Apophis: The Space Rock That Started a Panic

asteroid similar to Apophis
This is a typical near-Earth asteroid that could be deflected into our orbit with possible catastrophic consequences. Asteroid 99942 Apophis is similar to this one, and will pass near Earth on close trajectories in 2029 and 2036. NASA

Our planet has undergone many close calls with invaders from space throughout its history. A few have even smacked into our world, causing widespread damage. Just ask the dinosaurs, whose end was hastened 65 million years ago by a piece of errant space rock a few hundred meters across. It could happen again, and scientists are the lookout for incoming impactors.

Enter Apophis: The Earth-orbit-crossing Asteroid

In 2004, planetary scientists discovered an asteroid that looked like it was on a collision course with Earth within a few decades.

Since there's not really a way to deflect incoming asteroids (yet), the discovery was a stark reminder that Earth shares space with lots of objects that hit it.

The discoverers, Roy A. Tucker, David Tholen, and Fabrizio Bernardi, used Kitt Peak Observatory to find the rock, and once they confirmed its existence, assigned a temporary number to it: 2004 MN4. Later on, it was given a permanent asteroid number of 99942 and they suggested it be named Apophis after a villain in the show Stargate, and harks back to ancient Greek legends about a serpent that threatened the Egyptian god Ra.

A lot of very deep calculations took place after the discovery of Apophis because, based on orbital dynamics, it seemed very possible that this little bit of space rock would be aimed squarely at Earth on one of its future orbits. No one was sure if it would hit the planet, but it seemed clear that Apophis would pass through a gravitational keyhole near Earth that would deflect its orbit just enough that the asteroid would collide with Earth in 2036.

It was a scary prospect and people began observing and charting the orbit of Apophis very closely.

Searching out Apophis

NASA's automated sky search called Sentry made further observations, and other astronomers in Europe used a program called NEODyS to track it as well. As the word got out, many more observers joined the search to contribute as much orbital data as they could.

All observations point to a very close approach to Earth on April 13, 2029 — so close that a collision could occur. During that flyby, Apophis will be closer to the planet than some of the many geosynchronous communications satellites we use, passing within 31,200 kilometers.

It now appears that Apophis will not slam into Earth that day. However, the flyby will change Apophis's trajectory slightly, but it will not be enough to send the asteroid on an impact trajectory in 2036. First, the size of the keyhole Apophis has to pass through is only going to be about a kilometer across, and astronomers have calculated that it will completely miss that keyhole. That means Apophis will sail on by Earth, at a distance of least 23 million kilometers.

Safe, For Now

The detection and refinement of Apophis's orbit by a world-wide skywatching community was a good test of the observational systems that NASA and other agencies have in place for near-Earth asteroids that might stray into our orbital path. More could be done, and groups such as the Secure World Foundation and the B612 Foundation are researching further ways that we can detect these things before they get too close. In the future, they hope to have deflection systems set up to ward off incoming impactors that would significantly damage our planet (and us!).

More about Apophis

So, what is Apophis? It's a massive space rock about 350 meters across and part of a population of near-Earth asteroids that regularly cross our planet's orbit. It's irregularly shaped and looks fairly dark, although during its passes by Earth it should be bright enough to spot with the naked eye or a telescope. Planetary scientists call it a Class Sq asteroid. Class S means it is mainly made of silicate rock, and the q designation means it has some metallic features in its spectrum. It's very similar to carbonaceous-type planetesimals that formed our Earth and the other rocky worlds. In the future, as humans branch out to do further space exploration, such asteroids as Apophis might well become sites for mining and mineral extraction. 

Missions to Apophis

In the wake of the "near-miss" scare, a number of groups at NASA, ESA, and other institutions began looking at possible missions to deflect and study Apophis.

There are several ways to change an asteroid's path, given the right time and technology. Attaching rockets or explosives to gently nudge an asteroid slightly off its path is one, although mission planners need to be very careful not to take it into a more dangerous orbit. Another idea is to use a so-called "gravity tractor" to orbit a spacecraft around the asteroid and use the mutual gravitational pull to change the asteroid's trajectory. No specific missions are underway right now, but as more Near-Earth asteroids are found, such a technological solution may well get built to prevent a future catastrophe. Currently, there are somewhere between 1,500 known NEOs orbiting out there in the dark, and there could be many more. At least, for now, we don't have to worry about 99942 Apophis making a direct hit.