New photos show Jupiter’s small moon Amalthea

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Amalthea (arrowed) passes Jupiter. Credit: NASA/JPL-Caltech/SwRI/MSSS. Image processing by Gerald Eichstädt

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Amalthea (arrowed) passes Jupiter. Credit: NASA/JPL-Caltech/SwRI/MSSS. Image processing by Gerald Eichstädt

It’s small, but it’s there. By now we’re all used to seeing great photos of Jupiter on a routine basis, thanks to NASA’s Juno mission. Many of these are processed by volunteer ‘citizen scientists’, and they show Jupiter’s swirling cloud tops in stunning detail courtesy of the spacecraft’s JunoCam.

JunoCam recently captured something special. Look closely at the side-by-side images of Jupiter from March 7, 2024, and you’ll see a small dot passing through the Great Red Spot in the main left image, which is not on the right. That is the small inner moon Amalthea, only 84 kilometers in diameter. The image was captured during the 59th perijove (close flight) of the ‘King of the Planets’, at a distance of 265,000 kilometers (about two-thirds of the Earth-moon distance).

Amalthea: an origin story

The elusive moon was discovered by prolific astronomer and observer E.E. Barnard on the night of September 9, 1892. Barnard used the Lick Observatory’s 3-foot-diameter refractor telescope to spot the moon at magnitude +14, which never strays far from Jupiter (less than the planet’s apparent diameter) during its 12-hour orbit.

Amalthea has the distinction of being the last moon discovered through direct visual observation, and the first moon of Jupiter discovered since Galileo first saw the four large Galilean moons in 1610. Today, Jupiter has 95 known moons, most of which are captured asteroids. These were discovered mainly photographically and during the flight of spacecraft.

Like other small moons, Amalthea is not large enough to pull itself into a true sphere. Instead, like the Martian moons Phobos and Deimos, Amalthea is a potato-shaped captured asteroid.

Jupiter, as seen from the surface of Amalthea. Credit: Stellarium

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Jupiter, as seen from the surface of Amalthea. Credit: Stellarium

Amalthea: Nothing red anymore

The moon is also the reddest object in the solar system and is undoubtedly undergoing some serious tidal bending thanks to the enormous gravitational field of nearby Jupiter. Amalthea is located 180,000 kilometers from Jupiter, just over 100,000 kilometers outside Jupiter’s Roche limit radius. If she were closer to Zeus, Amalthea would be torn apart. The innermost moon Metis just passes this boundary.

Voyagers 1 and 2 gave us the first blurry images of the moon. NASA’s only other Jupiter orbiter Galileo has given us the best images of Amalthea yet, with a flight from 230,000 miles (374,000 kilometers) away on November 26, 1999. Those images reveal a deformed world, similar to Mars’ moon Deimos. From the surface of Amalthea, Jupiter would offer an astonishing view, covering almost half the sky at 42 degrees across.

Juno and the current status of the mission

Juno was launched from the Cape on August 5, 2011 and arrived at Jupiter on July 5, 2016. The mission explores Jupiter’s interior and its magnetic and radiation environment. Juno will answer important questions, including whether the planet has a solid core.

Juno is the first solar-powered mission (as opposed to a nuclear/plutonium-powered mission) to the outer planets, meaning its nominal far-reaching orbit was intended to prevent radiation damage to the solar panels. Engineers allowed the spacecraft to pass past Jupiter’s inner moons only during the extended and final phase of the mission. Juno will be active until at least September 2025.

There are two more missions on their way to Jupiter; ESA’s JUICE (Jupiter Icy moons Explorer) was launched on April 14, 2023, and NASA’s Europa Clipper was scheduled to launch in October 2024.

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