All the Myriad Ways

The asteroid Apophis is extremely unlikely to hit the Earth any time soon, but we do know that it’s slated to make two close passes, closing to a distance of 36,000 kilometers or so in 2029 and again in 2036. These events should give us pause — this is an object some 335 meters in diameter weighing an estimated 25 million tons. It’s 90 stories tall, if you like to think in skyscraper terms, which is what Greg Matloff probably likes to do, given that the physicist and asteroid deflection expert works at New York City College of Technology (City Tech).

Of Apophis, Matloff says, “We don’t always know this far ahead of time that they’re coming, but an Apophis impact is very unlikely.” A good thing, too, for a strike by an object of this size would be catastrophic. This City Tech news release offers a look at Matloff’s ideas on what to do if we find a Near-Earth Object on a collision course. He’s a proponent of diverting rather than destroying such objects because of the potential for debris striking the Earth after an explosion.

Asteroid Deflection the Slow Way

Wouldn’t we need a huge nuclear explosion to divert an asteroid’s trajectory in the first place? Not necessarily. Matloff worked with a team from Marshall Space Flight Center (Huntsville, AL) in 2007 to study methods for deflecting NEOs, finding that heating the surface could alter an object’s trajectory. That heating project is another potential use for the solar sail technologies Matloff has been investigating for the past thirty years, going back to the days of a seminal paper in interstellar studies (written with Eugene Mallove) called “Solar Sail Starships: Clipper Ships of the Galaxy,” which appeared back in 1981 in the Journal of the British Interplanetary Society.

But when it comes to asteroids, solar sails play a different role than leading humanity’s push to the stars. The idea is to configure twin solar sails to act as concentrators of sunlight. Imagine a highly reflective sail that faces the Sun, focusing solar photons on a smaller thruster sail. Both sails would be stationed alongside a Near-Earth Object, with the thruster sail focusing sunlight on its surface. In his book Paradise Regained: The Regreening of Earth (with Les Johnson and C. Bangs), Matloff notes the result:

> [The] thruster directs a concentrated sunbeam on the NEO’s surface. If the NEO is coated with layers of dust, soil, or ice, a jet of superheated material (like a comet’s tail) may be raised in the direction of the thruster sail. The reaction force to this jet pushed the NEO in the opposite direction.

The potential is to create a jet stream of sufficient strength that, over time, it would nudge the NEO into a different trajectory. Creating a steerable jet involves penetrating the object’s surface with photons, but by just the right amount to create the deflection. According to Matloff, it could be as little as a tenth of a millimeter.

Probing an Asteroidal Surface

Here the need for missions to one or more NEOs again comes into focus, but while we wait for the development of the necessary tools and funding, Matloff and colleagues at City Tech are working with red and green lasers to study how deeply they penetrate asteroidal rock, using meteorite samples from the Allende meteorite that fell in Mexico in 1969. Their first results were presented at the recent gathering of the Meteoritical Society, which met in New York last July. “To my knowledge,” says Matloff, “this is the first experimental measurement of the optical transmission of asteroid samples.”

And given the significance of the work, we can assume it won’t be the last. We won’t know whether creating a jet stream by long and slow application of light reflected off a solar sail will work on an actual object without analyzing a wide variety of Near-Earth Objects. And that raises the question of how to proceed. The City Tech story quotes Matloff on the matter:

> “At present, a debate is underway between American and Russian space agencies regarding Apophis. The Russians believe that we should schedule a mission to this object probably before the first bypass because Earth-produced gravitational effects during that initial pass could conceivably alter the trajectory and properties of the object. On the other hand, Americans generally believe that while an Apophis impact is very unlikely on either pass, we should conduct experiments on an asteroid that runs no risk of ever threatening our home planet.”

In any case, further City Tech work by physicist Lufeng Leng has shown through scans of the Allende sample that the composition of the surface material through which the light passes governs the depth of the light’s reach. The results show that lasers from a space vehicle placed near an NEO can help us understand its composition, allowing subsequent sail missions to focus solar photons with the precision needed to create the trajectory-bending jet stream. It’s an ingenious use for solar sails, but we’d better be sure we understand the objects we’re heating well enough to ensure a successful result.

Consider: We have much to learn about the mechanics of keeping a twin solar sail mission deployed on station near an NEO for long periods of time. Moreover, a deflection option like this one (or a similar idea Matloff discusses, in which astronauts land on an asteroid and set up a highly reflective thin film sail on the surface to exert a small but constant force on the NEO), are optimised only for certain kinds of NEOs. Would they work on a ‘rubble pile’ asteroid that’s barely held together by its own gravity? Other options, like the so-called ‘gravity tractor,’ seem more useful in that context, a reminder that NEO deflection may have many potential solutions.

Related: Ray Villard on asteroid deflection. The next Sputnik moment?


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