ESA JUICE probe targets dark Jovian moon Kallichore in 2031

Jupiter hosts more than 100 moons, with most attention focused on the four Galilean moons: Io, Europa, Ganymede and Callisto. This is primarily due to their active geology, including Io being the most volcanically active planetary body in the solar system and Europa having a vast subsurface liquid-water ocean beneath its icy crust. However, Jupiter’s smaller, lesser-known moons could provide astronomers with key insights into the history of the solar system because they have remained largely unchanged, unlike the Galilean moons.

Now, an international team of researchers has shed light on one of Jupiter’s smallest moons, Kallichore, which has an average diameter of 3.8 kilometers (2.4 miles) and is known as an irregular moon because of its extremely noncircular orbit. For context, the smallest Galilean moon, Europa, has a diameter of about 3,122 kilometers (1,940 miles).

The primary motivation behind the study, whose findings were recently published in Nature, was to use a variety of space- and ground-based telescopes to gain new insight into Kallichore in preparation for a planned flyby of the small moon by the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) spacecraft, which is slated to arrive in the Jupiter system in July 2031.

To accomplish this, the researchers obtained data from NASA’s Hubble Space Telescope and the Gran Telescopio de Canarias, a multinational, multi-institution-operated telescope located on the island of La Palma off the coast of West Africa.

In the end, the researchers found that Kallichore’s area-equivalent diameter—the diameter if its irregular shape were a perfect sphere—was about 3.8 kilometers (2.4 miles). Its geometric albedo (brightness) was calculated at 0.037 (3.7%), meaning the tiny moon is incredibly dark. For context, albedo is measured on a scale from dark to bright, from 0 to 1.

Finally, the researchers put tighter constraints on Kallichore’s orbit, enabling JUICE mission planners to better estimate a flyby trajectory. Before the study, the estimated location of Kallichore upon JUICE’s planned arrival had a margin of error of about 272 kilometers (169 miles). However, these new measurements reduce this error margin by about 80%, with JUICE needing a margin of error of about 30 kilometers (19 miles) to substantially increase its chances of a flyby of Kallichore in October 2031.

Irregular moons have solar system secrets

While the ultimate goal of planetary science is to find life beyond Earth, Europa is a prime target for astrobiologists. However, because these larger and more geologically active moons have changed significantly since their initial formation, irregular moons can provide scientists with a more pristine sample of the ancient solar system, potentially allowing them to study an object as old as the solar system itself.

Of the 456 currently known solar system moons, an estimated 396 (about 87%) are irregular moons. This provides scientists with a trove of scientific data regarding the formation and evolution of the solar system. However, their extreme orbits make studying irregular moons extremely difficult. In the case of Kallichore, it orbits Jupiter at an average distance of 23 million kilometers (14.3 million miles) from the solar system’s largest planet, but its distance ranges between 17.7 million kilometers (11 million miles) and 30.4 million kilometers (18.9 million miles) from Jupiter.

JUICE to unlock Jupiter’s moons’ secrets

As noted, the purpose of this study was to prepare for a potential flyby of Kallichore by ESA’s JUICE spacecraft in 2031. The spacecraft was launched in April 2023 and is slated to arrive in the Jupiter system in July 2031. The primary goal of JUICE will be to study the icy Galilean moons to ascertain whether they host conditions habitable for life. However, JUICE is also slated to become the first spacecraft to orbit a moon other than Earth’s moon when it enters orbit around Ganymede, an event slated to occur in December 2034.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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