Radar spacecraft can see the planet's surface in all weathers, day and night.

BBC: Business questions

Today, thanks to radar and interplanetary spacecraft, the astronomical unit is known with a precision of one part in 50 billion.

ECONOMIST: Transits of Venus

When scientists used the spacecraft's antennae to analyse radar echoes from the lunar surface, they found that within certain deep canyons near the south pole the echoes increased and were altered in just the way they would be if dirty icerather than rock were filling the caverns.

ECONOMIST: Divining water

The latest 3-D maps, painstakingly pieced together from radar data collected by numerous spacecraft passes, have already changed scientists' notions: The channels are deeper than expected, a sign that flooding was more extensive than generally believed.

NPR: Radar Reveals Apparent Buried Channels On Mars