Don Yeomans, a senior research scientist at JPL, keeps a watchful eye on near-Earth objects -- asteroids, comets and other space rocks. Yeomans heads a group charged by NASA to watch for objects whose orbits bring them close to Earth. He's pulled together his top ten asteroid factoids.
1. Thanks to Asteroids ...
After the solar system formed about 4.6 billion years ago, comets and asteroids hitting Earth probably carried water and carbon-based materials -- the building blocks of life - to our young planet. Once life formed, later collisions, like the asteroid impact that killed off the dinosaurs 65 million years ago, altered the evolution process. This created an environment where only the most adaptable species, such as mammals, evolved further. We humans likely owe our existence and current position atop the animal food chain to these space rocks.
2. Asteroid Attack
Every day, Earth is pummeled by more than 100 tons of material that spewed off asteroids and comets. Fortunately the vast majority of this "spillover" is just dust and very small particles. We sometimes see these sand-sized particles brighten the sky, creating meteors, or shooting stars, as they burn up upon entry into Earth's atmosphere. Roughly once a day, a basketball-sized object strikes Earth's atmosphere and burns up. A few times each year, a fragment the size of a small car hits Earth's atmosphere. These larger fragments cause impressive fireballs as they burn through the atmosphere. Very rarely, sizable fragments survive their fiery passage through Earth's atmosphere and hit the surface, becoming meteorites.
3. The Asteroids Next Door
At times, asteroids are our closest neighbors, even closer to us than the moon. That's because some asteroids have orbits that bring them closer to Earth than the moon's distance from Earth. (The moon is about 384,000 kilometers, or 238,606 miles from Earth.) This happens several times a year.
4. Rich Neighbors
Asteroids may literally pave the way to building future structures in space. Examination of meteorites suggests that the average near-Earth asteroid has a higher concentration of precious metals, such as platinum, than the richest known ore on Earth. These raw materials may also be more accessible, since some asteroids are easier to reach and return from than the moon. Comets may be about 30 percent water ice, which could be broken down into hydrogen and oxygen - the most efficient form of rocket fuel.
5. The Name Game
Comets are named after their discoverers while asteroids are named for a wide variety of scientists, geographic locations, celebrities and all manner of other things. Asteroids are also given a number, for example 99942 Apophis. With so many asteroid names in play, there are ample opportunities to string together several asteroid names for unabashed foolishness. By stringing together the names of the asteroids numbered 904, 673, 449, 848 and 1136, we get Rockefellia Edda Hamburgia Inna Mercedes. And yes, there is even 2956 Yeomans, named after yours truly.
6. It's an Asteroid, It's a Comet ... It's an Apollo Stage
After the launch of Apollo missions 8, 9, 10, 11 and 12 to the moon in the late 1960s, the launch vehicles' upper stages went into orbits circling the sun. The orbit paths of these upper stages are just inside Earth's orbit of the sun. Some of these stages might be mistaken for natural near-Earth asteroids. So we check out small objects to verify what they are. Should we discover they indeed are old upper stages, we would catalog them as such.
7. Asteroid Mini-Me
In 1993, NASA's Galileo spacecraft flew past asteroid Ida, a resident of the asteroid belt between Mars and Jupiter. Images taken by Galileo revealed that Ida, 60 kilometers (almost 40 miles) long, had a small moon accompanying it. The asteroid-orbiting moon, named Dactyl, was just 1 kilometer (a little over one-half mile) long. Since Dactyl's discovery, more than 150 "asteroid moons" have been found, some of which are partners to near-Earth asteroids. There are even a few objects that have two moons. Some theories as to how these small bodies form include collisions with other asteroids and the accumulation of material thrown off of a rapidly rotating asteroid to form a small companion.
8. A Diverse Population
Asteroids were once thought to be whirling rocks in space but they are actually diverse in structure and composition. At one end of the spectrum are weak asteroids, ones that were once comets. These asteroids have either run out of ices to vaporize -- and lost their comet-like activity -- or have entered a dormant period where none of their ices are exposed to sunlight. These space rocks are highly porous and relatively fragile. Then we have asteroids that appear to be piles of rubble in terms of their structure. The Near-Earth Asteroid Rendezvous (NEAR) spacecraft's flyby of asteroid Mathilde in 1997 found this type of structure and so did Japan's Hayabusa spacecraft when it visited asteroid Itokawa in 2005. The NEAR spacecraft's rendezvous with asteroid Eros in 1999 to 2000 found a shattered rock structure, while the iron meteorites found around Arizona's Meteor Crater suggest that the impacting asteroid was solid nickel-iron. Talk about diversity!
9. Sunlight Affects Asteroid Motion
After sunlight reaches an asteroid, it is absorbed by the surface and, after a short time delay, re-emitted back into space. The energy from the re-emitted heat, or thermal radiation, adds a tiny thrust to the asteroid's motion. This effect is called the Yarkovsky acceleration, named for the Russian scientist who first described it in 1900. Over hundreds of thousands of years, this tiny thrust can move an asteroid's orbital path into a so-called resonance region where the asteroid gets repeatedly pulled by the gravitational tugs of Jupiter and Saturn. Eventually, this can alter the asteroid's orbit and move the asteroid into the near-Earth object neighborhood of the solar system.
10. Possible Origin of Asteroid Moons
Almost all asteroids are irregularly shaped and marked with varied surface features. This means that one side of the asteroid's sun-facing surface will absorb a bit more sunlight and re-emit more thermal radiation than the other. A rotational push, or torque, can thus be introduced that will either slowly increase or decrease the asteroid's rotation rate. Over a few million years, a small, loosely bound rubble-pile asteroid, which has very little gravity to begin with, could spin up to the point where surface material flies off its equatorial region and comes together again to form a small moon of the asteroid. This process is the favored theory for explaining how asteroid "moons" are created. This "YORP" effect is named after four scientists who investigated the effect of sunlight on small solar system bodies: Ivan Yarkovsky, John O'Keefe, V. Radzievskii and Stephen Paddack.