China plans to crash a spacecraft into a distant asteroid

SPACE ROCKS NASADARTDARTXFNASA ESADARTESA DART ESA DART DARTXFXFcollide with Earth’s atmosphere all the time. The vast majority are too small to do any damage, but the risk of a larger asteroid (or comet) wiping out a city is one that space agencies around the world are taking seriously. The first goal of such would-be planetary defenders is detection: finding all objects capable of wreaking such damage and confirming that they are not on a collision course with Earth. The other goal is testing technologies that could either destroy such asteroids or else, more plausibly, deflect them away from the planet.Only one nation has thus far tested such technologies in deep space: America. In 2022 its space agency, , collided a semi-autonomous spacecraft (capable of self-steering as well as being remotely piloted) into a 160-metre-long asteroid named Dimorphos, itself orbiting a larger asteroid called Didymos. () mission successfully nudged Dimorphos off course while also keeping it from flying off on an unknown, and potentially hazardous, trajectory—proof that, in a genuine emergency, deflection is the better part of valour.Two years on, seems keen to do something similar. According to a presentation at a space science conference in South Korea earlier this year, and a paper published in the spring, China plans to crash a -style spacecraft into an asteroid while another vehicle floats nearby and chronicles the collision. The current target is the near-Earth object 2015 261, a 30-metre-long asteroid tens of millions of kilometres away from Earth. Such small asteroids are an important area of study. They strike Earth far more frequently (relatively speaking) than the Dimorphos-size “city killers”, for a start. And, although they do not pack enough punch to annihilate a metropolis, they could cause shock waves to radiate out from the site of impact that would be similar to those produced by a mid-air nuclear detonation. China has been signalling its intention to conduct such an experiment for some time. “Every time I look into it, it seems like the details have changed,” says Harrison Agrusa, a planetary scientist at the Côte d’Azur Observatory in France. “[But] they do seem serious about testing something.” The mission, tentatively set to launch no later than 2030, will involve the observer craft, remotely controlled from Earth, arriving at the asteroid between three and six months ahead of its impactor partner, and documenting the fallout for up to a year afterwards.This mission architecture is reminiscent of an earlier concept put forward by and the European Space Agency (): the latter was originally supposed to send an observer spacecraft to Dimorphos in advance of . But funding disagreements within tanked the observer portion of the mission, and largely relied on Earth-based observations to monitor the impact. (has since developed , a spacecraft that launched in October, to examine the impact had on Dimorphos.) For China’s mission, however, having a nearby observer spacecraft in place may be critical to its success. For one thing, the intended target will be considerably smaller and dimmer, making it harder for a -like spacecraft to autonomously track and follow ahead of a deliberate, high-speed collision.There are other challenges. Unlike Dimorphos, 2015 261 doesn’t orbit a larger asteroid, so if it is deflected by the spacecraft, it isn’t clear which trajectory it will wind up following. And because the asteroid is so tiny, and the impactor spacecraft will crash into it at breakneck speed, “I think the biggest concern is that they completely destroy the asteroid,” says Dr Agrusa. This is a chaotic outcome that planetary-defence researchers usually want to avoid, to minimise the risk of asteroidal shards flying in all directions. Fortunately, if this were to happen to 2015 261, the fragments would probably be too small to pose a threat to life on Earth.