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 NASA’s Juno spacecraft is set to make the closest flyby of Jupiter’s moon Io that any spacecraft has made in over 20 years.

Juno will pass within roughly 1,500 kilometres from the surface of Io -- the most volcanic world in our solar system -- on Saturday.

“By combining data from this flyby with our previous observations, the Juno science team is studying how Io’s volcanoes vary,” said Juno’s principal investigator Scott Bolton of the Southwest Research Institute in Texas, US.

“We are looking for how often they erupt, how bright and hot they are, how the shape of the lava flow changes, and how Io’s activity is connected to the flow of charged particles in Jupiter’s magnetosphere,” Bolton added.

Launched in August 2011, the spacecraft arrived at Jupiter in July 2016. The orbiter has so far performed 56 flybys of Jupiter and documented close encounters with three of the gas giant’s four largest moons.

A second ultra-close flyby of Io is scheduled for February 3, 2024, in which Juno will again come within about 1,500 kilometres of the surface.

The spacecraft has been monitoring Io’s volcanic activity from distances ranging from about 11,000 kilometres to over 100,000 kilometres, and has provided the first views of the moon’s north and south poles. The spacecraft has also performed close flybys of Jupiter’s icy moons Ganymede and Europa.

“With our pair of close flybys in December and February, Juno will investigate the source of Io’s massive volcanic activity, whether a magma ocean exists underneath its crust, and the importance of tidal forces from Jupiter, which are relentlessly squeezing this tortured moon,” said Bolton.

Now in the third year of its extended mission, with a total of 18 flyby’s, to investigate the origin of Jupiter, the solar-powered spacecraft will also explore the ring system where some of the gas giant’s inner moons reside.

After the close Io pass on February 3, the spacecraft will fly by Io every other orbit, with each orbit growing progressively more distant: The first will be at an altitude of about 16,500 kilometres above Io, and the last will be at about 115,000 kilometres.

The gravitational pull of Io on Juno during the December 30 flyby will reduce the spacecraft’s orbit around Jupiter from 38 days to 35 days. Juno’s orbit will drop to 33 days after the February 3 flyby.

After that, Juno’s new trajectory will result in Jupiter blocking the Sun from the spacecraft for about five minutes at the time when the orbiter is at its closest to the planet, a period called perijove.

Although this will be the first time the solar-powered spacecraft has encountered darkness since its flyby of Earth in October 2013, the duration will be too short to affect its overall operation.

With the exception of the February 3 perijove, the spacecraft will encounter solar eclipses like this during every close flyby of Jupiter from now on through the remainder of its extended mission, which ends in late 2025.

Starting in April 2024, the spacecraft will carry out a series of occultation experiments that use Juno’s Gravity Science experiment to probe Jupiter’s upper atmospheric makeup, which provides key information on the planet’s shape and interior structure.