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In four years, on 13 April 2029 to be exact, a large asteroid named Apophis will fly by Earth just 32,000 kilometres above the surface. Apophis, like the recently identified asteroid known as 2024 YR4, was once thought to be on a crash course with our planet. After its likelihood of hitting us was lowered to zero, and some presumed sighs of relief, astronomers began planning for the day it would make its exceptionally close flyby.

When it does, a fleet of spacecraft 鈥� featuring missions by NASA, the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) 鈥� is going to chase it down.

鈥淭his kind of approach of a body of Apophis鈥檚 size occurs only once every few thousand years,鈥� says at C么te d鈥橝zur University in France, a member of ESA鈥檚 Apophis-hunting project. 鈥淭his is extraordinary.鈥�

To meet this moment, astronomers and planetary defence researchers from around the world gathered at the University of Tokyo from 7 to 11 April for a . Each space agency shared its mission鈥檚 objectives, while making sure that they would coordinate with one another to get the most out of this opportunity.

Although multiple spacecraft will effectively be racing to get to Apophis first, 鈥渘obody is winning or losing鈥�, says Michel. If the historic team-up goes according to plan, everybody will have a better understanding of how to defend the planet from killer asteroids.

Shortly after Apophis was discovered in 2004, it caused a bit of a fright. According to early orbital calculations, this 450-metre-long asteroid 鈥� big enough to cause damage on a continental scale 鈥� stood a 2.7 per cent chance of impacting Earth in 2029. But follow-up observations this calamitous possibility. Instead, it will safely pass by Earth.

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However, it will get extremely close to the planet, passing below the height of geostationary satellites. Weather permitting, people in Europe and Africa will be able to see it with the naked eye, as a hastily moving glimmer. 鈥淚t [will be] evening twilight skies when Apophis is brightest,鈥� says at the Massachusetts Institute of Technology.

There are thousands of asteroids just like Apophis in near-Earth orbits yet to be discovered. If one is found to be heading our way, any mission attempting to deflect or destroy it would greatly benefit from knowing what their structure is like. Are they rigid objects that could absorb the impact of a ram-like spacecraft, or rubble piles prone to chaotic fragmentation?

Apophis won鈥檛 just passively pass by the planet. It鈥檚 likely to shift to a slightly different orbit, while its surface geology and interior structure will be disturbed. 鈥淭here is going to be seismic shaking on Apophis,鈥� says at the University of Arizona. These asteroid quakes will hint at its geologic make-up.

Throughout the close encounter, a bevy of , including radar observations, will give us a precise measure of the asteroid鈥檚 size, shape and trajectory. But planetary defenders have more ambitious hopes: they want to send several spacecraft after Apophis to get the best look possible before, during and after the Earth flyby.

The first of the fleet is the successor to NASA鈥檚 OSIRIS-REx mission. This spacecraft extracted pieces of rock from the asteroid Bennu in 2020, before returning to Earth in 2023 to drop a capsule containing them. This mission was primarily scientific; those stolen shards are being studied to understand the origin of both planets and life itself. But OSIRIS-REx also helped anti-asteroid research efforts: it studied the geology of the asteroid and helped us better understand how this was being affected by sunlight, which can very gently but significantly alter an asteroid鈥檚 orbit.

OSIRIS-REx has since been repurposed for planetary defence. While the spacecraft was on its way back from Bennu, its operators realised they could pilot it in such a way that it could also rendezvous with Apophis. It can no longer gather samples with its extendable arm, but it could examine the asteroid with its suite of cameras and sensors.

So, 20 minutes after the sample capsule was jettisoned toward Earth, the spacecraft fired its thrusters, sending it on a circuitous trajectory around the solar system to catch up to Apophis in 2029. The mission, now named OSIRIS-Apophis Explorer, or , is taking a gamble: it wasn鈥檛 designed to get as close to the sun as it has been. Fortunately, its instruments are holding up to the scorching heat. 鈥淓verything appears 鈥榞o鈥� for Apophis,鈥� says DellaGiustina, the principal investigator for OSIRIS-APEX.

OSIRIS-APEX will begin observing Apophis on 2 April 2029, from 5 million kilometres away, closing the gap to just 50,000 kilometres during the 13 April Earth encounter. It will finally arrive alongside Apophis on 5 June, and fly in formation with it into 2030, mapping out its surface. In September 2030, it will approach Apophis before firing its thrusters at it. Known as the Spacecraft Thruster Investigation of Regolith, or , manoeuvre, this is designed to expose the pristine geology just under the surface of the asteroid.

OSIRIS-APEX鈥檚 mission ends in December 2030. What happens then? 鈥淲e鈥檙e pretty interested in trying some daring things,鈥� says DellaGiustina. They may prod Apophis with OSIRIS-APEX鈥檚 extendable arm or even try landing on the surface. 鈥淲e would love to do something crazy, but right now, we鈥檙e just hoping to get through our nominal mission plan.鈥�

ESA鈥檚 contribution to the fleet is the Rapid Apophis Mission for Space Safety, or , spacecraft. Unlike OSIRIS-APEX, this is a work in progress. The preliminary design work has already been funded but awaits a critical go-or-no-go decision at an ESA gathering in late 2025. But there was considerable optimism in Tokyo that RAMSES would be green-lit.

at ESA, the mission鈥檚 project manager, says RAMSES will rendezvous with Apophis in February 2029, flying alongside it for at least six months. As well as monitoring it with cameras, RAMSES will deploy two miniature satellites called CubeSats. One will use ground-penetrating radar to probe the interior of Apophis, while another may land a seismometer on the asteroid, says Michel.

鈥淲e can take more risks with CubeSats, while the mother spacecraft remains at a safe position,鈥� says Michel.

Beating OSIRIS-APEX and RAMSES to the punch 鈥� if it launches in 2028, as planned 鈥� will be JAXA鈥檚 mission. Although its primary target is another asteroid named Phaethon, which it will reach in 2030, it will make a in early 2029, taking photographs to aid the incoming RAMSES and APEX spacecrafts, says at the Chiba Institute of Technology in Japan, the principal investigator of DESTINY+.

Assuming both RAMSES and DESTINY+ make it off the launch pad, the three-piece Apophis squadron must ensure it avoids an inadvertent destruction derby. 鈥淥ne coordination task is collision avoidance between spacecraft,鈥� says , an ESA planetary scientist.

But if they succeed, NASA, ESA and JAXA will get to watch Apophis transform as it wrestles with Earth鈥檚 gravity 鈥� revealing vital secrets to planetary defenders in the process.

鈥淲e have spent years considering what happens to an object when it makes such a close approach to a planet,鈥� says at Northern Arizona University. 鈥淲e finally have the opportunity to see.鈥