Frontiers of Science: It’s a New Moon! Or Is It?

It’s a New Moon! Or Is It?

Nearly everyone on Earth is familiar with the moon. In fact, astronauts from Earth have already been there. But the moon—our moon—is only one of many that revolve about the planets of the solar system. Mars has two, Neptune two, Uranus five, Saturn ten, Jupiter fourteen, and Pluto one, maybe.

Maybe? Yes, maybe. That qualification arises for two reasons. First, new discoveries in astronomy are being made so rapidly today that between the writing and reading of this article, there may well have been other moons discovered. Second, the announcement on July 7, 1978, of a newly discovered moon of Pluto has thrown the very nature of that celestial body into question. So, it is now debatable whether Pluto is a true planet. And moons always orbit planets, don’t they? Well, maybe.

To put the problem in perspective, let’s review the recent discovery of Pluto’s companion satellite. In April and May of 1978, a scientist using a 61-inch reflecting telescope at Flagstaff, Arizona, noticed a small bulge in the image of Pluto on a series of photographs. Checking pictures taken in 1965 and 1970, he found the same thing. Further observations then led to the determination that the bulge was a separate object, so close to the planet that it could only be seen at special times.

Calculations that followed showed the satellite to be about 500 miles in diameter and separated from Pluto by only 12,000 miles. It is believed to circle Pluto once every 6 days 9 hours and 17 minutes, the same length of time that it takes Pluto itself to rotate on its axis. Thus, an observer on one side of Pluto would always see the object in the same position in the sky, while someone on the other side would never see it at all.

The most interesting aspects of the discovery, however, are not those relating to Pluto’s “moon” but to the planet itself. When first discovered in 1930, little was known about this most distant planet from the sun. Pluto was originally thought to be ten times more massive than Earth, but observations in the early 1970s lowered this estimate to about one-fifth of Earth’s mass. The new discovery has allowed much more accurate calculations to be made, and they indicate that Pluto cannot have more than two-tenths of one percent of Earth’s mass. So it is very small indeed, being only two to three times larger than its moon.

The fact that we can see Pluto at all, being as far away and as small as it is, means that it must reflect a large amount of sunlight. And what is highly reflective? Ice. And what heavenly bodies are believed to be composed mostly of ice? Comets. Consequently, there is a real question about the true nature of Pluto and its companion.

Perhaps they are dead comets that now orbit the sun like a double planet or maybe a double asteroid. For on the very same day the announcement of Pluto’s new companion was made, a similar announcement was made with respect to the asteroid Herculina.

The 135-mile diameter asteroid Herculina had been predicted to pass between a certain star and the Earth on June 7, 1978. Both professional and amateur astronomers in southern California and Arizona readied their telescopes for the occasion, in an attempt to better measure the asteroid’s size. Just as in the discovery of the rings of Uranus, they noted a small reduction in starlight two minutes before Herculina itself obscured the star. When all of the data was analyzed, the most reasonable explanation for the observations was that a second object, about 30 miles in diameter, was orbiting Herculina at a distance of about 600 miles from it. If further observations bear out this interpretation, it will prove to be the first such moon of an asteroid ever discovered.

But is such an object properly classified as a moon? Once again we have a situation where a set of new discoveries creates almost more questions than it answers. So how many moons are there in our solar system? Thirty-six and counting … maybe.

[illustrations] Illustrated by Mike Rogan

[photo] 1 Earth’s moon photographed by crew of Apollo 17. One-third of surface in this view is the lunar far side, not visible to Earth-based observers. (NASA photo.)

[illustration] 2 Comparative sizes of some of solar system’s larger moons and planet Mercury. (Adapted from NASA artwork.)

[illustration] 3 Diagram of Ganymede, one of the large moons of Jupiter, showing how scientists think it may be constructed. (Adapted from artwork courtesy of Jet Propulsion Laboratory, California Institute of Technology.)

[photo] 4 Famous photograph that led astronomer James W. Christy to discover a moon, now called Charon, that orbits Pluto. Charon is so close to Pluto that it appears here to be hardly a bump on left side of planet’s image. (Photo courtesy of the U.S. Naval Observatory.)

[photo] 5 Three-picture mosaic of Mars’s inner moon, Phobos, taken by orbiter Viking I on February 18, 1972, when it flew within 300 miles of this small satellite that measures only 13 miles across and 11.8 miles from top to bottom. Scientists speculate that Phobos may look much like an asteroid and that it may indeed be an asteroid that was “captured” by the planet. (NASA photo.)