Science1 publisher2 min readPublished
A Beihang preprint sends a diffractive solar sail backward around the sun to hit asteroids head-on
Researchers at Beihang University simulated a sail that steers by diffracting sunlight sideways, and report that a head-on impact at about 100 km/s would deliver 230 times DART's energy per kilogram of impactor.
The Scientist · Science desk
What happened
- Researchers at Beihang University propose deflecting a potentially hazardous asteroid by hitting it head-on with an impactor carried on a new type of solar sail, in work posted to the arXiv preprint server.
- DART approached its target from behind, traveling the same direction as the asteroid, and struck Dimorphos at about 6 km/s.
- The paper puts a head-on closing speed at around 100 km/s, which it says would impart something like 230 times the energy per kilogram of impactor mass that DART delivered.
- Reaching a retrograde orbit means using the sail as a brake until angular momentum hits zero, diving toward the sun, then firing at perihelion to slingshot back outward the wrong way.
- Among the sail configurations the team simulated for that trajectory, reflection-type diffractive sails came out best.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint The rocket equation puts a retrograde orbit out of reach for chemical rockets, so a head-on impactor mission exists only if sail hardware can fly the reversal.
- capability Because the thrust direction no longer requires tilting the sail off the sun line, a trajectory that was impractical for mirror sails becomes a candidate design.
- decision The advantage is quoted per kilogram of impactor, so the design question a planner faces is payload mass: a fast sail that arrives light delivers less than its speed suggests.
- exposure With mirror-type sails the ones that have flown, qualifying grating film becomes the first hardware milestone for any mission built on this concept.
The 230 is worth checking against the scaling it rests on. Kinetic energy grows with the square of speed [6]. Against DART's roughly 6 km/s, a 100 km/s impact is a speed ratio of about 16.7, and squaring that gives about 278 times the energy per kilogram [16][17]. The paper's number is lower. Both input speeds are given as approximations in the phys.org account [3][5], and running 230 backwards gives an effective ratio near 15, or a closing speed of about 91 km/s against 6 km/s [19].
Moving a rock is a momentum problem, though. A deflection is a change in the asteroid's own velocity, and momentum goes with the first power of speed: at equal impactor mass, a 100 km/s hit brings about 17 times DART's momentum, not 230 [18]. The figure is stated per kilogram of impactor mass [5], so the total depends on how much mass the sail carries through the reversal and into the target.
The film matters for a geometric reason. A mirror sail's thrust points perpendicular to its surface [11], so pushing sideways at perihelion means tilting the sail far off the sun line, and a tilted sail presents a smaller target for sunlight and loses efficiency [12]. Diffractive film steers optically instead. Microstructured gratings throw the light out sideways while the sail holds its whole face to the sun. That also makes attitude control easier [13].
phys.org writes that DART's trailing approach could work given enough warning but would require years of orbital maneuvers [15]. How long the reversal itself takes is missing from the account: braking angular momentum to zero, falling to perihelion, then climbing back out retrograde [10]. Impact speed and arrival date are different quantities. Until a cruise duration is published next to the impact speed, this is evidence about energy delivered at impact, and the warning-time question stays open.
What to watch
- The cruise duration for the H-reversal in the preprint itself. That number would let planners compare it against the years of orbital maneuvers a trailing intercept needs.
- Peer review of the Beihang simulations, and whether the advantage of reflection-type diffractive sails survives it.
- Any in-space test of grating film, since the sails flown so far steer by tilting a mirror.