We’ve come a long way since the second half of the 20th century in terms of how well we know the planets and their systems in our solar system. While groundbreaking theories that defined the behaviour of various features of the universe were already in place, observational data corresponding to the structure and surface of the planets were yet to be collected.
The World War II brought along with it a slew of technological advances, including rapid development of radar. Once we were through with the war, scientists were quick to recognise the potential of this new technology and the possibility of employing it to learn more about our solar system.
Birth of radar astronomy
The plan was to detect radar echoes by bouncing them off bodies, both small and large, in the solar system. The first target naturally was our own satellite and Project Diana bounced and received radar signals off the moon in 1946. Radar astronomy was born.
Next up was Venus, a planet shrouded in mystery owing to the featureless dense clouds that surrounded the planet. With space probes to the planet in the pipeline, it was of absolute interest to find out as to where exactly the planet would be at any given time.
First attempt
On February 10, 1958, the first such attempt for a precise measurement was made when radar signals from MIT Lincoln Laboratory in Massachusetts, the U.S. were directed towards Venus. Scientists believed that an inferior conjunction with Venus allowed ideal conditions for observing from the Earth. (When seen from a superior planet - an outer planet, an inferior conjunction takes place when two planets lie in a line on the same side of the Sun.) The results, however, were ambiguous, allowing interpretations that varied from weak radar returns to even random noise.
The experiments were repeated by the U.K.’s Jodrell Bank in 1959 and their results were in fact similar to those obtained by the Lincoln Lab. But since those at Lincoln were unable to repeat their result, even with a more powerful radar, it didn’t convince everyone.
It had to wait for another two years as it was only on March 10, 1961 that NASA’s Jet Propulsion Laboratory successfully bounced off radar from Venus. Six-and-a-half minutes after beaming the first radar signals, they started receiving the echoes in return. The first confirmed radar observations of Venus was followed by hundreds of hours of further observation over the next two months.
A mistake that magnifies
Apart from pinpointing the position of Venus, these measurements proved critical in defining the astronomical unit (AU) more accurately. It was found by those working on the project that the then conventional value of AU adopted by the astronomical community was actually off the mark by almost one part in one thousand when Venus’ position was charted out.
While that might seem like a small number, it magnifies when working with big numbers. For instance, the old value of AU would have meant that the Mariner 2 probe, which became the first robotic space probe to conduct a successful planetary encounter when it flew past Venus in 1962, would have been off its course by thousands of km.
With the influence it had on defining an AU, radar astronomy gained in prominence and it was used to study Mercury, Mars and also the moons of Jupiter and Saturn. As there is a limit to how powerful radars can be, they are mainly used in studying near-Earth asteroids these days.
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What is an astronomical unit?
In our pursuit to deal with the unknowns in the universe in a familiar setting, we often relate them to those that we already know about. Take exoplanets for instance. When we classify them, we more often than not compare them to planets in our own solar system.
In a similar way, when we are thinking of distances across space, we tend to employ the astronomical unit (AU), which corresponds to the distance between the Sun and the Earth. This unit allows us to define distances, vast and small, either as a multiple or fraction of one astronomical unit.
As the distance between the Earth and the Sun varies during Earth’s revolution and with improving technology affording better measurements, the definition of an AU by the International Astronomical Union (IAU) has varied with time.
The latest definition was adopted in a meeting of the IAU in 2012, when members voted to make the AU an exact 149,597,870,700 metres, which is the average mean distance between the Earth and the Sun when viewed from the Earth.
