Spitzer Samples an Assortment of Asteroids

Close-ups of asteroids captured by robotic spacecraft.

When you hear the word "asteroid," what image is evoked in your mind? A confusion of tumbling, skittering mountains in space suitable for a breath-defying spaceship chase? Blue-white digital outlines of big rocks flying around a video screen, ready to be blasted into pixel dust by your torpedo cannon? Or perhaps, as some science fiction stories would have it, a tiny barren world complete with a breathable atmosphere and giant space worms living in deep crater caverns….

Ah, gotta love science fiction.

As Sci-Fi is often more colorful than reality, are the facts about asteroids a bit more down to Earth (so to speak)? Big gray rocks, perhaps quite large, but otherwise less interesting even than the granite mountains of an Earthly place like Yosemite? Well…try scooping up a handful of sand; at first glance you may see only a pile of tawny-gray grains. Look more closely—with a magnifying glass or microscope—and a veritable treasure trove of colorful jewels is revealed.

NASA's Spitzer Space Telescope is just such an instrument, and has revealed that asteroids may have more variety than once imagined.

Spitzer recently ran out of the supply of coolant that kept its infrared cameras cold—at least, cold enough to sense the subtle heat radiation emitted by distant celestial objects. Now that Spitzer's instruments have "warmed up" to a balmy negative 406 degrees F, observers have shifted their attention to other objectives, including asteroids.

In particular, Spitzer has observed the infrared emanations from a hundred or so Near Earth Asteroids: those that cross Earth's orbit in the course of their own elliptical routines. There are plans to observe 600 or more NEAs in the future—out of about 7000 currently known Near Earth Objects (which includes comets and meteoroids).

The observing program aims to give us a clearer focus on the individual characteristics of these flying mountains that Earth shares space with: the sizes, the composition, and even the age and origins of what is proving to be a diverse and rich population. Spitzer's infrared cameras see past the façade of mere visible light, which, at the distance of most NEOs, doesn't tell us much more than the amount of light they reflect. A big, dark asteroid and a small, light-colored one may reflect the same amount of light, giving those distant specks in the telescopic image the same appearance.

But throwing in an infrared measurement of an asteroid's heat emissions can reveal much more: the temperature of an asteroid is governed by the amount of sunlight it absorbs, which is in turn governed by size, color, composition, and its distance from the Sun.

Already Spitzer's initial 100 asteroid observations have revealed a wide variety of characteristics—maybe not unlike that handful of sand grains scrutinized under a microscope.

Meteorites, having fallen to Earth and been examined up close, have long revealed that their "source mountains" (their parent asteroids, from which they were broken away by collisions) are diverse in composition. We find stone meteorites, and meteorites of solid nickel-iron, sometimes embedded with crystalline gemstones of great beauty.

Combining the compositional data on NEOs gleaned from Spitzer with their orbital characteristics determined through ground-based observatories--such as Chabot's own Asteroid Search program—we are learning a great deal about the NEO neighborhood, and what exactly may be passing quietly in the night. It's all good information, giving us a better handle on how to protect our planet from possible impacts, and laying the groundwork for future missions of exploration to Earth-passing asteroids.

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