NASA's Juno orbiter arrived at Jupiter on July 4, 2016, but the unmanned explorer is already approaching the halfway mark of its science mission. On December 21 at 8:49:48 am PST (11:49:48 am EST), the spacecraft will be marking the midpoint of its Jovian mapping and data collection mission with a close flyby on its 16th of 32 scheduled science orbits that will bring it within 3,140 mi (5,053 km) of the planet's cloud tops while traveling at 128,802 mph (207,287 km/h).
The Juno mission was launched from Space Launch Complex 41 at Cape Canaveral Air Force Station, Florida atop an Atlas/Centaur rocket on August 5, 2011. It took almost five years to reach Jupiter by way of a flyby of Earth in 2013 in a slingshot maneuver to gain enough velocity to match orbits with the giant planet.
The spacecraft was originally meant to go into a 14-day elliptical orbit around Jupiter, but a faulty valve prevented the probe's engines from firing to make the necessary course correction, leaving Juno in a much larger 53-day polar orbit at a distance of between 3,000 mi (5,000 km) and five million mi (eight million km) of the cloud tops. This meant that the craft was spending too much time in the outer Jovian system, so NASA approved a 41-month extension to allow the mission to run through 2022 and make more orbits to complete the required observations.
"With our 16th science flyby, we will have complete global coverage of Jupiter, albeit at coarse resolution, with polar passes separated by 22.5 degrees of longitude," says Jack Connerney, Juno deputy principal investigator from the Space Research Corporation in Annapolis, Maryland. "Over the second half of our prime mission – science flybys 17 through 32 – we will split the difference, flying exactly halfway between each previous orbit. This will provide coverage of the planet every 11.25 degrees of longitude, providing a more detailed picture of what makes the whole of Jupiter tick."
According to NASA, Juno has already sent back enough valuable data to keep scientists busy for decades as they try to learn more about Jupiter's atmosphere, origins, interior structure, magnetosphere and history. In addition to the suite of scientific instruments aboard, there have been unexpected dividends, like the Stellar Reference Unit (SRU) that was intended to send back navigation and engineering data, but has helped the space agency learn more about the planet's ring and radiation belts. And then there is the JunoCam, which was first intended for public relations.
"While originally envisioned solely as an outreach instrument to help tell the Juno story, JunoCam has become much more than that," says Candy Hansen, Juno co-investigator at the Planetary Science Institute in Tucson, Arizona. "Our time-lapse sequences of images over the poles allow us to study the dynamics of Jupiter's unique circumpolar cyclones and to image high-altitude hazes. We are also using JunoCam to study the structure of the Great Red Spot and its interaction with its surroundings."
The Juno mission will continue its science mission until 2021. By 2022 it will have suffered too much radiation damage to operate much longer, so it will be destroyed by making a controlled entry into the Jovian atmosphere.
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