The *Real* Star Wars

The first steps in a Planetary Defence System for this ball of rock of ours.

From The Australian :

NASA will soon fire its "first strike" against a deadly interstellar enemy - comets and asteroids on a collision course with Earth.

NASA'S Deep Impact spacecraft will be launched early next year to intercept and blast a hole in a distant comet.

The main point of the six-month journey to Comet Temple 1 is to look inside a comet for clues to the formation and evolution of the solar system.

But it is also a major manoeuvre against comets and asteroids, which, unlike Temple 1, are on a collision course with Earth.

"We need to know a lot more about our enemy," said Duncan Steel, a space scientist with the Ball Solutions Group in Canberra and vice-president of the Rome-based Spaceguard Foundation, a consortium searching the skies for threatening objects, and devising ways to protect the planet from them. "We don't know if we need an elephant gun or a butterfly net to deal with them. We need to find these things out."

When the spacecraft reaches the comet on July 4 it will hurl a 372kg projectile toward its hard "nucleus" at 10km a second. As the copper-tipped "impactor" hits - with the equivalent of three tonnes of TNT - optical and infrared data will be sent to earth.

With the usual degree of journalistic Acumen, the headline reads (and I quote)

NASA attack on comet lights up Star Wars fear

Booga Booga Booga!

As for how a 372kg chunk of metal can be the equivalent of 3 tonnes of TNT, here's a quote from Project Rho (An indispensible resource for those interested in High-Energy events in Space) :

Rick Robinson's First Law of Space Combat states that "An object impacting at 3 km/sec delivers kinetic energy equal to its mass in TNT." In other words there are 4,500,000 joules in one kilogram of TNT (3,000²m/s * 0.5 = 4.5e6). This means a stupid bolder travelling at 2,000 km/sec relative has about 400 kilo-Ricks of damage (i.e., each ton of rock will do the damage equivalent of 2e12 / 4.5e6 = 400 kilotons of TNT or about 20 Hiroshima bombs combined).

Ricks = (0.5 * V²) / 4.5e6
where:
V = velocity of projectile relative to target (m/s)
Ricks = kilograms of TNT worth of kinetic energy per kilogram of projectile

"4.5e6" means "4.5 x 10e6" or "4.5 x 1000000", in other words, 4,500,000.

In the NASA mission, Ricks = 0.5 x (1.0e4)² / 4.5e6, or about 11. So a 372 kg projectile at 11 Ricks will be the equivalent of about ~4000 kg of TNT : 4 tonnes, not 3 (as the article states).

And the reason why people like myself are a little concerned about "falling rocks" is that they're usually at a relative velocity of 20 km/sec - call it 40 Ricks. A cubic kilometre of rock masses about 5 times the same volume of water. A cubic kilometre of water weighs 1.0e9 kilograms, call it a million tonnes. So a relatively small rock would do 40 x 5 x 1 Megatonne. 200 Megatons equivalent. The largest man-made explosion was the Soviet Tsar Bombka, at 60 Megatons, and the largest warhead in the US arsenal (last time I checked) is about 5 Megatons, with most warheads 1/20 that size or smaller.

That is why we're a little concerned. Because that 1 cubic kilometer chunk of rock I used as an example is a little one compared with some that have come pretty darn close to Earth recently. The larger ones would mass on the order of 500 Million tons not 1, and ones of any size could come in with a velocity multiplier of 400 Ricks, not 40.

The University of Arizona has a nice little Impact Calculator, that allows you to specify a distance from Ground Zero, and the characteristics of the impacting body.

Space.com has a nice little factual article on the subject, with a neat quote that sums up the whole situation:

Prepare to be scared

If you're not, you haven't been paying attention. High Energy events like this happen relatively frequently - every few thousand years, at most.