The Big Question: Is there any realistic chance of an asteroid smashing into Earth?

Why are we asking this now?

Scientists at the US National Aeronautics and Space Administration (Nasa) have taken the first radar images of Asteroid 2007 TU24 which yesterday passed within 538,000km (334,000 miles) of Earth – about one and a half times the distance between the Earth and the Moon. The asteroid is roughly 250m (800 feet) in diameter, which is big enough for it to have caused a global environmental catastrophe, had it collided with our planet.

Many asteroids that pass close to Earth have orbital trajectories that make it possible for them at some point in the future to hit the planet, with potentially devastating consequences. Calculations, however, suggest that this was the closest approach to Earth that this particular asteroid will make in the next 2,000 years. So we are safe for the moment – at least from Asteroid 2007 TU24.

Are there many of these objects out there?

Yes, but the risk they pose is comparatively small, although not negligible. So far, scientists have identified more than a hundred asteroids that are large enough to pose a threat if they collided with Earth, and, crucially, which also orbit within a certain distance to the orbital trajectory of the Earth around the Sun. Nasa scientists have begun a project to identify, catalogue and track 90 per cent of all potentially hazardous objects greater than 140m in diameter by the end of 2020.

For these objects to be deemed potentially hazardous, their orbits around the Sun must pass within a certain distance to the Earth. This distance has been set at 0.05 astronomical units from the Earth – in other words, within 5 per cent of the distance between the Earth and the Sun.

How do scientists assess the risks?

They use a network of telescopes and radar installations. It takes a number of observations over a period of time to assess the orbit of the object around the Sun, and hence the risk that it may at some time make a close approach to Earth. Sometimes, when scientists discover a new asteroid, they can look at archived images of the sky to assess where the object had been before it was detected. This "pre-recovery" process is useful when determining the risk of a future collision.

The first thing scientists want to know is roughly when the asteroid will next cross the Earth's orbit around the Sun.

At this early point in assessing the risk of a direct hit, the scientists draw up an oval "target" known as the "error ellipse", which is the area of space adjacent to the Earth that the asteroid will pass through. If the Earth falls within this oval target, then there is a risk of a collision. As more observations are made, this error ellipse progressively decreases in size and the probability of a collision increases. Eventually, the Earth falls out of this error ellipse and the scientists can then confidently predict that there is zero risk of a collision.

This is why an initial observation of a new asteroid is sometimes followed by assessments of an increasing probability of a collision, followed by a sudden drop to zero when scientists refine their calculations and find that the Earth no longer falls inside the error ellipse.

What are asteroids?

Asteroids are essentially large chunks of rock, or space debris. Some scientists have likened them to "builders' rubble" left over from when the planets of the solar system were created more than 5bn years ago. The word asteroid means "star-like" because they can shine in the sky like points of light – in fact, they reflect sunlight, as they are cold, inert objects. They can range in size from specks of dust to giant boulders several miles in diameter. The largest known asteroid, Ceres, is 913km across.

Anything else we should worry about?

Comets are also space objects that pose a risk to Earth. These "dirty snowballs" are made of rock, ice and organic compounds. They may, in the past, have deposited water on Earth and even the building blocks of life in the form of simple organic substances. Comets are less likely to hit the Earth than asteroids, but when they do collide they can cause even greater damage because of their size.

What would happen if an asteroid struck the Earth?

It depends on how big it is. Objects less than a metre in diameter are quickly destroyed by the Earth's atmosphere, which burns them up as flashes of light. We call these meteor showers or "shooting stars". Larger objects can explode in the atmosphere, releasing enough energy to cause damage on the ground. Perhaps the most famous example of this was the explosion over Tunguska in Siberia in 1908 which flattened some 80m trees over an area of 2,100sq km. Luckily, the explosion occurred over a relatively uninhabited region. If it had occurred over London or Moscow, these cities would have been totally destroyed.

When an asteroid is larger than about 50m in diameter, there is a strong possibility of it reaching the ground to form a crater. This would result in millions of tons of soil and rock being vaporised or sent as debris into the atmosphere. In addition to the huge explosion, equivalent to several hundred nuclear bombs going off, the atmospheric debris would encircle the globe, cut out sunlight and cause an extended winter that would kill off many plants and animals. This is thought to have happened about 65m years ago, when a giant asteroid caused a mass extinction, including the demise of the dinosaurs.

Does anyone take the asteroid risk seriously?

The US Government is certainly taking it seriously by ordering Nasa to complete its survey of near-Earth objects as soon as it can – with 90 per cent of the survey completed by 2020. The Near-Earth Object Survey Act was signed into law by George Bush at the end of 2005. The British Government established its own Task Force on Potentially Hazardous near-Earth Objects in 1999, and the resulting report made 14 recommendations on how to track objects and assess the risks.

What can we do about it?

Scientists can only speculate on what we could do if a large object was ever identified as being on a collision course with Earth. Some have suggested the use of nuclear missiles to shatter an asteroid into pieces, but this could make matters worse, with tens or hundreds of sizeable fragments heading our way.

Another possibility is to nudge it off course with non-nuclear, "kinetic" missiles. A further possibility is to use a "slow push" method of deflection, such as the deployment of a space sail that uses the force of the solar wind, but this would require many years or even decades of planning and preparation.

So should we be worried?

Yes

* It has happened in the past, with devastating consequences for life on Earth

* The probability is higher than most people think, roughly equivalent to the risk of death in an air crash

* It is certain to happen at some point in the future; the only question is when

No

* In climate change and terrorism, the world faces much more pressing problems than asteroids

* There is nothing much we can do about the asteroid threat anyway, so why worry?

