NASA announced on Monday that its Juno space probe has reached the halfway mark on its voyage to Jupiter after covering a distance of 9.464 astronomical units (879 million mi / 1.4 billion km). This seems odd when you consider that Juno will pass just 347 miles (559 km) from Earth in October. Why both of these facts are true is due to the complex orbit that Juno is following in order to reach its destination.
Juno has indeed traveled half the distance to Jupiter, but only if you measure the total distance the probe must travel as if it were a straight line. In fact, Juno is, at the moment, 34.46 million miles (55.46 million km) from Earth and since its launch August 5, 2011 has remained in a terrestrial orbit. This is because the Atlas V rocket that sent it into space didn’t have enough power to provide the probe with enough velocity to send it to Jupiter.
To make up for this, NASA sent Juno on a long elliptical orbit out past Mars and then back to Earth. When Juno returns, its close pass to its home planet will speed it up so much that it will produce a slingshot effect and hurl the craft to its rendezvous point with Jupiter. It’s effective, though it does mean that Juno will have spent six years of its seven-year mission reaching its destination.
"On Oct. 9, Juno will come within 347 miles of Earth," says the mission's Project Manager Rick Nybakken of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The Earth flyby will give Juno a kick in the pants, boosting its velocity by 16,330 mph (about 7.3 km/sec). From there, it's next stop Jupiter."
Built by Lockheed-Martin, Juno will use imagers, spectrometers, plasma and energetic particle detectors, magnetometers, and gravity instruments to study the interior of Jupiter. The objective of the mission is to measure the amount of water and ammonia in the deep atmosphere, to determine if the giant planet has a solid core, and to collect clues as to Jupiter’s structure and origin. It’s also the first Jupiter probe to use solar panels as the primary source of power instead of radiothermal generators.
Juno is currently carrying out a series of deep space maneuvers in anticipation of the October flyby. After the slingshot maneuver, Juno will arrive at Jupiter on July 4, 2016 and go into polar orbit around the planet. It will circle Jupiter for one year while it makes 33 highly elliptical orbits, which will take it within 4,300 kilometers (2,672 mi) of the surface and then out beyond the orbit of the moon Callisto. Once it has completed its mission, Juno will be deliberately sent into Jupiter’s atmosphere in October 2017 to burn up in order to ensure that it cannot contaminate any of the moons with Earth microorganisms.
The animation below shows Juno’s October flyby.