Amateur astronomers help decide the path of Juno



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Prior to a decision on whether NASA's spacecraft was to fly over the Great Red Spot of Jupiter, Earth enthusiasts provided crucial information about the effects of an intrusive disturbance.

Juno Jupiter Images

A southern tropical disturbance has just passed Jupiter's iconic Great Red Spot and is caught stealing wires of orange haze in this series of colorful images of NASA's Juno spacecraft.
NASA / JPL-Caltech / SwRI / MSSS / Gerald Eichstadt / Sean Doran

Planning observations for the spacecraft Juno on Jupiter is a group effort, and Candice Hansen (Institute of Planetary Science) should know. As a scientist in charge of JunoCam – the only camera aboard the spacecraft – she spends a lot of time talking to amateur astronomers, noting what is happening in Jupiter's turbulent atmosphere. Your intelligence is key to figuring out where to aim at JunoCam each time the spacecraft makes another passage near the planet.

But lately, this worldwide network of Jupiter enthusiasts in the backyard has helped mission scientists find out where exactly Juno should fly. Speaking last week at a meeting of the American Geophysical Union in Washington, D.C., Hansen described how Juno's army of amateurs were key players in deciding whether or not to proceed with an imminent overpass of the Great Red Spot of Jupiter.

Hansen sat down with Sky and telescope to talk about the next flyby and the role that amateur astronomers played in planning.

Juno, which has been in polar orbit since July 2016, is halfway through its mission to map the deep interior of Jupiter. The spacecraft spends most of the time away from the planet, but once every 53 days, it comes low and fast, going from the north pole to the south pole in about two hours. Near the equator, the probe advances about 250,000 kilometers per hour (150,000 mph) just a few thousand kilometers above the clouds.

For each of these approaches closer, mission scientists can adjust Juno's speed to precisely control which lengths will fly over. When planning the eighteenth spacecraft overfly – referred to as perijove 18 in the jargon of the mission – the scientists realized that they had the chance to make a second passage through the Great Red Spot if they changed the original plane by perijove 18 with the plane of perijove 25.

But amateur astronomer John Rogers, chief of the Jupiter Section of the British Astronomical Association, called for caution. "John warned us that before making this exchange, you might want to make sure the Great Red Spot will be there," says Hansen.

A disturbance in the sky

Image of Jupiter

This terrestrial observation of Jupiter and the Southern Tropical Purple, approaching the Great Red Spot, was captured on January 26, 2018.
Christopher Go

Back during perijove 9 – on October 24, 2017 – JunoCam imagined an atmospheric disturbance not far from the Great Red Spot. Rogers, who maintains contact with Jupiter's worldwide amateur network, acknowledged what happened immediately: a resurgence of a turbulent swamp known as Southern Tropical Disturbance. This dark, bumpy spot occasionally appears at the same latitude as the Great Red Spot. . . and has a history of poking his most famous cousin.

The Southern Tropical Disturbance was discovered in 1901 by another British amateur military officer, Percy Molesworth. It stayed until 1939, beating the Great Red Spot eight times. Writing in the Journal of the British Astronomical Association in 1997, Richard McKim described Molesworth's disruption as a "dark stretch of the tropical zone south of Jupiter, through which the normal pattern of alternating dark belts and light zones is disrupted." appeared seven times, with one in 1979 well timed for the overflight of the Voyager 2 spacecraft.

Rogers knew this story well and also knew that the Southern Tropical Disturbance had a propensity to change the speed of the Great Red Spot on its east-west journey through the skies of Jupiter. He was worried that this appearance would cause Red Spot to miss the expected meeting with Juno in February 2019.

"We needed to wait as long as we could to see how this was going to happen," says Hansen. She and her colleagues persuaded project managers to give them as much time as possible while amateurs sent updates on how the Red Spot and Disorder were doing well.

Terrestrial observations showed the disturbance cuddling the Great Red Spot in February 2018. Over the next few months, the dark gasses swirled around the southern border of the Spot, while the Southern Tropical Disturbance once again licked the reddish storm. During April 12, Juno passed just 10 degrees eastward from the Great Red Spot and watched the disturbance drift away, sending rows of mist into its wake.

Data from amateur astronomers have been invaluable, says Hansen. In the end, the team decided to go ahead and try the Red Spot on approaching February. "We may not fly well in the middle of it," she says, "but it will be close enough to do the science we want to do."

Green light for the red spot

Close-up of a South Tropical Disturbance

A southern tropical disturbance that has just passed Jupiter's iconic Great Red Spot is captured in this colorful image of NASA's Juno spacecraft.
NASA / JPL-Caltech / SwRI / MSSS / Kevin M. Gill

This is not the first time Juno plays the Great Red Spot. On July 2, 2017, (perijove 7) the spacecraft moved forward with its microwave antenna probing the roots of the gigantic storm. These data revealed that the Spot – measuring 1.3 times the width of the Earth – stretches at least 300 km (200 miles) below the cloud.

During the next passage, Juno will be guided to fathom the gravitational field on the storm, which will reveal how much excess mass is spinning down there. Researchers map Jupiter's gravity by tracking small changes in Juno's speed revealed by Doppler changes in the radio link back to Earth. By combining microwave and gravity data, scientists must learn a lot about the vertical structure of Spot, says Hansen.

The amateurs would always be an integral part of the mission. The JunoCam was designed to be a citizen science camera, with observations from Earth enthusiasts providing a continually updated "big picture" view from where to look next. And without a dedicated team of professionals cleaning JunoCam images, the mission provided the raw data for an eager audience that produced surrealist Jupiter scenes that often confuse the line between science and art.

"One of the good things about amateur astronomers is that they can go out in their backyard any night and decide what they want to see," says Hansen. "They are, in fact, the best positioned community to see when something changes. . . . We have been counting on them since the beginning of the mission to keep us updated on what is happening in Jupiter. "

So far, however, such interactions have been limited to decisions about where to aim the camera. But with the emergence of the Southern Tropical Disturbance and its potential effect on the Great Red Spot journey, notes Hansen, "this is the first time this really affects our mission project."

References:

C. J. Hansen et al. "Images of Jupiter: science and art." 2018 Meeting of the American Geophysical Union. December 11, 2018.

J.H. Rogers et al. "The New South Tropical Disturbance and its interaction with the Great Red Spot." Conference on European Planetary Science. September 18, 2018.

R. J. McKim. "P. B. Molesworth Discovery of the Great Southern Tropical Disturbance in Jupiter, 1901. " Journal of the British Astronomical Association. October 1997.

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