When I was in elementary school, we learned the mnemonic, “Many very early men ate juicy steaks using no plates.” This was a way to remember the solar system: Mercury, Venus, Earth, Mars, Asteroids, Jupiter, Saturn, Uranus, Neptune, Pluto. Note that Pluto was still a planet and was in its “proper” place. This was presented as an orderly system with everything staying in its designated orbit.
The problem, it turns out, is that the solar system is far from orderly. Pluto moves in an orbit that takes it occasionally inside the orbit of Neptune. Comets are also part of the system and they turn up and swing around the sun willy-nilly.
The biggest worry is the, as I learned it, “asteroid belt.” Well, it’s not a belt. It’s not even a sash. It is just a loose mess of rocks, and some of them come very close to the Earth – close enough to be considered dangerous, as reported in this story from NASA which posted yesterday:
Observations from NASA’s Wide-field Infrared Survey Explorer (WISE) have led to the best assessment yet of our solar system’s population of potentially hazardous asteroids. Also known as “PHAs,” these asteroids have orbits that come within five million miles (about eight million kilometers) of Earth, and they are big enough to survive passing through Earth’s atmosphere and cause damage on a regional, or greater, scale.
The NASA astronomers are predicting that there are 4,700 potentially hazardous asteroids that are near Earth’s orbit – give or take 1,500. Yikes.
Gunsmoke and Knitting 