Our solar system is littered with asteroids and comets, debris left over from the planets鈥� formation. Although most are small 鈥� mere grains of sand or pebbles 鈥� others many metres or kilometres across lurk in the darkness. And as certain as death or taxes, some will, one day, have a fateful encounter with Earth.
Lesser lumps rocket across the sky, blazing brightly as meteors before being vaporised. Larger pebbles and rocks can survive this fiery journey and make it to the ground as meteorites, prized by collectors.
But scientists are worried about Earth鈥檚 potential impact with immense kilometre-sized asteroids. Travelling at tens of kilometres per second, these have enough explosive energy to plunge our planet into chaos and trigger mass extinctions.
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Hence the call at the meeting of the American Geophysical Union (AGU) in San Franciso, California, this week for , including measures to establish observer and interceptor craft. This came from researchers at Los Alamos National Laboratory in New Mexico and NASA鈥檚 Goddard Space Flight Center, who are worried that it currently takes five years from design to launch to make a reliable rocket.
Existential threat
We know the destructive potential of a large rock impact. The dinosaurs met their end when an 聽asteroid at least 10 km across smashed into Mexico鈥檚 Yucatan peninsula 65 million years ago, and we now know that something substantial hits Earth every few tens of millions of years. But we are the first creature to survey the heavens and determine how many of these potential extinction-causing objects are out there.
You can breathe a little easier for now, because the bigger ones are easier to see, and we think we have spotted most of them. Smaller rocks are harder to detect, and often arrive unannounced: testament to this is the 20 m Chelyabinsk meteor that flashed over Russia in 2013, exploding 30 km up, injuring hundreds of people and shattering many thousands of windows. We still face major blind spots, because even large rocks heading straight towards us are difficult to detect 鈥� but we are getting better at charting this potential death threat from the skies.
Even so, an important question remains. If we identified a threat, what could we do? If you believe Hollywood, all we need is Bruce Willis and his team to plant a nuclear weapon deep within the offending object, blowing it to smithereens and saving the day (). Yet as ever, science is harder than science fiction.
Simply smashing up a comet or asteroid may not be the best thing to do because the energy release would still be substantial if the fragments created were to hit Earth. Death would then come as a rain of fire rather than a single immense explosion. NASA pretty much ruled this approach out in a .
Averting Armageddon
The real secret to preventing Armageddon is time. Here, yesterday鈥檚 suggestion at the AGU meeting for of threats, and an interceptor rocket ready to fly, makes sense, potentially carving years off the response time.
Given long enough, we could gently nudge an asteroid off collision course onto a path that misses us. To do this, we could land little rocket boosters on the surface, or use immense laser beams.
When time is shorter 鈥� for example, if a threat appears in our blind spot 鈥� we may need to resort to 鈥渒inetic weapons鈥�, which essentially means giving the rock a thump with a high-speed 鈥渃annonball鈥� rocket to knock it out of the way. The other option is a nuclear bomb.
NASA鈥檚 preference in this scenario is for an explosion at a distance to vaporise surface material on the asteroid, thus creating a propellant effect to push it off course. Nothing is certain though: one hazard is that in both scenarios, the rock could still splinter.
For now, this is all wishful thinking. We do not at present have the technology in place for any of these potential solutions. No rockets sit poised on launch pads ready to respond to threats from above, and none of the world鈥檚 many thousands of nuclear weapons sit pointing towards the heavens ready to save humanity. Even Bruce Willis would be unable to help.
Now that telescopes allow us to peer into the darkness and discover space-borne threats to our very existence, we humans need to get in place the tools for our salvation. The scientists in the US were right to raise the need to properly prepare for averting this threat, even if it is an extremely rare event. Until then, we are simply a cosmic sitting duck. Merry Christmas!
