This is not correct. Optical telescopes are used, in conjunction with the WISE space telescope. https://www.nasa.gov/content/asteroid-grand-challenge/detect...

Radio only works for relatively close asteroids.

https://en.wikipedia.org/wiki/Sentinel_Space_Telescope

Which was sadly cancelled: http://www.dailymail.co.uk/sciencetech/article-3255059/Nasa-...

But the principle was based off infrared: http://aviationweek.com/space/ball-aerospace-ramps-sentinel-...

The canadian satellite also uses light: https://en.wikipedia.org/wiki/Near_Earth_Object_Surveillance...

Visible light is only one fraction of the spectrum, infrared is far more valuable for detection. Most asteroids are detected via passive observation. Radio waves are not conducive to detecting asteroids.

My mistake. Thank you for the correction.

Active radar is subject to n^4 (!!!) drop-off with distance and as such is not useful for astronomical observation of any sort. Only passive detection is useful, which has a much more manageable dropoff of n^2. And asteroids don't emit radio waves, but they do reflect the Sun's light.

I think they're talking about the Goldstone Array: https://en.wikipedia.org/wiki/Goldstone_Solar_System_Radar

It's used for high-res imaging of asteroids that pass close by.

Interesting, though it appears to be useful for imaging, not so much detection.

I've never heard this before. Can you provide a reference for more info?

Radar is how lots of asteroid imaging is done:

https://en.wikipedia.org/wiki/Radar_astronomy#Asteroids_and_...

However, I wasn't aware of the optical methods mitchty was referring to.