A new survey of asteroids near Earth by a NASA space telescope has found a much wider variety of the space rocks than previously thought, with some shiny and bright while others are dark and dull.
The discovery suggests that near-Earth objects are more diverse than scientists originally thought. The findings are based on infrared observations of 100 such space rocks by NASA's Spitzer Space Telescope.
"These rocks are teaching us about the places they come from," said the study's lead author David Trilling of Northern Arizona University, Flagstaff. "It's like studying pebbles in a streambed to learn about the mountains they tumbled down."
The new findings are part of an even larger Spitzer telescope project to survey 700 near-Earth objects to catalogue their individual traits. The research is detailed in the September issue of the Astronomical Journal.
Spitzer's infrared observations are helping astronomers gather more accurate estimates of asteroids' compositions and sizes than what is possible with visible light alone. Visible-light observations of an asteroid won't differentiate between a rock that is big and dark, or small and light. Both objects would reflect the same amount of visible sunlight.
Infrared data provide a read on the object's temperature, which then tells an astronomer more about the actual size and composition. A big, dark rock has a higher temperature than a small, light one because it absorbs more sunlight.
Trilling and his team have analyzed preliminary data on 100 near-Earth asteroids so far. They plan to observe 600 more over the next year. There are roughly 7,000 known near-Earth objects out of a population expected to number in the tens to hundreds of thousands.
"Very little is known about the physical characteristics of the near-Earth population," Trilling said. "Our data will tell us more about the population, and how it changes from one object to the next. This information could be used to help plan possible future space missions to study a near-Earth object."
NASA's new space plan is aimed at sending astronauts to visit an asteroid by 2025, though there are relatively few viable targets for such a mission known today, agency officials have said.
Asteroids of every flavor
The Spitzer observations show that some of the smaller objects have surprisingly high albedos.(An albedo is the measurement of how much sunlight an object reflects.
Since asteroid surfaces become darker over time due to exposure to solar radiation, the presence of lighter, brighter surfaces on some of them may indicate that they are relatively young, researchers said. This is evidence for the continuing evolution of the near-Earth object population, they added.
In addition, the fact that the asteroids observed so far have a greater degree of diversity than expected indicates that they might have different origins. Some might come from the main asteroid belt between Mars and Jupiter, and others could come from farther out in the solar system.
This diversity also suggests that the materials that went into making the asteroids the same materials that make up our planets were probably mixed together like a big solar-system soup very early in its history.
In the future, both Spitzer and NASA's Wide-field Infrared Survey Explorer, an all-sky infrared survey mission also up in space now, will tell us even more about the "flavors" of near-Earth objects. This could reveal new clues about how the cosmic objects might have dotted our young planet with water and the organic ingredients needed to kick-start life.
In May 2009, after nearly six years of operation, Spitzer used up the liquid coolant needed to chill its infrared detectors.
The space observatory is now operating in a so-called "warm" mode (the actual temperature is still quite cold at 30 Kelvin, or minus 406 degrees Fahrenheit). But the space telescope is still conducting useful science observations, NASA officials have said.