 On Tuesday, astronomers of the Carnegie Institution for Science in Washington, DC, United States, announced the discovery of ten small satellites orbiting Jupiter. With this discovery, Jupiter now has 79 known satellites.
The team led by Scott Sheppard had discovered twelve of the 79 Jovian satellites, including Tuesday's ten, mostly using a Blanco 4-meter telescope of Chile's Cerro Tololo Inter-American Observatory. The observatory is operated by the National Optical Astronomy Observatory in the US. The tiny satellites, none more than five kilometres in diameter, were first observed in 2017. Orbits of these new Jovian satellites were calculated by International Astronomical Union's Minor Planet Center's Gareth Williams. Williams explained, "It takes several observations to confirm an object actually orbits around Jupiter [...] So, the whole process took a year."
The astronomers were looking for planets much farther out than Pluto. Sheppard said, "Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant Solar System objects, so we were serendipitously able to look for new moons around Jupiter while at the same time looking for planets at the fringes of our Solar System".
Of the twelve satellites discovered by the team, nine were found to be retrograde, revolving around the gas giant in the direction opposite to the planet's spin. These nine new retrograde satellites take about two years to complete one revolution around Jupiter.
The remaining three satellites were prograde, spinning in the same direction as Jupiter's rotation. One of the prograde satellites, newly announced on Tuesday, took about one-and-half years to complete one revolution around Jupiter, and its orbit intersected with the outer retrograde satellites. Sheppard said, "Our other discovery is a real oddball and has an orbit like no other known Jovian moon [...] It's also likely Jupiter's smallest known moon, being less than one kilometre in diameter". The astronomer also said, "This is an unstable situation [...] Head-on collisions would quickly break apart and grind the objects down to dust."
Sheppard said of the composition of those satellites, they "started orbiting Jupiter, instead of falling into it. So we think they are intermediate between rocky asteroids and icy comets. So they are probably half ice and half rock."
"Valetudo" is the name suggested for the "oddball" satellite. Valetudo was the Roman god Jupiter's great-granddaughter, regarded as the goddess of health and hygiene.
Sheppard said, "Jupiter is like a big vacuum cleaner because it is so massive". Jupiter is the largest planet in the Solar System, with a diameter about 142,984 kilometres. The largest known satellite in the Solar System is Jupiter's Ganymede, whose diameter is approximately 5268 kilometres. Saturn has the second-most known satellites: 62, while Uranus has 27.

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US astronomers announce discovering ten tiny Jovian satellites