Don’t worry, the sky’s not falling in (Image: Sam Furlong/SWNS.COM)

On 30 June 1908, an asteroid struck the Tunguska region of Russia, devastating a forested area the size of a large city. Next Tuesday, on the anniversary of the Tunguska blast, a group led by astrophysicist and Queen guitarist Brian May and film-maker Grigorij Richters will host Asteroid Day, a celebrity-studded event in San Francisco and London designed to raise awareness of the potentially catastrophic risk of an impact.

But should we really be concerned about apocalyptic asteroids? There is no doubt that massive space rocks have the potential to harm life on Earth – just ask the dinosaurs.

Backers of Asteroid Day – including Chris Hadfield, Brian Cox and Richard Dawkins – call for a 100-fold acceleration of efforts to detect near-Earth asteroids in the next decade and increased funding to achieve this goal.


But asteroid-trackers say existing sky surveys already keep us safe, and events like Asteroid Day risk scaring people unnecessarily.

“The asteroid impact threat is very easy to overstate and misunderstand,” says Eric Christensen of the University of Arizona in Tucson. “The popular conception of asteroids is that they are menacing and going to kill us all, and it’s just not true.”

How do we know the risk?

Christensen heads the Catalina Sky Survey, a NASA-funded initiative tasked with identifying potentially hazardous asteroids using sensitive ground-based telescopes. In the past decade or so, Catalina, other ground-surveys, and NASA’s WISE space telescope have spotted thousands of near-Earth objects (NEOs).

As you would expect, larger asteroids are easier to spot, but there are fewer of them. Surveys show that we have found nearly all of the potentially world-destroying asteroids – those larger than a kilometre – but many smaller objects remain uncatalogued.

Here’s how we know: astronomers can estimate how many asteroids of a given size are out there, even if we haven’t seen them all, by looking at the rate of re-detection, or how often we see the same asteroid a second time. If we consistently see the same set of kilometre-sized rocks, and no new ones, then we can be pretty sure we have found them all.

When we look at rocks below that size, the gap between the number of asteroids we’ve seen and the number we think are out there widens (see graph below), according to a recent paper published in the journal Icarus that lays out the known unknowns.

Gap in our knowledge

Asteroid Day supporters think those smaller rocks are worth fretting over. This gap in our knowledge puts us at risk of being blindsided by an unseen threat, they argue.

“We’re talking about things that might ‘only’ set the world economy back by a thousand years, or might ‘only’ kill 100 million people, those are the kinds of things we haven’t yet found,” says Ed Lu, chairman of the B612 Foundation based in Mill Valley, California. B612 – named after the asteroid in The Little Prince – is a non-profit organisation aiming to build a private asteroid-hunting space telescope.

A meteor that exploded over Chelyabinsk, Russia (pictured below), in 2013 illustrated the potential risk. Although the rock was only around 20 metres across, it was the most energetic impact recorded since Tunguska. No one died, but about 1500 people were injured as the blast blew out windows and damaged buildings.

The meteor’s trail over Chelyabinsk just before it exploded (Image: Oleg Kargopolov/AFP/Getty)

The event was alarming, especially because no one saw it coming;: not only was it too small to detect in advance, most telescopes were aimed at a larger, unrelated object that passed Earth harmlessly later the same day. But astronomers still don’t expect another any time soon. Chelyabinsk-sized rocks are expected to hit Earth every few decades, but because the vast majority of the planet is either ocean or uninhabited, the risk to humans is minimal.

What will the next rock hit?

“It’s probably going to be centuries before an asteroid like that hits over a populated area,” says Christensen. “If you go up to the size of a Tunguska impactor, the next one will likely hit us within a few centuries and impact over the ocean.”

So if we’re safe from the really big asteroids, and the smaller, more frequent ones are likely to hit without major incident, what’s the problem? It all depends on how much risk you’re willing to accept, and how much money you’re willing to spend to mitigate it.

Martin Rees of the University of Cambridge, who has signed the Asteroid Day declaration, says it’s the same as calculating an insurance premium: you multiply the risk of impact by the cost of damage. By that measure, it is worth spending a billion dollars a year on reducing asteroid risk, he says.

How much should we spend?

That’s 25 times as much as what we spend now. NASA currently spends around $40 million annually on asteroid surveys and other asteroid-related activities, and has instructions from US Congress to find all space rocks above 140 metres by 2020. In a report last year, NASA’s Inspector General stated that the agency is unlikely to meet that without additional funding, so NASA has asked Congress to approve a further $10 million in 2016.

Linda Billings, a space policy researcher in Washington DC and consultant to NASA on NEOs, says this larger budget request is welcome, and another $200 million a year would fund a space-based survey telescope or an asteroid deflection mission every few years.

Teaming up with other organisations could bring that cost down. NASA is currently in the early phases of a test mission with the European Space Agency to smash a spacecraft into a harmless 800-metre asteroid in an attempt to deflect it. Dealing with larger bodies would likely require a nuclear explosion.

But Asteroid Day’s proposed 100-fold acceleration is not the way forward, Billings argues. “My colleagues in the observing community say this is not a reasonable goal,” she says. “It has nothing to do with available funding, it has to do with the way that observers find and track asteroids: it takes time.”

Christensen also questions this goal. “If the problem is trying to warn the people of planet Earth before every asteroid impact down to 3-metre sizes, then I would ask why are you doing that?” he says. “Within the next few days the Earth will be hit by an asteroid that is maybe a few metres in size, and all it will do is provide a nice light show. I don’t think we really even need to worry about the 20 to 50-metre asteroids. The risk is so small that I’m not sure the enormous cost to efficiently detect all of those asteroids is worth it.”

Another space telescope?

Rees is supportive of B612’s Sentinel space telescope, which the Foundation says will cost $450 million to build, money it hopes to raise through private donations. But the telescope, originally due for launch in 2017, has now been pushed back to 2019, says Lu.

B612’s tax filings show the organisation has some way to go. In 2011, before it began seeking donations for the telescope, the Foundation raised $90,000. That rose to $1.2 million and then $1.6 million in 2012 and 2013, the latest year for which filings have been published.

Billings says it would be better to pool resources with existing efforts. The 2014 figures are currently being audited and show increased funds, says Lu. “We raised more in 2014 than we have previously in all years combined, by a lot.”

The majority of B612’s increase in funding between 2012 and 2013 was spent on the salaries of Lu and Danica Remy, B612’s chief operating officers, who received $40,000 and $84,750 respectively in 2012, the first year the foundation paid them salaries. That rose to $240,000 and $204,279 in 2013.

The sky won’t fall in

Leaving aside salaries, an asteroid-hunting space telescope may not even be the best use of funds, Christensen says. “You could accomplish a lot of the same task with a couple of 4-metre dedicated telescopes from the ground, and that would not be half a billion dollars.”

Supporters say Asteroid Day will excite and engage young people. That’s good, says Billings – but don’t go to bed fearful that the sky will fall in. “I worry a lot more about my neighbour’s hemlock tree falling on my roof.”