Phoebe was the first of Saturn’s moons encountered by the Cassini space probe. In 2004, Earthlings got delicious images of the moon from just a few thousand miles away. Before Cassini, our view of this moon for the past 105 years had pretty much been a point of light on a photographic plate.
DeLisle Stewart was a Harvard astronomer taking images in Peru the summer of 1898. Several months later, in the other hemisphere, William Henry Pickering pored over the photographic plates and found movement in the outer reaches of the Saturn neighborhood. It was way out, as Phoebe orbits about four times as far out as Iapetus. Phoebe is about three times dimmer than the least-bright moon yet known–Uranus’ Umbriel.
Pickering and Stewart’s effort marked the beginning of the photographic age in satellite discoveries. Why take pictures? Two advantages make it an improvement over the human eye.
First, an astronomer can leave the lens open for long periods of time and gather more light than the unaided human eye can take in. A very dim moon will continue to send reflected photons onto a plate. Finding a moon means looking for a moving dim light.
One technique is not to develop the images, but compare negative plates, looking for a black spot that moves against the background stars. What could it be? Maybe a comet. Likely an asteroid. But if you’re probing the neighborhood of a planet, a satellite is the imaginative choice.
By the turn of the century, the known planets and their moon counts were as follows:
Earth-1, Mars-2, Jupiter-5, Saturn-9, Uranus-4, Neptune-1.
The largest planet was in second place in the satellite department. What gives with that? The first six satellites discovered in the 20th century were to be found in the Jupiter environs, seven total before the dawn of the space age. We’ll take a look at those moons in the next edition of Satellite Imagination.