Planetary defense experts use infamous asteroid Apophis to practice spotting dangerous space rocks

An animation shows Apophis' 2029 path compared to the swarm of satellites orbiting Earth.
An animation shows Apophis' 2029 path compared to the swarm of satellites orbiting Earth. (Image credit: NASA/JPL-Caltech)

Earth's most recent brush with asteroid danger was eight years ago, when a space rock the size of a six-story building came seemingly out of nowhere, injuring 1,200 people when it exploded over Chelyabinsk, Russia.

Now, scientists are using this month's flyby of the infamous asteroid Apophis to test their responses to potentially hazardous space rocks, honing the fine art of planetary defense. Planetary defense focuses on identifying asteroids and comets that hang out around Earth, mapping their precise paths and seeing how their orbits compare with Earth's.

If an orbital model shows that an asteroid and Earth are due to reach the same place at the same time, things get serious, particularly when the space rock is large. That's the sort of scenario that ended the dinosaurs' reign, after all. But planetary defense isn't hopeless: if humans identify a dangerous asteroid long enough before impact, we could theoretically do something to divert it.

Related: Scientists prepare for their last good look at asteroid Apophis before 2029 flyby

Successfully preventing damage from an asteroid impact will depend on spotting the threat in time, which takes practice. But although scientists have identified more than 25,000 near-Earth asteroids to date, the majority are too small to cause much worry. So while there are plenty of asteroids rattling around Earth's orbit, most aren't big enough or close enough to trigger realistic existential angst.

Apophis came to its fame because it isn't like most of these near-Earth space rocks. When scientists discovered it in 2004, it stood out right away. First, it is relatively large — more than 1,000 feet (300 meters) wide, around the height of the Eiffel Tower, according to NASA. And models based on early observations suggested a nearly 3% chance Apophis would collide with Earth on April 13, 2029.

More precise observations soon put the fear of impact that year to rest, but the early concern surrounding the asteroid prompted its name, which references an Egyptian "demon serpent who personified evil and chaos," as NASA put it. Right now, scientists are confident that Apophis is no threat to Earth for at least a few decades. But the space rock will still come visiting next month, offering scientists valuable opportunities to get a close look at a relatively large asteroid.

And, with a little imagination, these flybys can also serve as planetary defense rehearsals.

"The goal is to basically wrangle all the scientists from around the world, kind of the coalition of the willing," Vishnu Reddy, a planetary defense expert at the University of Arizona who is coordinating the project, told "Then we go on this months-long campaign, trying to observe this object."

Related: Huge asteroid Apophis flies by Earth on Friday the 13th in 2029, a lucky day for scientists

Apophis will fly past Earth on March 5. The asteroid will remain about one-tenth the average Earth-sun distance away — a downright mundane flyby compared to the 2029 event, when Apophis will pass by at about the altitude at which particularly high satellites orbit.

To mark this year's flyby, the International Asteroid Warning Network instituted its third such campaign. Previously, scientists have practiced on an asteroid called 2012 TC4 and on 1999 KW4, which is a pair of rocks circling each other. For Apophis, about 40 scientists from 13 different countries have signed on. These observers are pretending that Apophis has never been seen before, which means they are starting from scratch in terms of evaluating how much danger the asteroid poses to Earth.

"It's not a scientific goal," Reddy said. "The goal is to get new observations as if we don't know anything about this object and try and see where in the process we need to improve efficiency and also identify the human factor. Anybody dealing with scientists knows that it's like herding cats, and when you do that on an international scale, there's part diplomacy, part science, and part planetary defense."

Reddy said that the coincidence of the Apophis flyby occurring during the continuing COVID-19 pandemic offered an opportunity to understand how resilient the asteroid detection system is. At this point, most telescopes are managing to continue operating, although he said the pandemic likely would have interfered much more had the flyby occurred a year earlier, when institutions were still scrambling to respond.

"There's a reasonable amount of redundancy in planetary defense," he said. "Even if one telescope goes down or we lose a certain thing, it's not like the whole community goes down, to some extent."

A "discovery" moment

Campaign members first definitively identified Apophis around Dec. 19, thanks to a space-based instrument called NEOWISE. After the spacecraft, originally an astrophysics mission, lost the ability to keep itself cool, scientists reinvented it to identify new asteroids.

That the first discovery credit went to NEOWISE was unusual, however. These days, most new asteroids are discovered by ground-based surveys, particularly the Pan-STARRS telescopes in Hawaii and the Catalina Sky Survey in Arizona.

