New observations by NASA's Wide-field Infrared Survey Explorer, or WISE, show there are significantly fewer near-Earth asteroids in the mid-size range than previously thought.

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Amy Mainzer: The NEOWISE data has returned two very important findings. First, we've been able to determine
that we've found 93% of all the near-Earth asteroids that are out there that are larger than one kilometer.
And thus we've satisfied the goals of the Congressionally-mandated Spaceguard project.
We've also been able to tell that there are somewhat fewer near-Earth asteroids that are larger than
100 meters than we previously thought.
Before, surveys had indicated that there were about 36,000 near-Earth objects
that were larger than about 100 meters. With the NEOWISE data we've been able to determine that
there are really only about 20,500 in the total population. So: somewhat fewer.
However, 'fewer' does not mean 'none.' that leaves about 15,000 asteroids larger than 100 meters
that remain to be found.
The NEOWISE survey that we've carried out could be viewed as a pilot project for a larger survey.
When we look at asteroids from far away using a telescope, they just look like tiny dots.
So, something that is small, but very reflective, could look the same as something that's large but very dark.
And to a visible telescope they will look alike. But, if you use an infrared telescope, like WISE,
we can tell the difference.
We've shown with NEOWISE that infrared surveys are an efficient and effective way of learning
something about the true numbers, sizes and reflectivities of asteroids that are out there.
This is pretty amazingly cool. I am so pleased to finally see this answered
after working on this for years and years.
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NASA Jet Propulsion Laboratory, California Institute of Technology
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