Detecting natural satellites of Jupiter is darned difficult. The planet is bright. The satellites are small and dim. Most of Jupiter’s moons orbit in irregular paths, nudged by the sun. Some even orbit backwards. If you blinked, you might lose one. In fact, one moon was detected in 1975, only to be lost, then rediscovered in 2000.
One early twentieth century astronomer was up to the task, though. With big telescopes gathering a few reflected photons on photographic plates, Jupiter’s family of satellites grew and surpassed Saturn’s nine, thanks to the efforts of an Illinois-born astronomer working in California.
In 1914, a grad student found the ninth moon of Jupiter. While observing the newly discovered Jupiter VIII from Lick Observatory, Seth Barnes Nicholson found his PhD thesis topic: Jupiter IX, now known as Sinope. Over the course four decades, he added three to his tally, tying Galileo, Cassini, and Herschel for the most discoveries up to that time–four. Here are the others, all found at the Mount Wilson Observatory (image above and left), where Dr Nicholson spent his entire career:
Jupiter X, or Lysithea, in July 1938
Jupiter XI, or Carme, also in July 1938.
Jupiter XII, or Ananke, in September 1951.
Nicholson was more than an observer of dim, irregular satellites. He developed tools for assessing surface temperatures of solar system bodies (his moons as well as planets and the sun) and distant stars.
These seven outer satellites of Jupiter–Nicholson’s four, plus the three found from the first decade of the 1900’s are all thought to be captured from the nearby asteroid belt. Or they could be comets. Or they could be fragments from a few larger bodies that broke up. Since Jupiter’s outer moons move in similar orbits this last theory seems to fit observations pretty well. Don’t think clumps of moons, though. These bodies orbit millions of miles from Jupiter. A close association is more like them sharing a fairly wide pathway far out from the planet. Imagine Jupiter sitting on the thirty-yard line of a high school football stadium, its north pole pointed up, but at an angle, perhaps toward a near bank of lights. The satellite Carme and associated moons (discovered in the last ten years) would be at different places circling the field mainly on the surrounding track. Don’t imagine a handful of closely-bunched runners.
Nobody really knows for sure about any of this. No human-made spacecraft has ever flown anywhere near them. They aren’t big priorities for NASA or other nations’ space programs. It might be that the first robots or humans visiting these satellites will be lonely wanderers themselves.
In my imagination, I see a twenty-third or twenty-fourth century ship pushed by the pulse of ions. Looking around, Jupiter would be far away, only about the size of our moon as seen from Earth. Light from the sun would be only about four percent Earth levels. An unenhanced view would find us moving up next to a dark gray body with just a hint of red. Much of it might be water ice mixed in with rock. But don’t think of icebergs sticking out from soil–from Earth it seems these bodies are fairly homogeneous in surface. The ice will be dirty, laced with carbon dioxide and maybe ammonia, and studded with boulders and dust and particles of sizes in between.
As the explorer gets closer, the body–let’s say it’s Ananke–will loom large, dwarfing the living compartment and the ion engine. But even compared to asteroids, it would be small–only twenty miles across. Centuries after Seth Nicholson noted moving specks in the Jupiter neighborhood and plotted orbits, someone would finally see the moon, and not just in her or his imagination. It’s not likely there would be a landing. Maybe grappling hooks if the ship were sturdy. More likely a few small robots would tumble down to the surface. Dig around a bit. Send an optical feed back to the main ship. Taste the dust of the surface and clarify the makeup of the impurities.
It might be that the same mix of trace compounds and elements was detected at last month’s stop–ratios of cyanide, carbon monoxide, or silicate crystals. So it will be confirmed that Ananke and about one or two dozen mile-sized satellites were once a single asteroid captured by Jupiter. Then pulverized into fragments. If space explorers of the 2300’s feel the need to put down roots, Ananke would be an interesting place. There’s likely enough ice to turn into water for drinking and other industrial uses–more than on the moon, that’s for sure. Splitting water molecules into hydrogen for fuel and oxygen for breathing will be child’s play in future centuries. The sun being so far away, is only four percent as efficient as a power source as it would be near the Earth. On second thought, these intrepid explorers may have other places in their imagination, closer to the life-giving energy of our central star. After a few days, the ship will pull out its stakes, leave behind the robot probes, and move on to the next target.
It’s a lonely existence ten to twenty million miles out from Jupiter. No gazing back at Earth either–our home world will be lost in the glare of the sun–not unlike the way Jupiter’s reflected light washes out viewing some of its moons.