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The sky鈥檚 not falling in just yet (Image: Sam Furlong/SWNS.COM)
ON 30 JUNE 1908, an asteroid struck Earth, devastating an area the size of a large city. Luckily the Tunguska region of Russia where it landed was mainly forest.
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On the anniversary of the Tunguska blast next Tuesday, 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.
So should we be concerned about apocalyptic asteroids? May and Richters think so. They and other supporters of Asteroid Day 鈥� including Chris Hadfield, Brian Cox and Richard Dawkins 鈥� have signed a declaration calling for a 100-fold acceleration of efforts to detect near-Earth asteroids in the next decade and increased funding to achieve this goal.
鈥淲e鈥檙e talking about things that might 鈥榦nly鈥� set the world economy back by a thousand years, or might 鈥榦nly鈥� kill 100 million people,鈥� says Ed Lu, chairman of the B612 Foundation in Mill Valley, California. Named after the asteroid in the children鈥檚 classic story The Little Prince, B612 is a non-profit organisation aiming to build an asteroid-hunting space telescope.
There is no doubt that massive space rocks have the potential to harm life on Earth 鈥� just ask the dinosaurs. But asteroid-trackers say existing sky surveys already keep us safe, and events like Asteroid Day risk scaring people unnecessarily.
鈥淭he asteroid impact threat is very easy to overstate and misunderstand,鈥� says Eric Christensen of the University of Arizona in Tucson. 鈥淭he popular conception of asteroids is that they are menacing and going to kill us all, and it鈥檚 just not true.鈥�
聯The popular conception of asteroids is that they are menacing and going to kill us all, and it鈥檚 just not true聰
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鈥檚 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 鈥� those larger than a kilometre 鈥� but many smaller ones are uncatalogued.
Astronomers can estimate how many asteroids of a given size are out there, even if we haven鈥檛 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.
For rocks below that size, the gap between the number we have seen and the number we think are out there widens, according to a recent paper that lays out the known unknowns (Icarus, , see graph).
Asteroid Day supporters argue this gap in our knowledge puts us at risk of being blindsided by an unseen threat.
A meteor that exploded over Chelyabinsk, Russia, in 2013 illustrates the potential risk. Although the rock was only around 20 metres across, it was the most energetic explosion recorded since Tunguska. No one died, but about 1500 people were injured as the blast blew out windows and damaged buildings.
The event was alarming, especially because no one saw it coming. But astronomers still don鈥檛 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.
鈥淚t鈥檚 probably going to be centuries before an asteroid like that hits over a populated area,鈥� says Christensen. 鈥淚f 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.鈥�
So if we are safe from the really big asteroids, and the smaller, more frequent ones are likely to hit without major incident, what鈥檚 the problem? It all depends how much risk you鈥檙e willing to accept, and how much money you are willing to spend to mitigate it.
Martin Rees of the University of Cambridge, who has signed the Asteroid Day declaration, says it鈥檚 like 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.
聯It鈥檚 the same as calculating an insurance premium: you multiply the risk of impact by the cost of damage聰
That鈥檚 25 times as much as is allocated now. NASA currently spends around $40 million annually on asteroid surveys and related activities, and has instructions from US Congress to find all space rocks above 140 metres by 2020. In a report last year, NASA鈥檚 Inspector General stated that the agency is unlikely to meet that without additional funding, so it is seeking a further $10 million in 2016.
Linda Billings, a space policy researcher in Washington DC and consultant to NASA on NEOs, says this budget request is welcome, while another $200 million a year would fund a dedicated space-based survey telescope, or an asteroid deflection mission every few years.
Teaming up with other organisations could bring costs 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鈥檚 proposed 100-fold acceleration is over the top, Billings argues. 鈥淢y colleagues in the observing community say this is not a reasonable goal,鈥� she says. 鈥淚t has nothing to do with available funding, it has to do with the way that observers find and track asteroids: it takes time.鈥�
Christensen also questions this goal. 鈥淚f the problem is trying to warn the people of planet Earth before every asteroid impact down to 3-metre sizes, I would ask why are you doing that?鈥� he says. 鈥淲ithin 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. The risk is so small that I鈥檓 not sure the enormous cost to efficiently detect all of those asteroids is worth it.鈥�
Rees is supportive of B612鈥檚 Sentinel space telescope, which the foundation says will cost $450 million to build, money it hopes to raise through private donations. Originally due for launch in 2017, the scope has now been pushed back to 2019, Lu says.
B612鈥檚 tax filings show the organisation has some way to go to reach its funding target. In 2011, before it began seeking donations for the telescope, the Foundation raised $90,000. That rose to $1.6 million in 2013, the latest year for which filings have been published. Much of the 2013 funding went towards the salaries of Lu and Danica Remy, B612鈥檚 chief operating officer, who received $240,000 and $204,279 respectively.
Leaving aside salaries, an asteroid-hunting space telescope may not even be the best use of funds, Christensen says. 鈥淵ou 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.鈥�
Supporters say Asteroid Day will excite and engage young people. That鈥檚 good, says Billings 鈥� but don鈥檛 go to bed fearful that the sky will fall in. 鈥淚 worry a lot more about my neighbour鈥檚 hemlock tree falling on my roof.鈥�
This article appeared in print under the headline 鈥淒on鈥檛 fear apocalyptic asteroids鈥