Those Other Moons of Jupiter

After being periodically surveyed by robotic spacecraft in recent years, did those astronomers that discovered the other moons of Jupiter years before got the accolades that they truly deserve?


By: Ringo Bones


I tend to have a rather rigid definition on the difference between space exploration and astronomy because to me, sending space probes to distant celestial bodies – in my point of view – is space exploration. While peering though an astronomical telescope via the eyepiece or via a high-resolution computer grade monitor in front of me is what I define as true astronomy. The Voyager and Galileo spacecraft flybys of Jupiter and the planet’s retinue of satellites may have gathered data previously unknown to late 19th and early 20th Century astronomers. But the astronomers themselves who discovered those excruciatingly tiny non-Galilean satellites decades before those robotic spacecraft flybys seem to have been largely forgotten in this day and age.

Before the Pioneer X and Voyagers I and II flybys, there are only 12 moons of Jupiter that can be seen from Earth using existing astronomical telescope technology prior to the 1960s. Set side-by-side to the relatively large four Galilean satellites: Io at 2,000 miles, Europa at 1,800 miles, Ganymede at 3,120 miles, and Callisto at 2,800 miles, the other moons of Jupiter look like grains of sands in comparison. With diameters that range from as large as 70 miles to as small as 10 to 12 miles, discovering these Jovian moons without resorting to optical interferometry is probably next to impossible.

The first one of these non-Galilean satellites to be discovered was discovered by an American astronomer named Edward Emerson Barnard of Barnard’s Star fame in 1892 and was called / designated as V. This tiny Jovian moon at just 70 miles in diameter in unique in many ways. V orbits just 110,000 miles from Jupiter allowing Jupiter’s strong gravitation to make the tiny moon hurtle through space at 1,000 miles a minute – 26 times faster than the Earth’s moon.

On average, Jupiter is 629 million kilometers from Earth, which makes Barnard’s discovery of V somewhat of a remarkable feat in astronomy back in 1892 because astronomical telescopes having at least a 4-meter diameter mirror is the minimum needed to see V from Earth due to the Rayleigh Criterion limitations. The smallest celestial object that a 4-meter mirrored astronomical reflecting telescope when the Rayleigh Criterion is taken into account is around 103.5 kilometers – a little under 70 miles - in diameter from 629 million kilometers away using the visible spectrum centered around 550 nanometers. And most important of all, use of optical interferometry in astronomy to bypass the inherent Rayleigh Criterion limitations of existing reflecting telescopes in 1892 was still several years away. Probably a few years after 1910 when Albert A. Michelson used an optical interferometer of his own design to accurately measure the diameters of those newly-discovered tiny satellites or moons of Jupiter.

Even the discovery of the next non-Galilean satellite designated as VI (diameter 50 miles) in 1904 and VII (diameter 20 miles) in 1905 by American astronomer Charles Dillon Perrine is also miraculous given that optical interferometry for astronomical use is still years away. Even back in 1908, when English astronomer Philbert Jaques Mellote discovered Jupiter’s moon designated as VIII with a diameter of 10 miles without an aid of optical interferometry is nothing short of a miracle.

In 1914, an American astronomer named Seth Barnes Nicholson discovered one of the last few moons of Jupiter whose diameter averages between 10 to 12 miles designated as IX, probably with the aid of Albert A. Michelson’s newfangled optical interferometer. Not only will Nicholson discover the last of the tiny moons of Jupiter that can be seen from Earth before the Pioneer X and Voyager spacecraft flybys, he also made remarkable theories that almost accurately predicted the actual climate of the planet Venus. Together with fellow astronomer Charles St. John, their hot and dry Venus climate hypothesis that they proposed back in 1922 was proven to be closer to reality. Compare that to the one proposed in 1918 by Swedish chemist Svante August Arrhenius – who also discovered the mechanism behind the greenhouse effect – depicted planet Venus as covered by hot steamy tropical swamps.

In 1938, Nicholson discovered another two of Jupiter’s very tiny non-Galilean satellites. Designated at the time as X and XI, X because Nicholson declined suggesting names for the new Jovian moons that he discovered orbits in a region 7,400,000 miles away from Jupiter while XI orbits in a region almost 15 million miles away from Jupiter. X is part of the other 4 of Jupiter’s outermost satellites that orbit in a retrograde direction. Two of these outer satellites even have “open” orbits that are never repeated from one circuit to the next. Probably due to the Sun’s much stronger gravitational influence in comparison to Jupiter’s at this distance.

The last of Jupiter’s satellites / moons that can be seen by Earth-based telescopes have to await discovery until 1951 when XII – now known as Ananke – was yet again discovered by Seth B. Nicholson. XII or Ananke was a difficult find not only because of the Jovian moon’s small size – 10 miles – but also because it shines no brighter than the light of a burning candle seen from 3,000 miles away at night. This is primarily due to the Jovian moon’s extremely low visual albedo or reflectivity, not to mention the Jovian moon’s relatively small size of 10 miles in diameter. Sadly, these amazing astronomers, especially Seth Barnes Nicholson, are largely forgotten given their amazing feats in the science of astronomy.