Apophis

On 13 April 2029, the asteroid Apophis will pass about 32,000 km from Earth's surface, closer than many satellites in geosynchronous orbit.^[1] NASA puts a pass this close by an asteroid this large at about once every few thousand years; an earlier JPL analysis said about once every 800 years on average, and ESA gives once every 5,000 to 10,000 years.^[2][3][4] ESA sets that beside a total solar eclipse, which happens somewhere on Earth about every 18 months, and Halley's Comet, which returns every 76 years.^[4] For a short while, people across the Eastern Hemisphere will be able to see it without a telescope or binoculars.^[2]

From the top of the risk list to no risk

Apophis was discovered on 19 June 2004 by Roy Tucker, David Tholen and Fabrizio Bernardi at Kitt Peak National Observatory in Arizona, with the observatory's 2.3 m Bok telescope; technical and weather problems let them follow it for only two nights.^[2] Siding Spring Observatory in Australia found it again on 20 December 2004.^[2] With the new positions, NASA's Sentry system found a possible impact on 13 April 2029, and on 24 December 2004 the chance rose to about 1.6%, level 4 on the Torino scale, the highest rating any asteroid has had.^[5] For a short time the estimate reached 2.7%.^[3]

The answer came from old photographs. On 27 December 2004, Jeff Larsen and Anne Descour of the Spacewatch project near Tucson found faint images of the asteroid on archive frames taken on 15 March 2004, three months before its discovery; the longer track pinned the orbit down well enough to rule out an impact in 2029 with Earth or the Moon.^[6] Radar from the Arecibo Observatory in January 2005 then placed the 2029 pass at about 5.7 Earth radii from the planet's center, roughly 36,000 km, and found no later encounter of concern in this century.^[7]

Two dates stayed open longer. A possible impact in 2036 was ruled out in January 2013, after telescopes and NASA's Goldstone radar tracked Apophis during a distant pass that month.^[8] The last, in 2068, was ruled out in March 2021, after Goldstone tracked it from about 17 million km away.^[9] NASA now finds no risk of an impact for at least 100 years.^[9] Apophis remains a potentially hazardous asteroid, a label that describes its orbit, not a prediction of impact.^[1]

What it is

The name is the Greek form of Apep, the ancient Egyptian god of evil and destruction; the International Astronomical Union approved it in 2005, and before that the asteroid was 2004 MN4, number 99942 in the catalogue.^[2] David Tholen has said the television character of the same name was not the source, whatever has been reported.^[2]

NASA estimates Apophis at about 340 m across, roughly the height of the Eiffel Tower; ESA gives about 375 m, an example of how size estimates differ.^[9][10] Its spectrum resembles that of LL chondrites, a common kind of stony meteorite.^[3] NASA classes it as an S-type, or stony, asteroid, silicate rock mixed with nickel and iron, elongated and possibly made of two lobes, at least 450 m along its long axis.^[2] It tumbles end over end about once every 31 hours, with a slower rocking motion that repeats about every 264 hours.^[2] It formed about 4.6 billion years ago in the main asteroid belt between Mars and Jupiter, and the pull of the giant planets moved its orbit inward over time.^[2]

What the flyby will do to it

NASA describes what Earth will do to Apophis in four words: pull, twist, stretch and squeeze.^[2] The pull enlarges its orbit: today Apophis circles the Sun in about 0.9 years, inside Earth's orbit for most of each loop; afterward its year will be about 1.2 years, and it moves from the Aten group of asteroids to the Apollo group.^[2] The twist will probably change how fast it spins and which way it points.^[2] The stretching and squeezing may set off small landslides, most likely on its steepest slopes.^[2] ESA's scientists expect the same tidal forces to expose material from beneath the surface, and possibly to cause quakes.^[4]

That is why the flyby draws spacecraft. Measuring the asteroid before and after gives its mass, density, porosity and interior structure, the properties that matter most when planning how to push a hazardous asteroid off course.^[4]

Where and how to see it

The pass favors the Eastern Hemisphere, where, weather permitting, Apophis will be visible without a spacecraft, telescope or binoculars.^[2] ESA puts the audience at about two billion people in Europe, Africa and parts of Asia, under clear, dark skies.^[4] At closest approach it will look like a moderately bright point of light moving quickly across the sky, passing over the mid-Atlantic.^[3] A JPL estimate made after the 2005 radar work put it at about magnitude 3.3, the brightness of a middling star, moving about 42 degrees an hour through the constellation Cancer, fast enough that its motion against the stars shows within a minute or two.^[7]

The countdown at the top of this page gives the moment of closest approach in your own time zone. Plan for a dark spot away from city lights, and let your eyes adjust. Observatories around the world will be watching too, in a campaign coordinated through the International Asteroid Warning Network.^[2]

Two spacecraft for one flyby

ESA's Ramses mission is planned to launch in April 2028 and reach Apophis in February 2029, two months before the flyby, to watch these changes as they happen.^[4] ESA and Japan's space agency JAXA signed the agreement for it in May 2026; JAXA supplies the spacecraft's solar arrays, an infrared imager and the launch on its H3 rocket.^[11] JAXA's own DESTINY+ rides the same rocket and is to fly past Apophis first, as a rehearsal, before going on to the asteroid Phaethon.^[12]

NASA's OSIRIS-APEX is the spacecraft formerly called OSIRIS-REx, which returned samples from Bennu.^[13] On 27 April 2022 NASA extended its mission by nine years to redirect it to Apophis, with Dani DellaGiustina of the University of Arizona as its principal investigator.^[14] It is due to arrive in June 2029 to study the results, and may dip toward the surface and fire its engines to stir up loose rock and dust for a look at the material just below.^[13] Ramses also shows that a reconnaissance spacecraft can reach an approaching asteroid within a few years of the decision to go, a first step in answering a real threat.^[4] Why that matters for future deflection is covered in Planetary defense.

Questions people ask

Will Apophis hit Earth?

No. NASA finds no risk of an impact for at least 100 years, and the 2029 pass itself was ruled out within days of the alarm in December 2004.^[9][6] The "potentially hazardous" label it still carries describes an orbit that comes near Earth's, not a prediction.^[1] How such odds are worked out, and why they usually fall to zero, is explained in Impact risk.

Why was it once so feared?

Because for a few days in December 2004 the numbers were real: a 1.6% chance of an impact on 13 April 2029, level 4 on the Torino scale, the highest any asteroid has reached, and briefly 2.7%.^[5][3] The fear ended the way these cases usually do, with more observations shrinking the uncertainty until Earth fell outside it.^[6]

Can I see it with my own eyes?

Yes, from the Eastern Hemisphere, weather permitting, without a telescope or binoculars.^[2] ESA names Europe, Africa and parts of Asia.^[4] It will be a point of light moving quickly, not a streak or a disc.^[3]

Could it hit a satellite?

It passes inside the ring of geosynchronous satellites, but JPL's analysis finds that it does not threaten the satellites in that region.^[3]

Why do headlines call it the "God of Chaos"?

The name comes from Apep, the Egyptian god of evil and destruction, chosen when the asteroid still looked like a threat.^[2] "God of chaos" is the headline writers' version. The asteroid is not a danger; it is a rare chance to watch a planet reshape a small world.^[9]

What happens after 2029?

Apophis leaves on a larger orbit, taking about 1.2 years to circle the Sun, and is reclassified from the Aten group to the Apollo group.^[2] Its next pass by Earth on 13 April 2036 is distant, more than 49 million km away.^[3] OSIRIS-APEX stays with it from June 2029 to study how the flyby changed it.^[13]