“Our planet inhabits a hazardous environment. Earth is continually bombarded by cosmic objects.”

Cue some ominous music, and these words could have been spoken by a voice-over introducing a new Hollywood blockbuster. One about a meteorite heading straight for earth, causing millions to flee in panic.

Leave the music out, and these words are nothing but the first sentences of the a report published last Friday by the American National Academy of Sciences. The study, commissioned by American Congress, bears a title almost as sexy as Deep impact, Armageddon, or other Hollywood flicks about space debris. Defending earth, would make for a rare gem of a title for a scientific report, though the rest of it (“Near-earth object surveys and hazard mitigation strategies”) gives away its true nature.

91 die in asteroid strikes anually

Defending earth deals with exactly that: how to handle a piece of space debris hurtling towards our planet. Foremost amongst its findings, the committee, chaired by Irwin Shapiro, a professor at the Harvard-Smithsonian Center for Astrophysics, recommends an international body to be established which may take action if earth is ever threatened by a meteorite.

While our planet rarely collides with planetoids or comets larger than a couple of hundred metres in diameter, they have a tremendous appeal on the imagination, comparable perhaps to the shark, another age-old Hollywood darling. According to their report, only 3 to 6 swimmers die in shark attacks annually, while smoking costs five million lives annually, but does not do so well at the box office. Traffic worldwide takes 1.2 million lives every year.

Meteorites’ death toll clocks in at 91 people a year, according to the report. On average that is. An average reconstituted over millions of theoretical years. Commonly, a single strike might claim countless lives, while most years will remain without incident.

100 tonnes of asteroids a day

The main reason: most space debris that strikes the earth could be better termed ‘space dust', approximately 50 to 150 tonnes of it floats down to earth every day. Large impacts are rare in the solar system. Our sun and its orbiters have long left their wilder, younger days behind them, settling into a calmer existence after their first 700 million years.

The largest fragments of space debris almost all ended up in the Main Belt, a massive accumulation of asteroids between Mars and Jupiter. Most comets orbit our sun as a part of the Oort Cloud, which lies far beyond Neptune.

The only asteroids that pose a threat are those that have veered off course, and that might one day (nearly) cross paths with the earth – Near-Earth Objects, or NEOs, as they are known in the parlance of scientists.

Still, NEOs larger than 140 metres in diameter (sufficiently large to destroy an area the size of western Europe) strike only every 30,000 years on average – and when they do, three out of four times they crash into an ocean or uninhabited area.

Objects larger than a kilometre in diameter, which can have global consequences strike every 700,000 years on average. Objects larger than 10 kilometres, which could wipe out most living things on the planet, are only expected once every 100 million years or so.

But even though the threat resulting from meteorites is minimal, larger NEOs have been tracked since the 1970s to get an understanding of exactly how large the danger really is.

Shapiro’s report states some extra investments in existing telescopes should suffice to map the orbits of 90 percent of all NEOs larger than 140 metres in diameter before 2030.

Don Quijote

The report also deals with ‘mitigation strategies’ that may be used to deal with NEOs that are heading straight for earth. If such a NEO were discovered less than two years before impact, the only option left would be to evacuate the impact area. With more time to prepare, missiles could be useful. When fired at the NEO, they might be able to accelerate or slow it down. Another option would be to deflect the objects by crashing projectiles into them. For the life-threathening NEOs larger than one kilometre in diameter, the report gives only one viable option: nuclear missiles.

The committee recommends some practice at targeting asteroids, referring to an exercise planned by the European Space Agency. The exercise, which was never carried out, was part of the aptly named and now defunct Don Quijote project.

In its final considerations, the committee concludes that Hollywood might do much to “raise general public awareness of the threat from NEOs”. If only these filmmakers could be persuaded to make their movies more realistic, the report says, because “truth is usually stranger than fiction”.