Amalthea (arrow) transits Jupiter. Credit: NASA/JPL-Caltech/SwRI/MSSS. Image processing by Gerald Eichstädt
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Amalthea (arrow) transits 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 regularly seeing amazing pictures of Jupiter provided by NASA’s Juno mission. Many have been processed by volunteer “citizen scientists” and show Jove’s swirling cloud tops courtesy of the spacecraft’s JunoCam in stunning detail.
JunoCam captured something special recently. Look closely at the side-by-side images of Jupiter from March 7, 2024, and you’ll see a small speck crossing the Great Red Spot in the left lead image that isn’t in the right. This is the small inner moon Amalthea, only 84 kilometers in diameter. The image was taken during the 59th period (close flyby) 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 91-centimeter diameter refracting telescope at Lick Observatory to spot the +14th magnitude moon, which never moved away. of Jupiter (less than the planet’s apparent diameter) on its 12-hour orbit.
Amalthea has the distinction of being the last moon discovered by direct visual observation and the first moon of Jupiter discovered since Galileo first spotted the four large Galilean moons in 1610. Today, Jupiter has 95 known moons, mostly captured asteroids. They have been discovered mainly photographically and during spacecraft flybys.
Like other small moons, Amalthea is not large enough to be drawn into a true sphere. Instead, like the Martian moons Phobos and Deimos, Amalthea is a captured potato-shaped 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: No more red
The Moon is also the reddest object in the Solar System and no doubt undergoes severe tidal bending thanks to the massive gravitational field of nearby Jupiter. Amalthea is located 180,000 kilometers from Jupiter, just over 100,000 kilometers outside Jupiter’s Roche boundary radius. Any closer to Jupiter would tear Amalthea apart. The innermost moon Metis just straddles this boundary.
Voyagers 1 and 2 gave us the first blurry views of the Moon. NASA’s only other Jupiter orbiter, Galileo, provided us with the best images of Amalthea to date, with a flyby at a distance of 374,000 kilometers on November 26, 1999. These images reveal a deformed world not unlike the Mars moon Deimos. From the surface of Amalthea, Jupiter would provide a spectacular view spanning nearly half the sky at 42 degrees across.
Juno and the current state of the mission
Juno launched from the Cape on August 5, 2011 and arrived at Jupiter on July 5, 2016. The mission is exploring Jupiter’s interior and its magnetic and radiation environment. Juno will answer key questions, including whether the planet has a solid core.
Juno is the first solar-powered (as opposed to nuclear/plutonium-fueled) mission to the outer planets, meaning its nominal wide-area orbit is designed to avoid radiation damage to solar panels. Engineers allowed the spacecraft to pass through Jupiter’s inner moons only during the extended and final phase of the mission. Juno will operate until at least September 2025.
Two more missions are headed for Jupiter; ESA’s JUICE (Jupiter Icy moons Explorer), launched on April 14, 2023, and NASA’s Europa Clipper, due for launch in October 2024.
