on Earth. Most species, including the dinosaurs, became extinct in a

very short period of time.

Radar astronomy plays a critical role in studies of Near-Earth

Asteroids and especially in the context of the asteroid impact risk.

The first important contribution of radar is that it enables

considerable improvements to asteroid trajectory predictions. Radar

measurements allow us to locate an asteroid with superb precision,

about 1000 times better than optical measurements. With these

measurements, we can dramatically increase the interval of reliable

trajectory predictions. For instance, without radar measurements you

might be able to accurately predict the path of an asteroid over a

period of 90 years, with about 50 of those years in the future. With

the addition of radar measurements, the interval of reliable

trajectory predictions increases to 400 years, which is 4 times as

long, and with about 200 of those years in the future. This gives us

a much longer warning time should we have to mitigate against a

potential impact.

The second important role of radar astronomy in the asteroid impact

hazard has to do with the physical characteristics of the potential

impactor. Perhaps the most crucial thing we would want to know is

whether we are dealing with 1, 2, or even 3 objects? Radar

instruments have a unique ability to diagnose binary and triple

asteroids, and this knowledge is essential for impact hazard

mitigation efforts or any kind of spacecraft proximity operations.

Then we would want to get the best possible description of the

potential impactor. We would want to know as much as we can about the

size, spin, shape, mass, density, and porosity of the potential

impactor. Radar observations provide the most realistic ground-based

prospects of securing estimates of all of these quantities. This is

not only useful in the impact hazard context, but also for sending

astronauts to asteroids, or even for mining asteroids.

With optical telescopes, asteroids appear as unresolved points of

light. Radar observations allow us to obtain images of asteroids with

very high resolution, 10 meters or better. With a sequence of such

images we can create 3-dimensional shape models. Overall, radar

astronomy provides an exquisite description of near-Earth asteroids.

We use two facilities to make these observations. The Arecibo

Observatory in Puerto Rico is the largest telescope on Earth, it's 300

meters across and it's equipped with a megawatt transmitter. We also

use the Goldstone antenna in the Mojave desert in California, and that

antenna is 70 meters across and is equipped with a 450 kw transmitter.

We have detected over 400 near-earth asteroids with radar so far.

Last year we detected over 60.

To learn more about the power of radar astronomy to characterize

asteroids and their trajectories, go to radarastronomy.org. You will

find additional information on how radar can help protect our planet

and facilitate the future exploration of asteroids".