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Saturn's rings with moonlet interactions. Image credit: NASA/JPL/SSI
It is difficult to answer ‘what is the smallest moon in the Solar system’. As with many things in astronomy, the answer is subject to change based on the reclassification of other objects. At this time the title of smallest moon in the solar system would go to two small moons in orbit around Jupiter. S/2003 J 9 and S/2003 J 12 are each about 1 km across. Their hold on the title is tenuous at best, subject to the many moonlets within Saturn’s rings, remaining in a separate category.
S/2003 J 9 is a retrograde irregular satellite of Jupiter that was discovered in 2003 by a team of astronomers led by Scott Sheppard. An irregular satellite follows a distant, inclined, eccentric, and retrograde orbit around the primary. It is believed that these moons are captured by the gravitational pull of the primary where regular satellites form in place. S/2003 J 9 is about 1 km in diameter and orbits at an average distance of 23,858,000 km. It has an orbital period of 752.839 days, its inclination is 165° to the ecliptic of Jupiter, with an eccentricity of 0.276. It is part of the Carme group made up of irregular retrograde moons orbiting Jupiter.
S/2003 J 12 is a natural Jovian satellite. It was discovered by the same team of astronomers, led by Scott Sheppard, that discovered S/2003 J9. It is close to 1 km in diameter, and orbits at an average distance of 17,883,000 km. Its orbital period is 489.72 days. It has an inclination of 143° to the ecliptic of Jupiter’s equator, orbits in retrograde, and has an eccentricity of 0.4920. It does not appear to belong to the Carme group.
The Carme group has been mentioned twice, so it is only courteous to define it. According to NASA and other sources, the Carme group is a group of Jovian retrograde irregular satellites that follow orbits that are similar to that of the moon Carme and are thought to have a common origin. Their distances from Jupiter(semi-major axes) range from 22.9 and 24.1 Gm, their inclinations are between 164.9° and 165.5°, and their eccentricities range between 0.23 and 0.27 with a single exception. The very low variability of the average orbital elements among the core members suggests that the group may have been a single body and was broken apart by an impact. The parent body was probably about 46 km in diameter; the approximate size of the moon Carme which accounts for 99% of the group’s mass.
Until, and if, the moonlets around Saturn are reclassified, the answer to ‘what is the smallest moon in the Solar System’ is S/2003 J 9 and S/2003 J 12. You could also research Deimos as it is a close runner up.
Wondering what’s the biggest moon in the Solar System, and here’s how many moons there are in the whole Solar System.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.
References:
NASA Solar System Exploration: Planets
NASA Solar System: Jupiter
Wikipedia
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