Orbiting inside the orbit of Io, which is the innermost of the four largest moons of Jupiter (called the Galilean moons), are four smaller moons. These are:

Metis: 40 kilometers (25 miles) in diameter, orbiting 128,000 kilometers (79,500 miles) from Jupiter.

Adrastea: 20 kilometers (12 miles) in diameter, orbiting 129,000 kilometers (80,000 miles) from Jupiter.

Amalthea: 189 kilometers (117 miles) in diameter, orbiting 181,300 kilometers (112,700 miles) from Jupiter.

Thebe: 100 kilometers (60 miles) in diameter, orbiting 222,000 kilometers (138,000 miles) from Jupiter.

We don't know much about any of these moons. Even these size measurements may not be very accurate. They are all oddly shaped, lacking either the mass and/or fluidity of composition to pull themselves into a reasonably spherical shape. The Galileo spacecraft has revealed some surface features, including impact craters, hills, and valleys.

Since Io orbits about 422,000 kilometers above Jupiter and, at this close distance, is subjected to extreme tidal flexing from Jupiter's gravity, one would imagine that these even closer satellites would be pulled to pieces. However, because they are so small (the largest, Amalthea having a diameter that is just 1/19th that of Io), they are relatively immune to the effects of tidal forces. However, the two closest moons, Metis and Adrastea, orbit inside what is called the synchronous orbit radius of Jupiter. That is, they orbit Jupiter faster than Jupiter rotates on its axis. At this distance, the satellites' orbits will eventually decay and they will fall into the planet.

Metis and Adrastea also orbit inside Jupiter's main ring and are undoubtedly the source of the material in the ring. Amalthea and Thebe provide the material for the Gossamer ring.

Amalthea is the reddest object in the solar system and it appears to give out more heat than it receives from the Sun. This may be because, as it orbits within Jupiter's powerful magnetic field, electric currents are included in the moon's core. Or, the heat could be from tidal stresses.

Amalthea and Thebe rotate on their axes once for each orbit around Jupiter (which take around one-half an Earth day for Amalthea and two-thirds an Earth day for Thebe), always keeping the same side facing the planet. We do not know the rotational periods for Metis and Adrastea, but their orbital periods are 5 and 7 hours, respectively.