Analysis: the asteroid threat: should we worry?

Experts have identified over 900 asteroids and comets that pose a potential threat to Earth. E&T investigates what we can do about it.

In a world where so much emotional energy is expended on climate change and its possible mitigation, the status of planetary defence against asteroid impacts seems strangely underplayed. This is probably because it evokes images of Bruce Willis laying nuclear charges on a marauding space rock, rather than a room full of grey-haired academics debating risk corridors, probabilities and Monte Carlo simulations. In the real world, the latter is closer to the truth.

The recent IAA Planetary Defense Conference in Granada, Spain, was the third in the series and the first to be sponsored by the International Academy of Astronautics. It provided a forum for scientists, engineers and policy experts to discuss the discovery and tracking of near-Earth objects (NEOs), potential deflection technologies, and public and political preparedness for any unavoidable, possibly devastating impacts.

Threatening evidence

Any cynicism regarding the seriousness of the threat was immediately quashed by the facts. Not only has the Earth been subject to impacts of varying severity in the past - such as the Tunguska airburst of 30 June 1908 that flattened 2,000km2 of Siberian forest, and the 'K-T extinction event' some 65 million years ago that apparently killed the dinosaurs - but also continues to receive incoming projectiles.

Indeed, as recently as 6 October 2008, an asteroid measuring a few metres across was discovered by the Catalina Sky Survey from its observatory near Tucson, Arizona. As predicted, 2008 TC3 hit the atmosphere early the next morning over northern Sudan, producing what the Nasa/JPL Near-Earth Object Program Office called a "brilliant fireball" with an estimated energy equivalent to a kilotonne of TNT. Although the warning time was minimal, JPL's Don Yeomans described the response of professional and amateur astronomers as "rather extraordinary", with 570 observations from 26 observatories reported in the 19 hours between discovery and impact.

A year earlier, 'the Carancas Event' of 15 September 2007 produced a 15m-diameter crater on the Peruvian Altiplano near lake Titicaca. According to scientist Gonzalo Tancredi, the anecdotal evidence of the power of the impact included "a man riding his bicycle who was knocked over, and a bull with one horn turned down when it fell". Somewhat more scientifically, Tancredi added, the impact was the first to be recorded both by local infrasound equipment and by seismometers, and the resultant "smoke trail" was photographed. The asteroid itself is estimated to have measured between 1.6m and 2m across and to have weighed between seven and 12 tonnes.

NASA survey

Considering the difficulties associated with detecting small rocks in a large solar system, astronomers have been doing a pretty good job. For example, Nasa's Spaceguard Survey, initiated in 1998, was intended to find over 90 per cent of NEOs (asteroids and comets) greater than 1km in diameter, and has already identified 856 according to JPL's Lindley Johnson, while the count of all near-Earth asteroids has reached more than 6,000. Although it will always be difficult to know when 90 per cent of existing objects have been detected, Johnson was optimistic: "Five teams with nine telescopes were originally making seven or eight discoveries a month," he said, "but now the discovery rate has dropped to two per month, we believe we have found the better part of the population."

In 2005, a dedicated NEO programme empowered by a Nasa Authorisation Act upped the ante by aiming to "detect, track, catalogue, and characterise the physical characteristics of NEOs equal to or larger than 140m in diameter with a perihelion distance of less than 1.3 AU from the Sun" (1AU being the average Sun-Earth distance). A key aim of this survey is to reach the 90 per cent detection point by 2020, but this will require investment in new observing facilities.

According to delegates, the two leading contenders for the next-generation surveys are the Panoramic Survey Telescope and Rapid Response System (PanSTARRS) and the Large Synoptic Survey Telescope (LSST). A prototype 1.8m-diameter telescope for PanSTARRS sports a camera with almost 1.4 billion pixels in a 40cm square array and is expected to discover "at least 600 NEOs larger than 300m per year".

A problem, warned Marco Delbo from the French National Centre for Scientific Research (CNRS), was that size estimates are based on albedo, the brightness of the body, "which is known to vary by a factor of four". This results in a factor-of-two uncertainty in size, he said, which means "the impact energy is uncertain by a factor of eight". Conference co-chair Richard Tremayne-Smith said: "The errors bars on size determination are too large."

Another key uncertainty is, of course, a body's trajectory, which governs whether or not it will hit the Earth. This is characterised by the concept of the 'keyhole', a region of space through which an asteroid must pass to place the Earth in its sights. Unfortunately, at least three detections - separated in time - are required to fix the trajectory with any degree of accuracy, which has obvious implications for any public warning system. In fact, according to JPL's Paul Chodas, what the planetary defence community considers to be "a discovery" requires three detections within 20 days.

Steve Chesley, another JPL researcher, commented that although the uncertainty region collapses as more detailed information becomes available, impact warnings should nevertheless be "viewed as a weather forecast rather than a prediction". Chodas agreed: "A whole slew of factors affect impact probabilities." He also said that, theoretically, "some of the 'warnings' occur at the point of impact!".

Not surprisingly, interested parties are lobbying for investment in upgraded survey systems, such as PanSTARRS and LSST, to "retire the risk", as Chesley put it. Not the risk that a NEO will hit the Earth, but the risk that a warning of such an impact will be wrong.

Deflection

Some 900 NEOs designated potentially hazardous objects (PHOs) have been identified, but the conference focused particularly on one named Apophis, a 270m asteroid discovered in 2004.

Current predictions show that Apophis will pass below the geostationary orbit (where most communications satellites operate) in April 2029, with a return in 2036 when the probability of an impact is estimated at 1 in 45,000. Not worryingly high, one might think, but as Rusty Schweikart (former Apollo astronaut and head of the B612 Foundation) pointed out: "For every Apophis, there are more than 50 Tunguskas." This is one reason his Foundation's aim is "to significantly alter the orbit of an asteroid, in a controlled manner, by 2015".

In a review of potential deflection methods, Keith Holsapple from the University of Washington warned of what he called "the D-words: disrupt, disperse and destroy", which are used in the context of nuclear devices (euphemistically known as 'physics packages'). To vaporise a 100m asteroid requires a device of "about 30 megatonnes", he said, but depending on the physical constitution of the body - some are thought to be unconsolidated 'rubble piles' - the explosion might simply produce a number of smaller bodies on a similar trajectory.

An alternative method uses a 'gravity tractor', typically a spacecraft equipped with an ion propulsion system that imparts a low thrust to the asteroid over a long period. While industry representatives were keen to make proposals for such systems, since they are at least technically feasible and not as politically divisive as the nuclear option, Holsapple dismissed them as "very wimpy", before adding: "You are probably better just running the mass into it."

Nevertheless, the push to understand asteroids continues with a European Space Agency proposal for a rendez-vous and sample return mission called Marco Polo, currently being studied under industry contract for a possible launch in 2017. It's a long way from a deflection mission but, as Marco Delbo points out: "We have to know our enemy!"

So, should we worry about asteroid impacts? According to IAA secretary-general Jean-Michel Contant: "It is estimated that the danger of NEOs is greater than that due to shark attacks, botulism, H5N1 flu or terrorism." Is that worrying enough?

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