New Moon Found Orbiting Neptune 120
Dave Knott writes "A tiny, previously unknown moon circling Neptune has been spotted by astronomers using the Hubble telescope. The moon, which is currently known as S/2004 N1, was found on July 1 by Mark Showalter of the SETI Institute in Mountain View, Calif., NASA announced Monday. It is less than 20 kilometres wide and its orbit is 105,000 kilometres from Neptune, between those of Larissa and Proteus, two of Neptune's other 14 known moons. It circles Neptune once every 23 hours."
Re:But wait... (Score:5, Informative)
The dividing line between "moons" and "rings" seems to be shared orbits, otherwise every little rock and/or ice ball in the outer planets' rings would have to be a "moon". A 20 km rock (or whatever) has enough gravity to sweep the space through which it passes, either clustering smaller bits into rings, adding them to its mass, or ejecting them from the planetary system.
Remember, Pluto was only a "planet" because we didn't realize it was an instance of a much larger class of KBOs. Now it appears to be more like a cluster of bits orbiting a mutual center, different from the planets and their moons, which have an orbital center deep inside the respective planets. Even without the companion bits, though, it's still a KBO.
We had already separated the "asteroids" from the 8 planets.
Re:But wait... (Score:5, Informative)
The IAU uses "moon" and "natural satellite" synonymously, which in this context refers to any natural body in a bound orbit of Neptune. I'm not sure why you think a 20km rock would fail to meet that definition.
Re:But wait... (Score:5, Informative)
How about specifying it must have enough gravity to make it round to be called a moon and not just a satellite?
That would reduce the number of moons in the solar system rather dramatically. Mars, for example, would no longer have any "moons" as neither Phobos nor Deimos meet this definition. In fact both Phobos (11.1 km) and Deimos (6.2 km) are smaller than this newly discovered moon of Neptune (20 km).