Wednesday, 2 January 2008

Mars and a 1 in 25 Shot

2007 WD5. That's the name of a 50metre wide chunk of rock that now has a 1 in 25 chance of slamming into Mars at a relative velocity of nearly 50,000 km/hr early next year.

Plugging the approximate orbital data into the asteroid impact calculator, which gives results for an equivalent Earth Impact, we get:

Projectile Diameter: 50.00 m = 164.00 ft = 0.03 miles
Projectile Density: 3000 kg/m3
Impact Velocity: 15.00 km/s = 9.31 miles/s
Impact Angle: 60 degrees

Energy before atmospheric entry: 2.21 x 1016 Joules = 5.28 MegaTons TNT
The average interval between impacts of this size somewhere on Earth is 395.9 years

Atmospheric Entry:
The projectile begins to breakup at an altitude of 52000 meters = 171000 ft
The projectile bursts into a cloud of fragments at an altitude of 4290 meters = 14100 ft
The residual velocity of the projectile fragments after the burst is 4.06 km/s = 2.52 miles/s
The energy of the airburst is 2.05 x 1016 Joules = 4.89 x 100 MegaTons.
No crater is formed, although large fragments may strike the surface.

The atmosphere on Mars is a *lot* thinner, so that ~5 Megaton equivalent blast will be a ground burst, leaving a crater about a kilometre wide.
In the unlikely event of an impact, the time would be 2008 January 30 at 10:56 UTC with an uncertainty of a few minutes.
Odds are that Mars will dodge this particular bullet. But don't worry, they actually hit about once every 400 years. The same for Earth of course.

We now have the technology to detect these things, and even do something about them if need be. It wouldn't take much to nudge something so small so it doesn't break windows in highly urbanised areas, but goes "boom" over water. It's the ones that are bigger and rarer that we have to worry about. The Dinosaur Killers. Things like the one that hit Chicxulub

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