* The chances of the Earth being struck by an asteroid are extremely small

PROMOTED VIDEO
Arts and Entertainment
glastonbury
News
i100
Arts and Entertainment
Shock of the news: Jake Gyllenhaal in ‘Nightcrawler’
filmReview: Gyllenhaal, in one of his finest performances, is funny, engaging and sinister all at once
Arts and Entertainment
Shelley Duvall stars in Stanley Kubrick's The Shining
filmCritic Kaleem Aftab picks his favourites for Halloween
News
people
Arts and Entertainment
Kit Harington has been given a huge pay rise to extend his contract as Jon Snow in Game of Thrones
tv
News
ebooksAn unforgettable anthology of contemporary reportage
Life and Style
Taste the difference: Nell Frizzell tucks into a fry-up in Jesse's cafe in east London
food + drinkHow a bike accident left one woman living in a distorted world in which spices smell of old socks and muesli tastes like pork fat
Sport
Luke Shaw’s performance in the derby will be key to how his Manchester United side get on
footballBeating City is vital part of life at United. This is first major test for Shaw, Di Maria and Falcao – it’s not a game to lose
Life and Style
Google's doodle celebrating Halloween 2014
tech
Arts and Entertainment
Don’t send in the clowns: masks and make-up conceal true facial expressions, thwarting our instinct to read people’s minds through their faces, as seen in ‘It’
filmThis Halloween, we ask what makes Ouija boards, demon dolls, and evil clowns so frightening?
News
peopleFarage challenges 'liberally biased' comedians to 'call him a narcissist'
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Senior IP Opportunity at Major Firm

vary Attractive Salary: Austen Lloyd: MANCHESTER - AN OPENING AT A VERY HIGH Q...

Nursery Manager

£100 - £110 per day: Randstad Education Ilford: Nursery Manager Long term Ran...

Sales Consultant – Permanent – West Sussex – £24-£25k plus commission and other benefits

£24000 - £25000 Per Annum plus company car and commission: Clearwater People S...

SEN Teaching Assistant

£45 - £65 per day: Randstad Education Bristol: Supply SEN Support Jobs in Bris...

Day In a Page

The drugs revolution starts now as MPs agree its high time for change

The drugs revolution starts now as MPs agree its high time for change

Commons debate highlights growing cross-party consensus on softening UK drugs legislation, unchanged for 43 years
The camera is turned on tabloid editors in Richard Peppiatt's 'One Rogue Reporter'

Gotcha! The camera is turned on tabloid editors

Hugh Grant says Richard Peppiatt's 'One Rogue Reporter' documentary will highlight issues raised by Leveson
Fall of the Berlin Wall: It was thanks to Mikhail Gorbachev that this symbol of division fell

Fall of the Berlin Wall

It was thanks to Gorbachev that this symbol of division fell
Halloween 2014: What makes Ouija boards, demon dolls, and evil clowns so frightening?

What makes ouija boards and demon dolls scary?

Ouija boards, demon dolls, evil children and clowns are all classic tropes of horror, and this year’s Halloween releases feature them all. What makes them so frightening, decade after decade?
A safari in modern Britain: Rose Rouse reveals how her four-year tour of Harlesden taught her as much about the UK as it did about NW10

Rose Rouse's safari in modern Britain

Rouse decided to walk and talk with as many different people as possible in her neighbourhood of Harlesden and her experiences have been published in a new book
Welcome to my world of no smell and odd tastes: How a bike accident left one woman living with unwanted food mash-ups

'My world of no smell and odd tastes'

A head injury from a bicycle accident had the surprising effect of robbing Nell Frizzell of two of her senses

Matt Parker is proud of his square roots

The "stand-up mathematician" is using comedy nights to preach maths to big audiences
Paul Scholes column: Beating Manchester City is vital part of life at Manchester United. This is first major test for Luke Shaw, Angel Di Maria and Radamel Falcao – it’s not a game to lose

Paul Scholes column

Beating City is vital part of life at United. This is first major test for Shaw, Di Maria and Falcao – it’s not a game to lose
Frank Warren: Call me an old git, but I just can't see that there's a place for women’s boxing

Frank Warren column

Call me an old git, but I just can't see that there's a place for women’s boxing
Adrian Heath interview: Former Everton striker prepares his Orlando City side for the MLS - and having Kaka in the dressing room

Adrian Heath's American dream...

Former Everton striker prepares his Orlando City side for the MLS - and having Kaka in the dressing room
Simon Hart: Manchester City will rise again but they need to change their attitude

Manchester City will rise again but they need to change their attitude

Manuel Pellegrini’s side are too good to fail and derby allows them to start again, says Simon Hart
Isis in Syria: A general reveals the lack of communication with the US - and his country's awkward relationship with their allies-by-default

A Syrian general speaks

A senior officer of Bashar al-Assad’s regime talks to Robert Fisk about his army’s brutal struggle with Isis, in a dirty war whose challenges include widespread atrocities
‘A bit of a shock...’ Cambridge economist with Glasgow roots becomes Zambia’s acting President

‘A bit of a shock...’ Economist with Glasgow roots becomes Zambia’s acting President

Guy Scott's predecessor, Michael Sata, died in a London hospital this week after a lengthy illness
Fall of the Berlin Wall: History catches up with Erich Honecker - the East German leader who praised the Iron Curtain and claimed it prevented a Third World War

Fall of the Berlin Wall

History catches up with Erich Honecker - the East German leader who praised the Iron Curtain and claimed it prevented a Third World War
How to turn your mobile phone into easy money

Turn your mobile phone into easy money

There are 90 million unused mobiles in the UK, which would be worth £7bn if we cashed them in, says David Crookes