Today, 30th June is asteroid day, to raise awareness of the searches astronomers do to detect and eventually deflect asteroids. This is your chance also to actually do something about them by signing the 100x petition (which has been signed by many famous astronomers and astronauts).

An asteroid impact is one of the few natural events we can actually prevent with our technology (unlike volcanoes, earthquakes and tsunami). With a few years or decades of warning, we can deflect them rather easily. But to find them in good time, first we need to detect them.

The main focus here is on smaller asteroids, up to ones large enough to destroy a major city. For instance, an asteroid large enough to flatten the whole of London or New York. We are much more likely to spot one of those than the larger ones. 

The chance of those also is tiny. After all, there haven't been any records of an asteroid that flattened a city throughout the whole of recorded history. But there are many more people in the world now, and also many more cities for them to hit. It can happen. 

The famous Tunguska impact in the early twentieth century would have totally destroyed a city if it hadn't hit in such a remote place. These are the ones we should be most concerned about, not the largely fictional world destroying ones you see in movies (even the dinosaurs' asteroid was not nearly as large as most of the movie asteroids - we've had nothing as large as those hit any of the planets or moons in the inner solar system since the end of the "late heavy bombardment" well over three billion years ago). 

Luckily this one landed in a remote uninhabited area of Siberia and instead just flattened many square kilometers of forest, over a region of diameter about 30 km or so.

An asteroid this large will hit Earth only every few centuries. And most will hit the ocean or an uninhabited area. But you would want to know if one was headed our way. Given plenty of warning, we can stop it easily.

Then we can detect smaller ones eventually, down to the size of the asteroid that hit Russia last year. The way it works for those - that it's easiest to detect them during a flyby of Earth. But the Earth is a tiny target so an asteroid typically does many flybys of Earth before it hits it. 

With each flyby we refine the orbit more and more exactly. And if we see that there is a chance of it hitting Earth - well we can deflect it, maybe a year or so before the next flyby. A tiny deflection at that point, in some cases as small as a millionth of a meter per second change in velocity - could shift it enough so it misses Earth next time around, because these flybys are so sensitive to tiny shifts in position during its closest pass of Earth. 

As we continue this search, eventually, we might spot one of the larger ones also, up to 10 km or so. If so, the chances are it is predicted to hit Earth several centuries, or a few thousand years into the future. (only 1 in a million chance of a large impact before the end of the century). 

There are many methods now known that could deflect asteroids. Even sometimes just paint it white. But first we need to detect them.

This is the launch of the asteroid day campaign last year.

And a video about the importance of asteroid day:

And many videos of famous astronauts and astronomers who have signed the petition on facebook here: asteroid day (facebook).

And Phil Platt's talk about the dinosaur's "Really bad day"


You may have read recent asteroid scare stories. But these are generally nonsense. For instance the one that's been most in the news recently, as due to hit Earth on Sept 24th, will actually miss Earth on that day by over 20 times the distance to the Moon.

it is easy to look up its Closest approach table. Notice that it misses by 0.0625 AU. If you aren't used to astronomical units, that may seem a close miss. But that's a distance of 9.35 million kilometers. The Moon is only 0.3844 million kilometers away. So that's a big miss there. And the minimum distance is 8.33 million kilometers. So no need to be concerned about that one!

You can keep up to date with potential impact hazards here:

Current Impact Risks

It's easy to check. Just look and see what colour they are.

To find out more see The Torino Impact Hazard Scale

If there is one that is a significant risk it will be coloured orange or red in the Current Impact Risks table. 

If they are all white, blue or green, no need to be worried about it at all.

So far none of them have ever gone orange or red. If that does happen - we will get lots of media frenzy for sure, but be aware, that the chances are very high that it gets gets reclassified to white as more measurements are made.

We don't know of any asteroid headed our way at present. But they are easy to miss, especially the smallest ones. That's why we have the Space Guard program and why we now have asteroid day.


If you are curious about how painting an asteroid white can deflect it, here is a short video of professor Dave Hyland talking about the idea:

And article
Asteroids No Match For Paint Gun, Says Prof | Texas A&M Today
(or How to Deflect Killer Asteroids With Spray Paint | WIRED)

Amongst the most favoured approaches are, direct kinetic (non nuclear) impact (obvious thing to do really) - and the gravity tractor

But a huge number of other ideas have been explored, some in a great deal of detail. For some of them see the wikipedia article:  Asteroid impact avoidance


So far our asteroid detection is done from Earth. For instance with Pan-STARRS, dedicated wide angle telescope - that's what you need, not a high magnification telescope like Hubble.

But if we can send space telescopes into independent orbit around the sun, especially if they are closer to the sun, then they will be able to spot faint close NEOs much more quickly, so that we can complete the search for them sooner.

This is the idea of the B612 foundation (named after the "Little Prince"'s planet) which has been working for some time on its Sentinel telescope idea

It would sit inside of Earth close to Venus's orbit giving it a good field of view of NEOs close to the sun. It looks away from the sun to avoid being blinded by it - and it can then see faint NEOs that are in between the Earth and the Sun which is the hardest place to spot them from our current Earth based surveys. It looks in infra red because the asteroids are far more obvious in the infrared.

Eventually it would spot just about everything out there that's in the vicinity of the Earth orbit.

Idea is that it would find nearly all  potential impactors down to 40 meters diameter. And recently announced, that it should be able to spot them down to 20 meters diameter.

They hope to launch it in 2017 to 2018 on a Falcon 9. And to find 90% of NEOs down to 140 meters within ten years and a significant proportion of all NEOs down to 30 meters.

Anyone who is really keen to support them with their work can help them out with a donation on their website.

You can also sign the Asteroid day petition to increase funding of asteroid detection by 100 times 100x Declaration

For all of you who panic about the news stories and movies about the really big asteroids - well - there's no conspiracy, nobody hiding anything, it's not possible that there could be. This must be one of the silliest conspiracy theories out there when look into it in detail. See Is It True That An Asteroid Will Strike Earth On [Insert Date Here]? - Truth Behind Asteroid Scare Stories

But there is a real threat. Some day Earth will definitely be hit by these medium sized and small asteroids. Some time in the next hundred million years it will almost certainly be hit by a large one also (though not quite as large as in the movies). And this is your chance to do something to prevent these impacts.


I've written a kindle book, which goes into all this in more depth. It also covers the material in my  Is It True That An Asteroid Will Strike Earth On [Insert Date Here]? - Truth Behind Asteroid Scare Stories and several of my quora answers on this topic.

You can get it to read on your kindle, or kindle app (available for most operating systems)

Estimated length - equivalent to 97 pages in a printed book

How Can You Tell If an Asteroid Is Headed Your Way?: The Truth Behind Asteroid Scare Stories (Amazon kindle)