Three astronomers had just two days of feasible conditions at Kitt Peak National Observatory, which is located in the mountains southwest of Tucson, Arizona, on the evening of June 19, 2004, before weather and technical issues closed in. They saw something traveling through space in that small window that would soon cause planetary scientists to lose sleep for years. The item was recorded as 2004 MN4. Later, it was renamed Apophis, after the god of chaos and destruction in ancient Egypt. Over the next 20 years, it received more consistent scientific attention than nearly any other rock in the solar system.
The results of the initial orbital calculations were unsettling. It seemed likely that Apophis would hit Earth in 2029. Then, 2036 and 2068 became additional potential impact windows as more data was gathered and the models were improved. For a short while in late 2004, the probability assessments created the kind of quiet alarm within the planetary science community that spreads through emails and conversations in conference rooms rather than being announced in press releases. The orbital picture became clearer and the probabilities decreased when improved observations from Australia’s Siding Spring Observatory were received. NASA is now certain that there won’t be any impact risk for the next 100 years or more. The alarm went off. However, the interest did not.
Key Information: Asteroid 99942 Apophis
| Official Designation | 99942 Apophis — previously called 2004 MN4 |
| Name Origin | Apophis = Greek name for Apep, ancient Egyptian god of evil and destruction; also a character in Stargate SG-1 |
| Discovery Date | June 19, 2004 — Kitt Peak National Observatory, Tucson, Arizona (2.3-meter Bok telescope) |
| Discoverers | Roy Tucker (1951–2021), David Tholen, and Fabrizio Bernardi; confirmed December 20, 2004 by Siding Spring Observatory, Australia |
| Size | Mean diameter: ~1,115 ft (340 m); long axis: ~1,480 ft (450 m) — roughly the height of the Eiffel Tower |
| Composition | S-type (stony) asteroid — silicate rock, metallic nickel and iron; shape possibly peanut-like with two lobes |
| Close Approach Date | April 13, 2029 — closest known approach to Earth by an asteroid of this size in recorded history |
| Close Approach Distance | ~20,000 miles (32,000 km) from Earth’s surface — closer than geosynchronous satellites at ~22,236 miles |
| Visibility | Visible to naked eye across Eastern Hemisphere — no telescope or binoculars needed, weather permitting |
| Impact Risk | No risk of impact for at least 100 years — originally feared for 2029, 2036, and 2068 impacts before orbit was refined |
| Orbit Type | Currently “Aten” group (orbit < 1 year); after 2029 flyby, reclassified to “Apollo” group (orbit ~1.2 years) |
| Rotation | ~31 hours per spin (short axis); rocking motion ~264 hours (long axis) — non-principal axis rotation |
| NASA Mission | OSIRIS-APEX (formerly OSIRIS-REx) — redirected to rendezvous with Apophis after 2029 flyby |
| ESA Mission | Ramses (Rapid Apophis Mission for Space Safety) — will accompany Apophis through Earth flyby |
| Historical Context | Event of this scale occurs only every few thousand years — likely unprecedented in all of recorded human history |
In the years since its discovery, Apophis has evolved into something less dangerous. It has evolved into an opportunity that, according to current estimates, only happens once every few thousand years. The asteroid will travel about 20,000 miles from Earth’s surface on April 13, 2029. The satellites in geosynchronous orbit above us, which are currently relaying GPS coordinates and television signals, hover at a height of roughly 22,236 miles, to put that in perspective and make the number seem realistic. Apophis will go beneath that. Threading through the area where our satellites reside is a mountain-sized rock, roughly shaped like a peanut and as tall as the Eiffel Tower. An asteroid this size has never been observed so close to Earth in human history.
It will be apparent. It’s worth taking a moment to consider that detail. Without a telescope, binoculars, or any other equipment, anyone standing outside on a clear evening in the Eastern Hemisphere on April 13, 2029—across Europe, Africa, Asia, and Australia—will be able to see Apophis traveling across the night sky. Approximately 4.6 billion years of solar system history are carried by a dot of reflected light that crosses the darkness. That is genuinely peculiar in some way. A remnant of the planets’ formation, constructed from the same raw material that eventually became Earth, that was visible to the unaided eye. The United Nations General Assembly has already designated 2029 as the International Year of Asteroid Awareness and Planetary Defense, in part because of the significance of this event for public participation in space science.
In some ways, the science surrounding this flyby is just as fascinating as the visibility. Apophis will experience effects from Earth’s gravity as it passes that have never been measured up close for an asteroid this size. The internal composition of Apophis, including its solidity, mass distribution, and presence of voids or fractures, will determine how the rock is stretched and compressed by the gravitational pull. The way the asteroid reacts—whether its spin quickens or slows, whether surface material changes, and whether landslides happen on its steepest slopes—will reveal this internal structure. Scientists can basically work backwards to determine what is going on inside a rock they are unable to touch by accurately measuring those responses. It’s only possible because the flyby is so incredibly close; it’s like an X-ray by gravity.
Two spacecraft are en route to assist with its recording. NASA’s OSIRIS-APEX, formerly known as OSIRIS-REx, has been rerouted to meet Apophis soon after the 2029 flyby. In 2023, it successfully collected a sample from the asteroid Bennu. The Ramses mission of the European Space Agency will meet up with the asteroid and follow it through the entire close approach, monitoring the interaction with Earth in real time. In the meantime, radar stations and ground-based observatories worldwide are already working together through the International Asteroid Warning Network to make sure that every measurable aspect of the encounter is recorded from as many perspectives as possible.
When I think about April 13, 2029, I get the impression that it falls into a category of events that are truly uncommon—the kind where deep time, science, and spectacle all come together at once. It’s possible that those who named this asteroid after a god of chaos were unaware that it would turn out to be one of the century’s most eagerly awaited scientific discoveries. Apophis frightened us when it crossed Earth’s orbit. It is now providing us with something more beneficial: a few thousand years of rarity condensed into a single night, close enough to see without looking up through anything but the open air.
