How diffractive solar sails could stop a killer asteroid at 100 km/s

Asteroids don’t come with a warning label that they might one day hit Earth. While we don’t know of any currently on course to do so, we are finding thousands of new ones each year, and there’s always a chance that one could.

We proved a viable technique for dealing with that possibility: The DART mission successfully moved a small asteroid using a “kinetic impactor“—basically a big rod designed to push the asteroid off its trajectory. But DART had one big flaw: It was traveling in the same direction as the asteroid and approached it from behind, eventually hitting it with an impactor launched while in orbit around it. That technique might work if we have enough warning, but it would require years of orbital maneuvers. An alternative proposed by researchers at Beihang University is to use a new type of solar sail to deflect a potentially hazardous asteroid by hitting it head-on. The work is published on the arXiv preprint server.

Deflecting an asteroid using an impactor comes down to one simple factor: energy. The more energy transferred from the impactor to the asteroid, the more the asteroid will move. And the thing about energy is that it increases with the square of velocity. In other words, the faster you hit something, the more energy you impart to it.

DART smashed into Dimorphos, its target asteroid, at about 6 km/s—a respectable speed, but one limited by the fact that it was coming from the same direction. According to the new paper, an impactor using a solar sail could approach an asteroid from the opposite direction—akin to a wrong-way highway driver—and smash into its target at around 100 km/s, imparting something like 230 times the energy per kilogram of impactor mass that the DART test did.

The problem is getting the spacecraft moving in the opposite direction. Almost everything orbits the sun counterclockwise, including Earth. As such, when something launches from Earth, it’s already moving that way. Reversing direction and entering a clockwise (retrograde) orbit requires a lot of energy, to put it mildly. Chemical rockets aren’t capable of it—the tyranny of Tsiolkovsky’s rocket equation makes it impossible for them to carry enough fuel to complete that maneuver. But solar sails potentially can.

Solar sails work by harnessing solar radiation pressure—the tiny amount of force that a photon imparts when it hits something. In essence, they sail on sunlight rather than requiring fuel like a traditional chemical or ion rocket. Back in the 1990s, Italian engineer Giancarlo Vulpetti realized you could use this technology to get a solar sail into what he called an H-reversal trajectory—essentially a way to make the sail start orbiting backward.

This technique first uses the sail as a brake, slowing the craft’s angular momentum. After it drops to zero, the sun’s gravity does its work, pulling the craft toward our star in a deep dive. But in Vulpetti’s trajectory, the sail kicks back on during the craft’s closest approach (perihelion) and slingshots back outward into a retrograde orbit. It sounds like a solution for getting an impactor moving in the right direction to impart a massive amount of energy to an asteroid. There’s only one problem: Solar sails are notoriously clumsy.

We’ve successfully tested a few in space—JAXA’s IKAROS mission and The Planetary Society’s LightSail 2 are examples of traditional solar sails, which essentially act like giant mirrors. In their case, the light that bounces off them leaves at the same angle at which it arrives. The thrust is always perpendicular to the sail’s surface. That situation isn’t ideal for an H-reversal trajectory, as it means you have to tilt the sail at an extreme angle to the sun. That presents a much smaller target for sunlight, dramatically reducing the sail’s efficiency.

Enter the diffractive solar sail. Instead of traditional reflective film, the material these sails are made of uses microstructured optical gratings to diffract light sideways without the sail itself turning. That means the sail can keep its whole face toward the sun, even at perihelion, but direct all that thrust into sideways motion. This dramatically simplifies attitude control and still provides the significant push needed to pull off this tricky orbital maneuver.

The researchers simulated several solar sail configurations for this H-reversal trajectory and found that reflection-type diffractive sails were the clear winner. In their simulations, every other type either fell into the sun or was pushed out of the solar system entirely. But finding the right type of sail was just the first step—their second simulation explored what might happen with another famous asteroid: Apophis.

Apophis is a 340-meter-wide (1,115-foot-wide) asteroid that will come within 38,000 kilometers (23,600 miles) of Earth on April 13, 2029. While that in itself does not warrant a deflection, the researchers decided to see what they could do with this well-studied asteroid. They simulated a diffractive-reflective sail in two configurations—one set with a permanent diffraction angle, and one that could switch from a moderate angle for its dive toward the sun to an angle of around 90 degrees near perihelion.

Both versions were able to deliver a 100 km/s impactor directly to Apophis in around 200–300 days from launch. That shaves nearly a year off the mission time of a standard reflective solar sail—a difference that could determine whether an asteroid is deflected or strikes Earth.

Hopefully, we will never need to use this technology to deflect a potentially hazardous asteroid. But realistically, we will face one someday. Knowing that this technique could deliver hundreds of times more energy than the more mundane co-orbital impacts we’ve proved work so far is heartening. At least we know there’s another tool in our toolbelt to deal with these hazards, even if we never have to use it.

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Gaby Clark

MA in English, copy editor since 2021 with experience in higher education and health content. Dedicated to trustworthy science news.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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