Matthew Neighbors, age 11, of Skiatook, Oklahoma, for his question:
How many satellites has Uranus?
It is safe to say that Mars has two, and only two, junior sized satellites. Nobody expected our probing spacecraft to find any more, though its close up pictures of these Martian moons were wondrous to behold. It is safe to say that Venus has no moons at all and nobody expects to find one orbiting little Mercury. However, we cannot at this time announce the exact number of satellites that belong to Uranus because more may be discovered in the near future.
Uranus is barely visible if you have sharp eyes and know just where to look. The big planet is way out there beyond the orbit of Saturn and the closest it comes to us is about 1,700 million miles. Its average distance from the sun is 1,785 million miles and it takes 84 earth years to complete each of its yearly orbits. The five closer planets are clearly visible. They were discovered, named and charted in antiquity. But Uranus had to wait for the invention of the telescope and more especially for astronomers capable of computing its whereabouts.
It was introduced to the world in 1781, with much fanfare. Six years later it made more headlines, when astronomers discovered that the big planet had two satellites. After some debate, Uranus was named for the Greek god of the heavens. The newly found satellites were named for the king and queen of fairyland. Oberon orbits its parent planet once every 13 days, lI hours and seven minutes at an average distance of 364,000 miles. Titania's orbit is 272,000 miles from Uranus and it takes eight days and almost 17 hours to travel around it. Its diameter is estimated to be about 1,000 miles, perhaps 100 miles or so wider than that of the fairy king.
In 1851, the Uranus family of known satellites increased to four and the newcomers were named for a couple of mythological sprites. Both are closer to the planet. Ariel completes its orbit in about 2 1/2 days, at a distance of 119,000 miles. Its diameter is estimated to be about 600 miles. Umbriel completes its orbit in about four days and 3 1/2 hours at a distance of 166,000 miles and seems to be about 400 miles wide.
Until 1948, many observers boldly stated that Uranus has four satellites when they should have said four known satellites. Then Miranda was discovered, a satellite so small that astronomers have not estimated its diameter. This little moon is only 81,000 miles from Uranus and takes only one day, nine hours and 56 minutes to orbit around its big parent planet.
In telescopic pictures, these five satellites all appear fairly close to Uranus and the four larger ones look very bright. Little Miranda is a dim small blur, almost lost in the glow of the planet. We may discover that Uranus has still other outer satellites. This news may be relayed home during the next decade, when our space probes take their Grand Tours past Uranus and the other distant planets. Today we know Uranus has 27 known moons and faint rings. The following are the latest statistics on the Planet Uranus.
Uranus Statistics
Planetary Symbol: Name in Roman/Greek Mythology: Uranus/Uranus
Diameter: 51,488 km (32,000 miles) Rotation Period about Axis: 17.24 hrs (retrograde?)
Mass: 86.8x10^24 kilograms (14.5 x Earth's)
Revolution Period about the Sun: 84 years
Density: 1,270 kg/m^3 Tilt of Axis: 98o (or 82o?)
Minimum Distance from Sun: 2.7 billion km (1.7 billion miles)
Surface Gravity: 8.69 m/s^2 (0.89 x Earth's)
Maximum Distance from Sun: 3 billion km (1.87 billion miles)
Temperature at Cloud Tops: -200o C ( -328o F)
Orbital Semi-major Axis: 19.19 AU (Earth=1 AU) Average Cloud Top Temperature (K): 73K
Minimum Distance from Earth: 2.57 billion km(1.6 billion miles)
Satellites/Rings: 27 known moons, faint rings