idea is to use the “kinetic” method, which essentially uses one bullet to hit another. It requires sending a small spacecraft into an orbit around the sun in the opposite direction of that of the planets and most other objects. You then maneuver this spacecraft to hit the target head-on. It would take months to accelerate something into such a retrograde orbit. Still, the job could be done using either a solar-electric [PDF] (ion) engine or a solar sail, which would use the tiny pressure that sunlight exerts on it to maneuver through space. A craft that hit the Earth-threatening rock with a relative velocity of about 60 km/s would impart a kinetic energy of 1.8 billion joules per kilogram. If the aim was perfect (no small feat at such a relative velocity), the collision could significantly alter the orbit of an asteroid—one that’s sturdy enough to take the impact without falling apart. Of course, the thing could fragment, which might just make things worse.
But if we had several decades to plan the intervention, we could apply force more gently and with better control. We could wrap a solar sail around the offending object, like aluminum foil around a potato, changing the degree to which it reflects light and thus the effective pressure that sunlight exerts on it. In this fashion, the sail could gradually alter the object’s orbit around the sun, converting an impending Earth impact into a near miss. This wrapping method ought to work for any kind of asteroid or comet.
Apollo 9 astronaut Rusty Schweickart and Bong Wie of Iowa State University have proposed yet another universally applicable solar-sail technique, called the gravity tractor [PDF]. Here a solar sail would maintain position near the threatening body for decades, exerting a small but significant gravitational attraction on that object, which over time would alter its course. The sail would move into position and remain there using the pressure of solar radiation to maneuver. This method has the advantage of working equally well on all classes of objects—metallic, stony, or rich in ice. It would take a very long time, however, to do the job.
For the 2007 NASA Marshall study I began working on yet another scheme, called the solar collector. H. Jay Melosh [PDF] of the University of Arizona and Ivan V. Nemchinov and Yu. I. Zetzer, both then affiliated with the Russian Academy of Sciences, first proposed this approach in 1994. For this strategy, the spacecraft would deploy a large parabolic reflector that would always face the sun. Although the reflector would resemble a typical solar sail, its purpose would be solely to concentrate sunlight onto a smaller flat mirror, known as a thruster sail. The thruster sail would direct concentrated sunlight onto the offending asteroid. If the object contained volatile material, the intense beam would heat things up enough to vaporize part of the surface. The gas shooting into space as a result would, over time, impart enough momentum to nudge the body’s solar trajectory away from a projected impact with Earth. It wouldn’t take much of a push, because with asteroids, unlike horseshoes, a near miss doesn’t count.
The version of this approach that I worked on for the NASA Marshall study in 2007 assumed that gas would shoot off the asteroid at a velocity of about 1 km/s. This estimate drew on an experiment Melosh and his colleagues had done long before, using a pulsed laser to heat a simulated chunk of rocky intrasolar debris. But I had some suspicions that this number was too high—that it overestimated the efficiency of this approach. So I later looked into the thermodynamics of the problem more closely.
As I described in a 2008 paper in Acta Astronautica, it turns out that much of the energy in the concentrated beam of light would simply get conducted through the rock, away from the hot spot. The beam would have to be quite powerful to ensure that the hot spot could evaporate enough volatile material to really do the job. I found that what really mattered was how deep the concentrated sunlight penetrates. Existing studies showed that most soils here on Earth allow light into just the top 100 micrometers, but measurements on extraterrestrial samples were lacking.
As an associate at the Hayden Planetarium at the American Museum of Natural History, in New York City, I was able to collaborate with Denton Ebel, curator of meteorites there. He graciously prepared two samples of the Allende meteorite, which slammed into Mexico back in 1969. It’s a carbonaceous chondrite, as are about a third of all near-Earth objects. The first sample consisted of a 30-µm-thick section epoxied to a transparent slide; the second was a finely ground simulated minimeteorite weighing just a few grams.
