On 13 April 2029, an asteroid the size of a large skyscraper, roughly 375 metres across and named Apophis, will sweep past Earth closer than many telecommunications satellites orbit. And for once, we will be able to watch. Under clear, dark skies, an estimated two billion people across Europe, Africa and parts of Asia should be able to see it cross the night sky with the naked eye.
The single most important thing to say about that is also the most reassuring: it is a close pass, not a threat. Apophis will not hit Earth.
How close is close
The numbers are startling. Apophis will pass about 31,600 kilometres above Earth’s surface, which is roughly one tenth of the distance to the Moon and only about five times the radius of the Earth itself. Crucially, that is inside the ring of geostationary satellites, the ones sitting some 36,000 kilometres up that relay much of our television and communications. An asteroid this large will actually pass beneath them.
For an object of this size, that is extraordinarily near. Encounters this close by an asteroid a few hundred metres wide are thought to happen only about once every several thousand years, which is why 2029 has been designated a United Nations year of asteroid awareness and planetary defence.
It is not going to hit us
Given all that, the reassurance bears repeating and explaining. When Apophis was discovered in 2004, early calculations briefly suggested a small chance it could strike Earth in 2029, and later in 2036 and 2068. That is how it earned its ominous name, after an ancient Egyptian god of chaos.
Those fears have since been laid to rest. Sharper tracking, in particular precise radar measurements in 2021, refined the asteroid’s path and removed every one of those impact scenarios. Space agencies now state that Apophis poses no risk of striking Earth for at least the next hundred years. The 2029 encounter is a fly-by, and a safe one.
A naked-eye asteroid
What makes 2029 remarkable for ordinary people is the visibility. Almost all asteroids are far too faint to see without a telescope, mere specks even in professional images. Apophis, passing so close, will briefly brighten to about the level of a modest star, bright enough to pick out with the unaided eye.
It will not look like a dramatic fireball. It will appear as a star-like point of light, but a moving one, drifting steadily across the sky over a few hours rather than holding still like the real stars behind it. The best view falls to the Eastern Hemisphere, across Europe, Africa and western Asia, where up to two billion people could watch it, weather and dark skies permitting. Seeing an asteroid move across the sky with your own eyes is something no generation before ours has been able to do on this scale.
Why scientists are excited
For researchers, a close pass by a several-hundred-metre asteroid is a rare natural experiment. As Apophis skims by, Earth’s gravity will pull on it hard enough to change its orbit and its spin, and quite possibly to reshape its surface, triggering tiny landslides or seismic shaking on a body that is likely a loosely bound pile of rubble. Watching that happen would teach us a great deal about how such asteroids are built and how they behave.
It is also a chance to rehearse planetary defence in slow motion, on a real object we can study in detail rather than a hypothetical one. Understanding the structure of asteroids like Apophis is exactly the knowledge we would need if one ever were on a collision course.
Spacecraft are being sent to meet it
Two missions aim to be there. NASA has redirected the spacecraft that returned samples from asteroid Bennu, renaming it OSIRIS-APEX, and sent it toward Apophis, where it is due to arrive shortly after the 2029 flyby to study the freshly disturbed asteroid up close. The European Space Agency, meanwhile, has been developing a mission called Ramses, intended to reach Apophis before the encounter so it can observe the asteroid in detail as Earth’s gravity works on it, capturing the changes as they unfold rather than only their aftermath.
What to watch
The date to circle is 13 April 2029, which happens to fall on a Friday the 13th, a coincidence that will not be lost on anyone. Between now and then, the things to watch are whether Ramses is built and launched in time to arrive ahead of the flyby, the progress of OSIRIS-APEX on its way to a post-encounter rendezvous, and increasingly precise forecasts of exactly how bright Apophis will get and where its path will cross the sky.
For most of us, though, the event will need no equipment and no expertise. On one spring night, a mountain-sized rock will pass close enough to slip beneath our satellites, bright enough to see, and harmless. It is the kind of encounter that happens on the scale of millennia, and this time we will be watching.