Callisto is the second largest moon of Jupiter after Ganymede. With a diameter of over 4,800 km, it is the third largest satellite in the solar system and is almost 99% of the size of Mercury but has less than 1/3 of its mass. It is the outermost of the Galilean satellites and orbits beyond Jupiter’s main radiation belts. This moon is the most heavily cratered object in the solar system. It is thought to be a long dead world, with a nearly complete absence of any geologic activity on its surface. In fact, Callisto is the only body greater than 1000 km in diameter in the solar system that has shown no signs of undergoing any extensive resurfacing since impacts have molded its surface. With a surface age of about 4 billion years, Callisto has the oldest landscape in the solar system.
Maybe you are wondering what a Galilean moon(satellite) is? They are the four moons of Jupiter that were discovered by Galileo Galilei in January 1610. They are the largest of Jupiter’s moons. They are named after the lovers of Zeus: Ganymede, Europa, Io, and Callisto. They are among the most massive objects in the solar system other than the Sun and the eight planets. Their various radii is larger than the dwarf planets. They were discovered when Galileo made improvements to his telescope. These upgrades enabled him to observe celestial bodies more distinctly than had ever been possible before. Galileo’s discovery showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye.
Now, back to Callisto. It is composed of nearly equal amounts of rock and ice, with a mean density of about 1.83g/cm3. Several compounds have been detected, spectrally, on the surface: water ice, ammonia ice, carbon dioxide, silicates, and other organic compounds. Close investigation by the Galileo spacecraft revealed that the moon may have a small silicate core and possibly an ocean of liquid water at depths greater than 100 km under its surface. The moon has an albedo of .22, an apparent magnitude of 5.65, and an average surface temperature of 134 K. The possibility of microbial life in the moon’s ocean has been discussed. It is though to be somewhat doubtful because of the low amount of heat generated at the moon’s core.
In 2003, NASA conducted a conceptual study called Human Outer Planets Exploration (HOPE) regarding the future human exploration of the outer Solar System. The target chosen to consider in detail was Callisto, for the purposes of investigating the possible existence of life forms embedded in the ice crust on this moon and on Europa.
The study proposed a possible surface base on Callisto where a crew could “teleoperate a Europa submarine and excavate Callisto surface samples near the impact site”. In addition, this base could extract water from Callisto’s ample supply of water ices to produce rocket propellant for further exploration of the Solar System.
The advantages of a base on Callisto include low radiation (due to its distance from Jupiter) and geological stability. Such a base could facilitate exploration on other Galilean Moons, and be an ideal location for a Jovian system way station, servicing spacecraft that heading farther into the outer Solar System – which would likely take the form of craft using a gravity assist from a close flyby of Jupiter.
A report filed by NASA in February of 2003 stated that a manned mission to Callisto, as envisioned by HOPE, may be possible by the 2040s. In this report, such a mission would likely involve a ship using a Mangetoplasmadynamic (MPD) or Nuclear-Electric Propulsion (NEP) drive system and be equipped to generate artificial gravity.
Here are two good articles about Callisto: one here and the NASA page here. On Universe Today we have another great article about the moon. Astronomy Cast offers a good episode about all of Jupiter’s moons.