Both samples were loaned to the physics department at the New York City College of Technology, in Brooklyn, where I teach. There, Lufeng Leng and her student Thinh Le shone two laser beams onto the samples, one at a wavelength of 532 nanometers, in the green part of the spectrum, and the other at 650 nm, in the red part. It turned out that both samples had about the same light-penetration depths you’d expect to find in terrestrial soils. I presented those results at a meeting of the Meteoritical Society in July 2010.
Such measurements must be repeated at other wavelengths and on samples of other meteorites and, ultimately, on samples retrieved from the moon and from asteroids. That way we will be able to test extraterrestrial material that could not have been modified by a meteorite’s white-hot passage through Earth’s atmosphere.
At this stage of the analysis, it is difficult to determine how big a solar collector would be required for this strategy to work. The device would probably have to measure more than 50 meters. Building it from a thin plastic film would keep its mass down to no more than a few hundred kilograms. It would remain tightly folded on the voyage out and be unfurled only near the standoff point, at least a few hundred meters from the Earth-threatening rock. Electric propulsion would probably be necessary to maintain the collector’s position during the months or years it would take to divert the rock. Autonomous robotic control seems necessary, although astronauts could certainly monitor the process. And it might prove easier to use a number of smaller solar collectors rather than a single large one.
These are early days for designing such delicate space hardware, space sails included. But that solar sails can be manipulated in orbit and used for various purposes is no longer in doubt.
In 2010, two such sails flew in space. The more ambitious one, a square about 14 meters on a side, was launched by the Japanese space agency on an interplanetary trajectory between Earth and Venus. Called IKAROS, for Interplanetary Kite-craft Accelerated by Radiation Of the Sun, it proved that solar radiation pressure can be used both for primary propulsion and for attitude control. A follow-on craft, planned for around 2020, would use its solar sail during a close pass of the sun. There it could gain enough momentum from light pressure to swing into an orbit that would take it all the way out to explore the asteroids—called the Trojans—that trail Jupiter in its orbit.
NASA’s Nanosail-D2, approximately the same size as IKAROS but lighter, went up in late 2010 and deployed in January 2011, when the craft unfurled its sail in an Earth orbit low enough for amateur astronomers to see it. It was also low enough for atmospheric drag to affect the orbit. In this case, that was a feature, not a bug, because the point of the mission was to clean up space junk by docking with it and then using the sail to drag it down to a fiery disposal in the lower atmosphere.
It’s a good thing that solar sails have many possible uses. This helps to defray the costs of developing a technology that’s likely to be valuable should we ever discover a rogue asteroid headed our way. It also helps in overcoming the all-too-human reluctance to begin working on a problem that requires a planning horizon measured in generations or even centuries.
As with other approaches to asteroid deflection, the solar collector would not work well on all classes of interplanetary rock. If you tried to use it on an iron asteroid, the metal would instantly conduct the heat away from the hot spot. Besides, there would be no volatile material there to vaporize anyway. The asteroid would just continue on its merry way, undisturbed. A rocky body without any volatiles would also be impossible to shift in this way. So, clearly, other techniques for dealing with those kinds of asteroids must also be developed.
It does seem, though, that a solar collector could divert a 300-meter water-ice object enough to prevent an Earth impact, and while remaining on station for just a few months. Even more ambitious is the notion of using such deflection techniques to steer smaller water-ice-bearing objects into high Earth orbit, where we could mine them for materials for rocket fuel, life-support systems for space habitats, cosmic-ray shielding for such habitats, the construction of satellites to beam solar power to Earth, and other purposes. In 2010, President Obama directed NASA to prepare to send astronauts to explore near-Earth objects by the year 2025. While on that mission or on succeeding ones, astronauts could test a solar collector and other deflection techniques.
We have plenty of time to study the matter. But we do not have all the time in the world.
About the Author
Gregory L. Matloff, an emeritus associate professor of physics at New York City College of Technology, is a pioneer of what might well be termed celestial engineering. He discusses using solar sails to manipulate the rocks and ice balls that orbit the sun and occasionally collide with our planet. He has consulted with NASA on this idea, as well as on using sails for deep-space propulsion. He is a coauthor of Solar Sails: A Novel Approach to Interplanetary Travel (Springer, 2008).