Planetary Defence: We Have Now Moved an Asteroid

Check the dates. Survey completion and follow up mission timelines move with funding. Treat the years here as intent.

Every other natural disaster is something to survive. An asteroid impact is the only one that could, in principle, be prevented outright, given enough warning.

In September 2022 that stopped being theoretical. NASA's DART spacecraft struck Dimorphos, a small moon orbiting the asteroid Didymos, at roughly six kilometres per second. The goal was to shorten Dimorphos's orbit around Didymos by at least 73 seconds. It shortened it by about 32 minutes.

Why It Worked So Much Better Than Expected

A spacecraft hitting a rock transfers its own momentum. But the impact also blasts debris off the surface, and that debris flies away in the opposite direction, pushing back on the asteroid like a brief rocket exhaust.

That recoil did most of the work. Dimorphos turned out to be a rubble pile, a loose aggregation held together weakly by its own gravity rather than a solid body, so a great deal of material was thrown clear.

This is encouraging and complicating at once. Encouraging because the effect multiplies your spacecraft's punch several times over. Complicating because the multiplier depends heavily on what the asteroid is made of and how it is put together, and you cannot know that reliably from a telescope.

Finding Them First

Deflection is useless without warning, and warning means finding the object years ahead.

Surveys have catalogued the large majority of near Earth asteroids larger than a kilometre, the size that causes global effects. None of the known ones threaten Earth in the foreseeable future.

The gap is the middle range, objects between roughly 50 and 300 metres. Too small to end civilisation, easily large enough to destroy a city or generate a serious tsunami, and far more numerous. Most have not been found.

Detection is hardest for objects approaching from the direction of the Sun, which are lost in glare from ground telescopes. That is what happened at Chelyabinsk in 2013: a roughly 20 metre object arrived unseen and injured around 1,500 people, mostly from glass blown in by the shockwave. Infrared space telescopes positioned to look at the inner solar system are the answer, because asteroids are bright in infrared even when dark in visible light.

The Options If Something Is Found

Kinetic impact. Hit it, as DART did. Best with years of warning, since a small velocity change accumulates into a large miss distance over time.

Gravity tractor. Park a heavy spacecraft alongside and let mutual gravity tug it gradually. Very slow, very controllable, and it does not care what the asteroid is made of.

Ion beam or laser ablation. Vaporise surface material continuously to create thrust. Untested at scale.

Nuclear. A standoff detonation to ablate one side and push the object. The option of last resort for a large object with short warning, and legally and politically fraught.

The pattern is consistent: the more warning, the gentler and more reliable the method.

What Comes Next

ESA's Hera mission is going back to Dimorphos to survey the crater and measure the asteroid's mass and structure precisely. Without that, the efficiency of the DART strike can only be estimated. With it, the deflection can be turned into a calibrated model that would apply to a real threat.

The remaining work is unglamorous: complete the survey of the mid size population, keep the models honest, and maintain the international arrangements for who decides and who acts. The technical question has largely been answered. The rest is organisation and attention.