But Apophis was tricky for Earth-based telescopes to discover this time around, Reddy said. "The challenge was that its motion and the way it was moving, it was hard to do it with short observing arcs," he said. NEOWISE is never pulled away from a target by the Earth's rotation, and since it studies infrared light, it can observe during day and night alike.

"NEOWISE got it first, and that's because of where we look in the sky and the wavelengths that we use to search for it," Amy Mainzer, an astronomer at the University of Arizona and principal investigator for NEOWISE, told "To get to, quote-unquote, discover Apophis was really fun."

There's a perk to spotting a new asteroid with NEOWISE: The instrument can quickly determine the size of a space rock. Telescopes that identify asteroids using optical light have trouble distinguishing between larger dark rocks and smaller light rocks; NEOWISE's infrared sight eliminates that confusion.

Related: Defending Earth against dangerous asteroids: Q&A with NASA's Lindley Johnson

And for this "discovery" of Apophis, the initial optical appearance turned out to be more menacing than NEOWISE's calculated diameter. "You can quickly tell if it's going to be a global catastrophe or just regional because you can get the size quickly," Mainzer said. "It makes an orders of magnitude difference in the prediction of hazard."

While a newly discovered asteroid may initially sport a slight chance of impact, additional observations typically whittle that chance to nothing. "For Apophis, if you start afresh, assuming that we don't know anything about it and we just discovered it with NEOWISE and we're following it up, there is a reasonable impact risk from Apophis," Reddy said.

It's worth emphasizing that "reasonable impact risk" appears only in the scenario they're practicing, not in the reality of scientists' full knowledge of the asteroid. "Of course, if you include the historical observations, it goes away, because we know its orbit very well, it's not going to hit us," Reddy said.

Good news for humans, but that disappearing impact risk isn't helpful to the practice session. "That's not what we're looking for, we want to see what are the effects on the ground," Reddy said. So at some point, the campaign will part with reality, and, in the safe confines of computer models, scientists will nudge Apophis toward Earth.

Altering the asteroid's trajectory lets planetary defense specialists practice each stage of the asteroid response process, the point, after all, of the campaign.

Continuing observations

Apophis has become much easier to observe since its "discovery"; by now, Reddy said, amateur astronomers can spot it with a decent backyard telescope, and he looks forward to including them in the project.

Meanwhile, for scientists, the initial hustle of "discovery" has calmed somewhat. "There's a huge flurry of activity, and then there's not a whole lot in the middle for five or six weeks," Reddy said. The team, ignoring old data about Apophis, already has much of the information a planetary defense expert would want, data that helps scientists determine an object's shape or composition.

"The key event after this point, I think, is the radar observations," he said; these observations should begin March 3. Planetary radar observations, which bounce a beam of radio waves off an object then catch the echo, are the most powerful tool scientists have to study the precise shape and location of an asteroid.

These observations are often the final data scientists need to confirm that an asteroid will safely pass by Earth. The responsibility for those observations will fall to NASA's Goldstone Deep Space Communications Complex in California, after the December collapse of the radio telescope at Arecibo Observatory in Puerto Rico.

Related: Losing Arecibo's giant dish leaves humans more vulnerable to space rocks, scientists say

But NEOWISE isn't done with Apophis either, Mainzer said. Although the instrument moved onto other work after the "discovery," the asteroid will be back in the NEOWISE field of view after its close approach. "We're almost certain to detect it again in April," she said.

That will be another valuable science opportunity. "If we see it in December, we look at it from one direction; when we get it in April, we're probably going to see from a different direction, right?" Mainzer said. By seeing Apophis multiple times at different angles, scientists may be able to get a glimpse of the space rock's surface.

"We can combine that information to hopefully pull out some details, I'm hoping we can get some details of the surface roughness, the thickness of any rocks and dust on the surface of the object," Mainzer said. "It may help us learn quite a bit more about it, if we're very lucky."

Email Meghan Bartels at or follow her on Twitter @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook. Senior Writer

Meghan is a senior writer at and has more than five years' experience as a science journalist based in New York City. She joined in July 2018, with previous writing published in outlets including Newsweek and Audubon. Meghan earned an MA in science journalism from New York University and a BA in classics from Georgetown University, and in her free time she enjoys reading and visiting museums. Follow her on Twitter at @meghanbartels.