A WISE Survey of Nearby Space Rocks

There was a busy space probe out there called the Wide-field Infrared Survey Explorer (WISE, for short). As its name suggests, it was sensitive to infrared wavelengths of light and cataloged millions of objects before it went into hibernation in 2011.  Many things radiate in the infrared, including some potentially hazardous asteroids (PHAs) that have the propensity to stray across Earth’s orbit from time to time.   WISE has been sweeping its gaze across near-Earth space in search of these asteroids, which are detectable in infrared. Because the telescope detected the infrared light, or heat, of asteroids, it was able to pick up both light and dark objects, which gave astronomers a pretty good and pretty representative survey of what’s “out there”. The infrared data allowed them to make good measurements of the asteroids’ diameters and, when combined with visible light observations, how much sunlight they reflect.

So, what is it about these PHAs that are so intriguing?  First, they have the closest orbits to Earth’s  of many asteroids. Some of them come within five million miles (about eight million kilometers) of our planet in their orbits. However, if one of them strayed across our orbit andgot too close to our planet, it would be sufficiently big that it would survive passing through Earth’s atmosphere and smashing into the surface (or the ocean). This would cause damage on a regional, or greater, scale.

WISE sampled 107 PHAs that it actually observed, and used that sampling to come up with a decently accurate estimate of how many more are out there. Based on WISE’s data and the estimates, there are roughly 4,700 PHAs, plus or minus 1,500, with diameters larger than 330 feet (about 100 meters). That’s just an estimate of what’s out there. Not all of them have actually been observed — only about 20 to 30 percent of these objects have actually been found.

WISE’s analysis also suggests that about twice as many PHAs as previously thought are likely to reside in lower-inclination  orbits. That’s a fancy way of saying that their orbits are which are more aligned with the plane of Earth’s orbit. In addition, they appear to be somewhat brighter and smaller than the other near-Earth asteroids that spend more time far away from Earth.  Why the difference?  One possible explanation is that many of the PHAs may have originated from a collision between two asteroids in the main belt lying between Mars and Jupiter. A larger body with a low-inclination orbit may have broken up in the main belt, causing some of the fragments to drift into orbits closer to Earth and eventually become PHAs.

Asteroids with lower-inclination orbits would be more likely to encounter Earth and would be easier to reach. The results therefore suggest more near-Earth objects might be available for future robotic or human missions. And that’s kind of exciting, because traveling out to asteroids and studying them is something we’re learning to do, with experience from such missions as the NEAR project.

The discovery that many PHAs tend to be bright says something about their composition; they are more likely to be either stony, like granite, or metallic. This type of information is important in assessing the space rocks’ potential hazards to Earth. The composition of the bodies would affect how quickly they might burn up in our atmosphere if an encounter were to take place. You might wonder why all the fuss about PHAs.  The short answer is pretty obvious: they have a chance to hit Earth and cause significant damage.  There are people studying them, trying to figure out ways to deflect them if they do head for us. But, as I mentioned above, these asteroids also give us a chance to out and study them and learn more about the basic makeup of objects that, until late in the 20th century, were something of a mystery to astronomers. Now, we know that asteroids hold a lot of information that would help us understand the origin and evolution of our solar system — making them historical troves of great significance!