For most of our history, astronomy has been a strictly spectator sport.

We observed and studied the light and other radiation that objects in the cosmos happened to be sending in our direction. These days, we are exploring the Solar System using spacecraft to fly-by, orbit or even land on other planets and moons. Since the Space Age started in late 1957, intriguingly, the first time we reached out and touched another body in space was in 1946, when a radar in the U.S. successfully bounced radio waves off the Moon. That marked the beginning of radar astronomy.

Radar—short for Radio Detection and Ranging—involves detecting, identifying and tracking distant objects by bouncing radio waves off them. Today it is an important facet of modern life, in peace and war.

Without radar, commercial air travel would be very difficult, and marine trade a lot more risky. It played a critical role in the Second World War, with the National Research Council of Canada becoming, along with the U.S. and U.K., a major development centre for radar systems. Canada’s first radio telescope was made out of war surplus radar components.

Most radar systems work by transmitting high-power pulses of radio waves. The pulses are reflected by distant objects and are detected using a specially designed radio receiving system.

Light and radio waves travel at 300 metres per millionth of a second so by measuring the time interval between the transmission of the pulse and the reception of the echo, we can determine the range of the object reflecting the pulses. By noting the direction the antenna is looking, we can assign a position to the target. Moreover, we can do this at night and in bad weather.

However, there is one downside to radar. Because the signal has to reach the target and bounce back to us, increasing the range dramatically reduces the strength of the echo. If the target moves to ten times as far away, the echo becomes ten thousand times weaker. This meant that getting a radar echo off the Moon would be a challenge, and detecting objects even further away would be an even bigger one.

Thanks to improvements in electronics and signal processing techniques, and the availability of really big antennas, such as the 305-metre dish at Arecibo, radar observations have been made of Mercury, Venus and Mars, yielding information about their surfaces and rotation rates. Venus is covered by a permanent layer of thick cloud. Radar observations from the ground and then from the Magellan spacecraft as it orbited the planet revealed that hidden surface in high detail. It is not a place people are likely to visit any time soon.

Radar has also been used to study the rings of Saturn. In addition, it has been used to detect a number of comets and a good number of asteroids. It also enables us to track spacecraft, satellites and space junk in Earth orbit.

One very important application of radar in astronomy is to detect asteroids that could pose a risk of hitting Earth. Although radar systems cannot detect asteroids out to the distances achievable with optical telescopes, they have the huge advantage of producing almost immediate measurements of the speed and direction of any asteroids they detect. In addition, radar systems can be used during daylight, to detect anything coming from a sunward direction, and bad or cloudy weather does not affect them.

Because we now exploit our world very intensely, even an impact by a relatively small object, say 100 metres in diameter, would have very serious consequences. So astronomical radar systems are an important part of our planetary defences.

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• Venus and Mars lie very low in the dawn glow.

• Jupiter shines in the west after sunset, with Mercury hiding low in the sunset glow.

• The Moon will be full on March 25.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.