Asteroid Apophis - existential threat or spectacular sky show?

This blog was posted (slightly edited) as an article in Crux, the Magazine of the Astronomical Society of Victoria, vol.39 no.2 April 2021.

Our neighbours the asteroids

Asteroids are pretty much everywhere in our solar system. Typically, they have highly elongated orbits, meaning they can cross the orbits of the planets, as the planets have much less eccentric (that is, nearly circular) orbits.

There are several classes of asteroid, determined by the amount if time they spend inside or outside Earth's orbit.

  • Atira - spend their entire time inside the Earth's orbit
  • Aten - cross Earth's orbit but spend the majority of time nearer to the Sun
  • Apollo - cross Earth's orbit but spend the majority of time further out than the Sun
  • Amor - spend their entire time outside Earth's orbit

Most of the Atira and Amor classes are quite safe, although some of them can approach Earth's orbit closely.

However, the Aten and Apollo types don't only approach, but actually cross Earth's orbit. While astronomers know of no asteroids that are going to collide with the Earth, they do study the paths of these asteroids closely.

Meet Apophis

99942 Apophis (2004 MN4), or simply "Apophis" to its friends, is a large Aten type asteroid, about a third of a kilometre across. You can see an early image here (taken by the University of Hawaii). 

It looks small and unassuming, but it's about 370 metres across. That's as large as the MCG.

Apophis, like all asteroids, orbits the Sun, and it spends most of its time inside the Earth's orbit. Have a look at the orbit here (courtesy of the JPL Small-body Database Browser site). The vertical lines sprouting from Apophis' orbit show how it relates to the plane of Earth's orbit.

The places where the planes of the two orbits intersect are called the "nodes". The "descending node" is when Apophis stops being above the Earth and crosses to being below the Earth (in relation to the Earth's orbit). You can see from the diagram that Apophis' descending node is far from Earth's orbit.

Quite safe.

Mostly harmless

But have a look at the ascending node. The point where Apophis rises through the Earth's orbital plane is right on the Earth's actual orbit. The Earth is in this position in around April each year.

This is a problem. And this is when astronomers start looking really closely.

In order to precisely calculate the probability of a collision, astronomers need to develop a highly accurate understanding of Apophis' orbit.

...we think

But there's another problem - the Yarkovsky Effect. Sunlight exerts a pressure on everything it hits. That pressure is very slight, but it's enough to become significant when you're dealing with problems like this. The pressure exerted depends on a number of factors, such as reflectivity, regolith type and distribution, as well as rate and direction of rotation of the body.

It's complicated and we need to know a lot about Apophis to be able to estimate the Yarkovsky Effect on Apophis's orbit.

So what are we doing?

Close approaches, when Apophis is near the Earth, enable astronomers to get highly accurate observations of the Asteroid's orbit. Importantly, this will enable them to understand more about the Yarkovsky Effect. In turn, this will enable them to estimate the possibilities of collision in future.

Close approach March 2021

The next close pass is only a few weeks away, on 5 March 2021. It's not a super close approach, remember that collisions can only happen in April of each year.

Apophis will sweep past the star Alphard, which is around 45° above the horizon in the North East (from Melbourne) at 10pm. It won't be close, though, the nearest it will get will be over 40 times the distance to the Moon. It also won't appear to move quickly - it will cover only about a degree of sky in a day!

Astrophotographers should be able to pick up the asteroid in camera images. It will change its position gradually over time, like my photos of Ceres here. This is actually the same series of photos, but one tracks the asteroid, the other tracks the background stars.

Close approach 2029 - don't miss this one!

There are a couple more notable passes over the next few decades. The closest will be in April 2029, when Apophis will - safely - graze past, just 31,000 km above the Earth. Orbital calculations for this pass have taken our current understanding of the Yarkovski Effect into account, so sit back and enjoy the show, this one is quite safe.

And it will be a spectacular show, because at 31,000 km, the asteroid is close enough to knock out some of our higher communication satellites! During the approach, the asteroid will be as bright as magnitude 3.1, or similar in brightness to, say, Ginan, the "fifth" star in the Southern Cross. That is, it will be visible to the naked eye from everywhere but inner-city areas.

From SE Australia, at this point, Stellarium suggests that we should be able to see it between about 8pm (when it will be high in the North) until when it sets after midnight. At its closest point, the asteroid will appear to move about the width of the Moon every minute.

What we don't know is exactly what Apophis is going to do next. During the fly-by, it'll be under the gravitational influence of the Earth, meaning its orbit will be deflected. This deflection will be so much that Apophis will switch from being an Aten-type asteroid to an Apollo-type one, spending the majority of its time outside Earth's orbit. However, whether the asteroid will change its physical shape under the tug of Earth's gravity remains to be seen.

...and the next close pass?

The next very close pass will be in April 2068. I probably won't be around to see it (if I am, I'd be 104 years old). Although the probability of a collision is very remote, astronomers are less certain of how close it will come.

And what will we learn?

It already looks like the chances of a collision with Apophis in our lifetimes are close to zero. Even so, future fly-bys are not only an opportunity to see the asteroid, but also a chance to study its physical makeup and behaviour. Doing so, we'll learn more about the Yarkovsky Effect, and this sill enable us to make better predictions of where Apophis, and other asteroids, will travel.

Importantly, we'll get a better understanding of the risks facing the Earth.


JPL Small-Body Database Browser