Cosmic close shaves

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Astronomy / Geology / Myths / Physics

What were you doing at 3:19am Tuesday 27 January? I was sound asleep and had no idea of the ‘near miss’ that occurred. Even if you were on the other side of the globe and wide-awake, it’s unlikely you were aware of what happened. That’s when 2004 BL86 passed by Earth. Huh? 2004 BL86 is a huge asteroid that passed within 1.2 million kilometres of our planet. Fortunately that’s about three times the distance to the moon so we weren’t in danger. But how often do rocks from space actually collide with Earth? And what happens when they do?

What really happens when rocks from space crash into Earth?

What really happens when rocks from space crash into Earth?

Science fiction?

If you saw the movies Deep Impact or Armageddon, you’re familiar with Hollywood’s nightmarish scenario when it comes to deadly space rocks: we’ll know the asteroid is coming, we’ll have enough time to panic, fall madly in love, and try some hair-brained schemes to divert its course (or blow it up!). If the asteroid hits Earth, our entire planet will become uninhabitable in less than a day.

Here’s a Discovery channel simulation of such an event – and it’s set to Pink Floyd for added impact (pardon the pun). Global BBQ anyone?

Let’s hope Bruce Willis and his rag-tag team of deep-core drillers is available to save humanity if we find out annihilation by asteroid is imminent.

The lingo

Before we go any further, let’s be clear about the differences between an asteroid, a meteor and a meteorite.

Put very simply, an asteroid is a large rocky object in space that is smaller than a planet. There are millions orbiting the sun, and they can be hundreds of kilometres wide.

A smaller object orbiting the Sun is a meteoroid. These objects are all astronomically interesting because they were debris from the period when our solar system was forming.

A meteor — commonly called a shooting star — is an asteroid/meteoroid or other object that burns and vaporises when it enters Earth’s atmosphere. If a meteor actually makes it to Earth’s surface before completely burning up first, it’s called a meteorite. And the hole that the meteorite makes is known as a meteor crater.

A short history

The fact we haven’t had to rely on Bruce Willis to save us yet could lead you to think it’s rare for a meteorite to slam into Earth; but think again. Until 2012, a total of 34,513 meteorites had been officially registered. But most meteorites are small (96% of the registered meteorites weighed less than 10 kilograms), and most of them land in water or in uninhabited areas so they don’t cause much of a stir. For example, scientists think a two-kilometre wide circle in the Antarctic ice was caused by a meteorite the size of a house that hit in 2004.

There have been some big asteroid impacts during Earth’s history.

On 30 June 1908, a massive asteroid exploded above Tunguska, Siberia, releasing energy equivalent to about 185 Hiroshima bombs. On 15 February 2013, an 18-metre wide rock exploded above the Russian town of Chelyabinsk, causing many injuries and widespread damage. And of course, scientists think the large asteroid responsible for the Chicxulub crater on the Yukatan Pensinsula in Mexico was at least partly to blame for the demise of the dinosaurs.

Size does matter. The asteroid that missed us back in January was about 300 metres across. The asteroid responsible for the dinosaur’s demise was probably about ten kilometres wide. Last year, scientists published evidence that a massive asteroid, up to 58 kilometres wide smashed into South Africa some 3.26 billion years ago.

Scientists have calculated that an asteroid the size of the one that exploded in Russia in 2013 is likely to hit Earth once every 150 years on average. And models suggest an asteroid more than 1.7 kilometres wide will only collide with Earth once every 250,000 to 500,000 years.

How safe are we?

I found many websites wanting to convince me the end is nigh and we are soon to be obliterated by an asteroid. Last year the documentary Asteroid Attack was posted on the website of the Russian federal space agency, suggesting a 400-metre space rock 2014 UR116 could theoretically collide with Earth. NASA was quick to point out it poses no threat. NASA’s Near Earth Object (NEO) Observations Program suggests it has identified 96% of the asteroids out there that are one kilometre wide or larger.

It’s easy to dismiss as ridiculous the many sensationalist claims we are doomed but in fact this year the first official Asteroid Day will take place on 30 June. It’s a day to launch a ‘Global movement to protect earth from Asteroids’ and the Asteroid Day Declaration has been signed by more than 100 scientists, astronauts (and rock stars). The declaration basically says that we need to do whatever it takes to ensure that we prevent future asteroid attacks and preserve life on Earth.

How do we do that? The Sentinel Mission, launching in 2018–2019 is one way. It’s an infrared space telescope to identify well in advance any asteroids that may pose a threat. The NASA astronauts behind the Mission say it’s relatively easy to prevent an asteroid impact, if you know it’s coming. We have several promising technologies which could deflect an asteroid, including one called a gravity tractor, which I like the sound of.

Phew. Fortunately it seems Bruce Willis isn’t our only option.

Links and stuff

Radio on demand

 This post accompanies a radio segment on Triple R’s Breakfasters program on Wednesday 11 March 2015. 


  1. Yeah, a space post.

    I’ve done a bit of research too into the probability of meteorites hitting earth and the ranges are incredible – it just goes to show that it’s not an exact science when it comes to trying to predict the future.

    I remember reading somewhere that painting part of of the meteoroid’s surface with a reflective paint would give it a sideways kick as it absorbed the sun’s heat – enough of a kick to change its trajectory. I can see the next Bruce Willis movie now – Bruce the asteroid painter – probably would be a flop. 🙂

    • Ooh, I haven’t heard that approach. There’s quite a lot of info about what we could do but that’s a new one. And makes sense! I think I agree with you about the movie plot though 😉

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