Planetary defense

Of all natural disasters, an asteroid impact is the only one that humanity knows how it could prevent entirely, as NASA's planetary defense officer has put it.^[1] Prevention rests on two things: finding a dangerous asteroid years or decades ahead, and having a proven way to push it off course. In 2022, the DART mission showed that the second is possible.^[2] The missions that follow it aim to make the method reliable and the warning time longer.

DART: the first test

NASA's Double Asteroid Redirection Test (DART) tested a kinetic impactor: in the simplest terms, a spacecraft flown into an asteroid to change its motion.^[3] Its target was Dimorphos, a moonlet about 170 m across that orbits the larger asteroid Didymos, about 805 m across.^[4] Neither posed any hazard to Earth, and because Dimorphos orbits Didymos, a change in its motion could be measured from Earth by timing that orbit.^[2]

DART launched on 24 November 2021 and struck Dimorphos on 26 September 2022 at about 6.1 km per second, some 11 million km from Earth.^[4] Before the impact, Dimorphos took 11 hours and 55 minutes to circle Didymos, and NASA had set a change of 73 seconds as the minimum for success.^[2] The measured change was 33 minutes, plus or minus 1 minute.^[5] It was the first time humans had deliberately changed the motion of a celestial object.^[2]

The impact did more than the spacecraft alone could have done. It blasted more than 10 million kg of rock into space, and the recoil from that debris added to the push, roughly doubling it.^[6][7] It also reshaped Dimorphos from a squashed ball into an elongated shape, and it changed the pair's 770-day orbit around the Sun by about 0.15 seconds, a tiny shift that, given enough time, can grow into a significant deflection.^[4][7] The DART team concluded that warning time is what matters most: several years at a minimum, and preferably decades.^[3] More on the target pair is on the Didymos and Dimorphos page.

Hera: measuring the result

ESA's Hera mission is the follow-up investigation. It will make a detailed survey of Dimorphos after the impact, with particular attention to DART's crater and a precise measurement of the moonlet's mass, turning a single experiment into a technique that can be repeated with confidence.^[8][2] It is also the first probe to rendezvous with a binary asteroid.^[8]

Hera launched on 7 October 2024 and flew past Mars in March 2025.^[9] In October 2026 it began the braking burns that slow it to match the speed of the asteroids, and it is due to arrive at Didymos in November 2026.^[10][9] It carries two small CubeSats, Milani and Juventas, which will be released to make their own close-up studies.^[9]

NEO Surveyor: finding threats earlier

Deflection only works with enough warning, and the hardest asteroids to warn about are the ones not yet found. NASA's goal is to find at least 90% of near-Earth objects 140 m and larger.^[6] NEO Surveyor is the first space telescope designed specifically to find asteroids and comets that could be hazardous to Earth.^[11] It observes in infrared light, in which dark asteroids glow as the Sun warms them, and it can search toward the Sun's direction, where ground-based telescopes are blinded by glare.^[11] It is scheduled to launch no earlier than September 2027.^[11]

Ramses and OSIRIS-APEX: Apophis up close

On 13 April 2029, the asteroid Apophis, roughly 375 m across, will pass about 32,000 km from Earth, less than a tenth of the distance to the Moon.^[12] ESA's Ramses mission, built with Japan's space agency JAXA, is planned to launch in spring 2028 and meet Apophis before the flyby, to observe how its shape, spin and structure respond to Earth's gravity.^[12] NASA's OSIRIS-APEX, the spacecraft that returned samples from Bennu under its earlier name OSIRIS-REx, is due to reach Apophis in June 2029 to study the changes after the encounter.^[13] Apophis itself poses no risk. The flyby is a rare natural experiment, and ESA describes what it will reveal about how asteroids respond to outside forces as critical knowledge for future planetary defense.^[12]