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Jupiter probe Juno buzzes Earth to pick up speed

Paul Sutherland, Feature writer
Oct 9, 2013, 7:00 UTC

Sen—A spacecraft will pay a fleeting visit to the Earth today as NASA’s Juno probe flies by on its long journey to Jupiter.

Closest approach comes at 19.21 UT tonight when the unmanned craft swoops just 560 km (350 miles) over South Africa before heading off again into deep space.

Juno was launched way back on 11 August, 2011, from the Kennedy Space Centre, Florida. It may seem odd that it has popped briefly home, but it is all part of the plan to speed it on its mission to learn more about the giant planet of the Solar System.

The flyby will allow the Earth to give Juno a gravitational boost, firing it on its way like a slingshot. Such manoeuvres, which have been likened to interplanetary snooker, allow the spacecraft to build up momentum on its lengthy journey. It will not reach its destination until 4 July, 2016.

When it gets to Jupiter, Juno will go into a low, elliptical orbit and circle the planet from pole to pole, slowly spinning as it flies. Hugging the planet means Juno will avoid the regions where Jupiter pumps out its highest levels of radiation that could destroy the instruments, particularly a belt around the equator where tiny charged particle of ions and electrons zip about at close to the speed of light.

Even with the special flight plan, radiation levels will still, over a year, be equivalent to more than 100 million dental X-rays and high enough to fry vital circuitry. To avoid this, the most sensitive electronics have been shielded within a titanium vault a metre wide and with walls a centimetre thick.

Juno also has a set of three vast solar panels that will soak up sunlight even at Jupiter’s great distance where it is 25 times weaker than at Earth. With panels extended the craft will be 20 metres wide.

The weight of such a vault and those large solar panels helped determine that it would need an additional fillip on its journey to Jupiter.

A video about Juno's close flyby of the Earth. Credit: SwRI

The probe’s principal investigator, Dr Scott Bolton of Southwest Research Institute, said: “Juno is a large, massive spacecraft. Even a large rocket couldn’t provide enough propulsion to get us all the way to Jupiter, so we are flying by the Earth for a gravity-assist that will provide about 70 percent of the initial boost provided by the Atlas V 551 rocket. The gravity assist essentially provides as much propulsion as a second rocket launch.”

Juno’s launch rocket provided it with enough speed to reach the asteroid belt, where the Sun’s gravity pulled it back toward the inner solar system. The Earth flyby gravity assist increases the spacecraft’s speed relative to the Sun from 126,000 km (78,000 miles) per hour to 140,000 km (87,000 miles per hour).

Juno will brighten to around magnitude 10 when at its closest, bringing it in range of amateur astronomers’ small telescopes, but it will rapidly fade as it departs. Predictions for its position are on the Heavens Above website.

Bolton added: “While we are primarily using Earth as a means to get us to Jupiter, the flight team is also going to check and calibrate Juno’s science instruments. As another bonus, Juno is approaching the Earth from deep space, from the sunlit side.

“Juno will take never-before-seen images of the Earth-Moon system, giving us a chance to see what we look like from Mars or Jupiter. We plan to release a movie of this unique perspective of the Earth-Moon system shortly after the flyby.”