On Friday, April 13, 2029, Earth will experience a dramatic close encounter with the asteroid 99942 Apophis. The 1,120 feet (340-meter) wide object will pass within just 19,000 miles (31,000 km) of our home planet — a distance that brings it closer than most geostationary satellites.
Thanks to the tremendous size of Apophis, its close-passage will be so bright that around 2 billion people will be able to witness it with the naked eye. First becoming visible in the southern hemisphere, the asteroid will appear as a bright star streaking across the sky from east to west, initially passing over Australia, then the Indian Ocean, and eventually crossing the equator over Africa.
Fortunately, this spectacular and historic event won't be as severe as experts once thought. Initially, scientists were unsure whether the passage of Apophis would result in a collision with Earth.
Even when this risk was negated, researchers still could not rule out the possibility that Earth's gravitational effects could influence the asteroid in such a way that one of its next visits could result in an impact — something that was only ruled out this year, Space.com previously reported.
"Apophis is in the category of Potentially Hazardous Asteroids (PHAs) — asteroids with orbits that bring them very close to Earth now and for centuries in the future," said Richard Binzel, professor of planetary sciences at the Massachusetts Institute of Technology in Cambridge. "What makes Apophis the poster child for potentially hazardous asteroids is that it will make the closest known approach to Earth of any large asteroid this decade," he said.
"But the three most important things about Apophis are: Apophis will miss the Earth. Apophis will miss the Earth. Apophis will miss the Earth."
Though we now know Apophis won't strike Earth in the near future, the asteroid has sat at the top of the European Space Agency's "impact risk list" of PHAs and NASA's Sentry Risk Table for 17 years. And because of its forthcoming visits, its time as an object of intense scientific interest is just beginning.
What we know about Apophis
Apophis was discovered on June 19, 2004, by Roy A. Tucker, David J. Tholen and Fabrizio Bernardi whilst working at the Kitt Peak National Observatory. The object was identified at the time as 2004 MN4. Though the team was only able to observe the asteroid for two days as a result of technical issues and unfavorable weather conditions, further observations quickly revealed a wealth of details about the asteroid.
In 2005, the asteroid was renamed "Apophis" after the Greek name for an ancient Egyptian representation of evil, a serpent that dwells in darkness and frequently attempts to devour the sun-god Ra as he makes his nightly passage across the sky.
In the same year that Apophis received its new moniker, Binzel was part of a team that used reconnaissance telescopic observations to measure the color properties of Apophis and determine its composition. The group found that the asteroid resembles the most common class of meteorites known as "ordinary chondrites," which are composed mostly of stone and silicates.
Classified as an "S-Type" or stoney-type asteroid, according to NASA, Apophis also contains a mix of metals like nickel and iron. Its shape is believed to be elongated and comprising of two lobes — almost like a rocky space peanut.
"Apophis is a piece of an early solar system planetesimal — a planetary building block that never coalesced into one of our solar system's planets. It has been stored in the asteroid belt for 4.6 billion years and may be a fragment of a larger asteroid that broke apart in a collision in the asteroid belt," Binzel said. "That collision may have cast the fragment we call Apophis on a trajectory that resulted in its current orbit that comes very close to Earth."
The excitement of Apophis' discovery quickly turned to concern when researchers calculated just how close the asteroid's orbit would bring it to Earth.
From visual observations taken in 2004, researchers calculated there was around a 2.7% chance that the PHA would hit Earth in 2029. These probabilities were refined with radar observations the following year. These estimates may seem low but were still extreme enough to give Apophis the highest values on two systems used by astronomers to calculate how dangerous an asteroid is to our planet, the Torino scale (created by Binzel) and the Palermo Technical Impact Hazard Scale.
By 2006, the probability of Apophis hitting Earth in 2029 was all but negated, yet future impacts, particularly in 2068, could not be dismissed as easily. The trajectory of Apophis after its 2029 flyby will depend on how Earth's gravity changes the asteroid's orbit, said Davide Farnocchia, an astrophysicist at NASA's Jet Propulsion Laboratory (JPL) who is studying the asteroid's trajectory. "If Apophis passed through specific gravitational keyholes, the gravity of the Earth would tweak its orbit by just the right amount — or should I say the wrong amount — to put it on a collision course in a later year," Farnocchia said.
Because astronomers were unable to track Apophis from 2015 to 2019 due to the fact it was located too close to the sun and therefore obscured by its light output, further progress on ruling out a 2068 impact was not forthcoming. Thus, the asteroid seemed almost stubbornly determined to live up to its apocalyptic name.
When the asteroid once again moved away from our stars and thus could be better observed, Farnocchia and his team resolved to tackle the problem head-on and better determine the asteroid's trajectory, finally resolving if it would impact Earth in 2068.
"At JPL's Center of Near-Earth Object Studies (CNEOS) we compute high-precision trajectories and assess the possibility of future impacts for all known asteroids," said Farnocchia, who relished the challenge that Apophis presented. "That's our daily bread and butter."
