Although there’s no basis to the new-age ‘Mayan Apocalypse’, there are two ways Earth can be dealt to, apocalyptically, from space. Neither will make the Earth disappear, but at worst most of us will die, and survivors will be back to living in caves.
Method No. 1 is gamma ray burst from an exploding Wolf-Rayet star. A 3-10 second burst from 10,000 light years away can destroy our ozone layer. The energy is mind blowing. NASA’s SWIFT satellite detects them. We’d be unlucky to be caught in the beam at lethal distance, but afterwards life would be nasty. One might have caused the Ordovician-Devonian extinction event, 440 million years ago. Eta Carinae, a star radiating 5,000,000 times the energy of the Sun, circa 7500 light years distant, could cause one.
Method No. 2 is being hit by an asteroid about 10 km in diameter or more, at a speed of more than 11-12 km/sec. One of those impacts, 65 million years ago, helped trigger the ‘Cretaceous extinction event’. So far, nothing big has hit a city – though had the Tunguska meteor of 1908 been five hours earlier, it would have destroyed St. Petersburg.
On 11-12 December – this week– Asteroid 4179 Toutatis passed us at 4.3 million miles. It has a diameter of 4.5 x 2.4 x 1.9 km and escape velocity is 0.0019 km/s, about 6.8 km/h or 4.27 mph. You could jump into orbit from the surface. A handy asteroid impact simulator tells me impact with Earth would release energy equal to 365,000,000,000 tons of TNT. It’ll be back at the end of 2016. Will it hit us? No chance in the next 600 years. The orbit – with 4:1 resonance to ours and 1:3 to Jupiter’s – is well known.
Smaller things hit more often. The thing that exploded over Tunguska in 1908 was likely a fragment of Comet Encke, which has shed material as two Taurid meteor stream. The Tunguska fragment likely came from the B-stream and was mostly ice. Compression heating when it hit the atmosphere meant one side was white hot while the other was cold. Lots of thermal stress. Boom!
For obvious reasons, we’ve been looking for potential rocks. Most of the risk-objects have been found – uh, we think. Well, we’ve even found the S-IVB stage from Apollo 12. Pretty cool. The only objects of any danger are Ag5, a 140-metre rock which has a 1 in 500 chance of hitting us in 2040; and Vk-184, a 130-metre rock that has an 0.055% chance of hitting us in 2048.
Can we deflect one coming for us? Yes, if we get years of warning. Even spray-painting black on one side would do it, because of differential radiation effects.
Suppose we found one just four weeks off, like in the movie? Could Bruce Willis save us by landing on the asteroid and nuking it? Uh… no. Supposing you could get something on the launch pad instantly, you’d still have to accelerate away from Earth to meet the intruder, needing fuel to change velocity (delta-V) by at least 12-15 km/sec. Then you have to stop, requiring about 1-4 km/sec delta-V (Earth’s gravity slows you on the outward leg). Then you need to accelerate back towards Earth to catch the interloper. If it’s incoming at 15 km/sec per-second (like Vk-184), you need that much delta-V. Mission energy is 34 km/sec delta V. Oh…and you’re on course to hit Earth with the asteroid. More delta-V, please.
The Saturn V moon rocket could accelerate Apollo to 11.2 km/sec; and the Apollo SM had 2.8 km/sec delta V in vacuum. The xenon-ion electric driven Dawn probe that’s just departed Vesta, en route to Ceres in 2015, has 10 km/sec delta-V. (Hey folks – we visited one of the biggest asteroids, past Mars!) But ion thrusters have less puff than human breath and take months to get the probe into a new orbit.
There’s always Project Orion, a US idea from 1959 that would have pushed a spacecraft by lobbing atomic bombs out the back. Dumb? Absolutely. The worst part is, it worked on test. There was even a paper done on the world increase in cancer rates they’d induce by launching Orion spacecraft from the ground. (Were they MAD in the Cold War? Arrrrggghhhh!)
But all this is academic for our four-week warning asteroid. What does the damage is the net kinetic energy the asteroid delivers to Earth. If you hit an incoming asteroid with a nuke, it’ll turn into a cloud of fragments, most still heading our way with virtually all the original kinetic energy, this time spread over a larger area. Neatypoos.
So there you have it – a couple of astronomical doom scenarios that involve proper science. When will they happen? Nobody knows. But I don’t lose any sleep over it. Do you?
Copyright © Matthew Wright 2012