"Sir, the possibility of successfully navigating an asteroid field is approximately 3,720 to 1!"
|Harrowing! Awesome John Williams soundtrack here.|
Turns out, the real asteroid belt in our own solar system is much less exciting. About half of the total mass in the belt is held in four very large asteroids, the largest of which, Ceres, holds a third of the total mass all to itself. At 950km in diameter, Ceres technically classifies as a 'dwarf planet' (1); for comparison, our Moon is 3500km in diameter (2).
|Ceres, the largest member of the asteroid belt. Is this seriously the best photo we have? NASA needs a raise!|
Most of the rest of the asteroid belt is very, very empty - as described on Wikipedia, "Collisions between main-belt bodies with a mean radius of 10 km are expected to occur about once every 10 million years." (3). We've sent a number of unmanned satellites right through the asteroid belt with no problems. Contrast that with the asteroids bouncing off of eachother every twenty seconds in every movie and video game ever made.
In the space in between planets and stars, things get even emptier. More than 90% of the interstellar space in our galaxy, the Milky Way, contains only thousands to hundreds of thousands of atoms per cubic meter, 90% of which are lone hydrogen atoms (4).
Emptiness aside, things do very occasionally crash into Earth. Today, a meteor entered Earth's atmosphere and exploded over the Ural Mountains in Russia, injuring on the order of 1,000 people in the Chelyabinsk Oblast district. Eyewitnesses captured incredible video of the event:
This was not the first time Russia has been struck by space stuff in recorded history. In 1908, a meteor or comet exploded over the Tunguska region in central Russia, releasing 1,000 Hiroshima atomic bombs' worth of energy and flattening 2,000 square kilometers of forest (5). Fortunately this 'Tunguska Event' occurred in a remote area; an urban area would have been completely destroyed. The airburst left no crater and there were no eyewitnesses.
It's not the first record of injuries by falling space debris, either: in 1954, Ann Hodges, age 31 of Oak Grove, Alabama, received some very nasty bruises when a 4kg space rock smashed through her roof, bashed up her console radio, and hit her in the hip while she was napping on the couch (6). In 1992, a tiny 3g piece of the Mbale meteorite is claimed to have hit a Ugandan boy in the head (7). There are other stories on the Internet of meteorites killing an Italian friar and an Egyptian dog.
And this wasn't the only space rock in the news this week: Asteroid 2012 DA14 passed within 27,700 km of Earth's surface earlier this morning (8). That's over 10 times closer than the moon (385,000 km) and even our geosynchronous satellites (36,000 km). That's right - this asteroid was between Earth and our satellites. It's a pretty big rock too - on the order of 50m in diameter - and would have caused a Tunguska Event-sized explosion had it impacted. Perhaps this will jump-start the debate about astronomy funding!
I wonder if it's technically feasible - and what it would cost - to detect an incoming object the size of the one that crashed into Russia today?
(There are, by the way, not one but two astronomical object impact hazard rating scales - the Palermo Scale and the Torino Scale (9,10) - that attempt to assign a threat value by quantifying an object's impact probability and damage potential.)
(3) Backman, D. E. (March 6, 1998). "Fluctuations in the General Zodiacal Cloud Density".