Apophis will not hit Earth
Unseating Apophis from the top of various space agencies' risk lists was no easy feat and required a much better determination of the asteroid's orbit. Farnocchia and his colleagues weaved together radar and optical tracking data collected in late 2020 and early 2021 to come up with a precise trajectory for Apophis, according to a statement from NASA. The team then took this newly acquired data and combined it with years of intense observation to come up with a clearer picture of Apophis' trajectory.
"Apophis has been extensively tracked since its discovery by both optical and radar telescopes," Farnocchia said. "Thanks to these data collected by astronomers all over the world we can precisely predict the motion of Apophis through the April 2029 flyby."
The team was able to calculate that the orbital period of Apophis, or the time it takes to orbit the sun, will be stretched from 0.9 years to 1.2 years as a result of its 2029 encounter with Earth.
Perhaps more significantly, data from 2020 and 2021 observations allowed the CNEOS crew to model how close Apophis will come to the gravitational keyholes that correspond to potential impacts with Earth in both 2036 and 2068. The CNEOS team was also able to rule out any chance of a potential impact for the next 100 years, Farnocchia said.
What if Apophis hits Earth?
Predicting what an impact between our planet and an asteroid the size, shape and density of Apophis would look like is not an easy task. There certainly isn't anything like it in the geological record of our planet — other impacts have either been much larger, such as the Chicxulub that struck Earth 66 million years ago — or much smaller. Additionally, there are many factors that have to be considered when estimating the damage that could arise from such a collision. These factors include asteroid size, density and mass, as well as the angle and velocity at which the asteroid strikes.
"We have run simulations of an impact between Apophis and Earth, but at 340 meters across, the outcome of an Apophis-scale impact with Earth is very uncertain," said Gareth Collins, a professor in the Department of Earth Science & Engineering at Imperial College London, United Kingdom. "Size and speed are the main factors, but the nature of the target site is also important . If the impact occurs in the ocean, it can generate hazardous tsunamis; on land, a lot more ejected dust is produced. And, of course, a major factor is how close the impact happens to human populations."
Collins estimates that were Apophis to strike Earth at 45,000 mph (20 kilometers per second), — the average speed of asteroid impacts — the energy released would be about 10 billion, billion Joules (that's 1 followed by 19 zeros). "This is equivalent to the explosive yield of the global nuclear arsenal," he said. "About 100,000 times more than the energy of the Chelyabinsk meteor and a million times more energy than the bombs dropped on Hiroshima."
The worst-case scenario for an impact this size would be if it were to occur close to a large population center or near a heavily populated coastline, Collins said. Were this to occur, devastating consequences would arise from a number of secondary effects, such as violent ground shaking, intense thermal radiation and atmospheric shock waves.
"Unless an asteroid similar to Apophis hits Earth and we can measure the consequences, our program will remain a 'best guess' and subject to large uncertainties," Collins said. "But, close approaches do help us better understand asteroids and their likelihood of striking Earth in future."
What will scientists learn from future fly-bys?
With the threat of an impact from Apophis negated for a while, researchers can now focus on the collection of scientific information from its flybys. With eight years of planning still remaining before its next visit, the sky really is the limit for Apophis investigation.
In particular, Farnocchia and his team will take advantage of this valuable scientific opportunity by observing how Apophis reacts to tidal forces when so close to Earth. "Among other things, the rotation state of Apophis could change," as a result of its close proximity, Farnocchia said. Observations of Apophis in 2029 should also help scientists better ascertain its size, shape, composition and even model its interior for the first time.
"By watching how Apophis might shake, rattle and roll, even just by a tiny amount, we will learn how it is put together on its inside," Binzel said, similar to how seismic waves traveling through Earth, the moon and Mars have been used to figure out what's beneath their surfaces. "Interior structure for a potentially hazardous asteroid is something we have never measured before. Apophis might just give us that chance."
Because Apophis seems to be similar to around 80% of the PHAs identified so far, collecting data on it could help scientists learn how we might divert future PHAs should they threaten Earth in the future.
"Knowing how PHAs are put together might be some of the most valuable space physics knowledge ever obtained, in the event we ever had to put that knowledge to use to defend our planet from some future asteroid impact," Binzel said.
Beyond this, however, the passage of Apophis in 2029 is a historic spectacle that shouldn't be missed. "Objects of the size of Apophis come this close to Earth approximately only once every thousand years, on average," Farnocchia said. "Don't miss the chance to see it. I know I won't!"
- Estimate what would happen when an asteroid of any size hits Earth using the Earth Impact Effects Program, created by Davide Farnocchia and his colleagues.
- Watch an animation of asteroid Apophis' 2029 close approach with Earth in this video from NASA's JPL.
- Learn more about other asteroids that will also make close approaches to Earth, from NASA's JPL.
Editor's note: This article was updated on Sept. 28, 2021 to include two corrections. First, the 2.7% probability calculated in 2004 was originally based on visual observations, not radar astronomy data. And second, the average speed of asteroid impacts to Earth is 45,000 mph, not 45 mph.