What is Asteroid Apophis?
Apophis is a near-Earth asteroid that’s estimated to be about 350 metres across. That may sound small in celestial terms but to a human it’s huge. Apophis is about ten percent taller than the Eiffel Tower in Paris, France. That makes it a large asteroid. It’s an S-type asteroid, thought to be composed of mostly stoney materials with little iron or other metals in the mix. So far, so ordinary.
When was it discovered?
Apophis was discovered on 19 June 2004 by astronomers Roy A Tucker, David J Tholen and Fabrizio Bernardi, using the Kitt peak National Observatory, Arizona. It was given the name Apophis by its discoverers on 19 July 2005. In classical mythology, Apophis was the enemy of the sun-god Ra, pursuing and trying to swallow Ra every night. So, a pretty menacing name to give an asteroid – but as astronomers quickly found out, it was maybe an appropriate one.
Yikes. Why were we worried about Apophis?
Soon after its discovery, Apophis had its orbit calculated. That’s when astronomers started to keep a watchful eye on it because it was going to make a number of very close passes to Earth. A few months after discovery, on 21 December 2004, Apophis passed Earth at 14.42 million kilometres. That’s not particularly close when you consider that the Moon orbits at 0.384 million km but as astronomers extrapolated the asteroid’s orbit into the future, it became clear that it was going to make a number of very close passes to Earth.
When you say close, how close was Apophis going to approach Earth?
Very close! A flyby on 13 April 2029 will take place at around 31,600 km above the Earth’s surface. That’s more than ten times closer than the Moon and closer even than some of the telecommunication satellites that are in orbit around the Earth. Initially, astronomers calculated that there was a slight risk of it actually hitting our planet in either 2029 or during another close pass in 2036. That was concerning since Apophis is at least one and a half times bigger than the object that devastated the Tunguska region of Siberia in 1908. Thankfully, further studies of the asteroid’s orbit ruled those possibilities out, but concern still hung over yet another close pass in 2068. Now, however, everything is hunky dory again.
Phew. Why aren’t we worried any more?
On 6 March, Apophis passed Earth at roughly 17 million km. Radar observations by NASA’s Goldstone Deep Space Communications Complex in California, and the Green Bank Observatory, West Virginia, provided such accurate data on the asteroid’s position and movement that its orbit could be calculated more precisely than ever. This shows clearly that Apophis has no chance of hitting Earth any time in the next century. As a result, the European Space Agency has taken Apophis off its ‘Risk List’.
So, can we just forget about Apophis now?
No! Its orbit will continue to evolve because of gravitational interactions during its close passes of the Earth, and because of the way the asteroid responds to the Sun’s heat but at least we can be confident of its orbit for the next century or so. And there is plenty of opportunity to use Apophis for science. Because it comes so close to Earth, it will provide astronomers with great chances to study it in comparative close-up and learn more about it: the details of its composition and internal structure for example. Anything we learn from one asteroid can help us understand the others. Now that we no longer need to fear Apophis, it can become our friend – and that means if you live in Europe, Africa or western Asia, you need to get your friends together for an Apophis party on 13 April 2029.
Always up for a party, tell me more!
From those locations Apophis will be visible to the naked eye in the night sky as it passes by Earth. It will appear as a moving point of light, about as bright as a moderate star. It’s a great opportunity – and an extremely rare one – to see the celestial dance of an asteroid’s orbit with your own eyes. Mark it on your calendar now!
Wow, now I think about it, this is all pretty neat isn’t it?
Yes. Asteroids rock. So do the people that study them. Asteroid Day congratulates all who took part in this work: great job!