Uranus

Voyager 2 Image of Uranus Credit: NASA Voyager

Uranus is the seventh planet from the Sun. While being visible to the naked eye, it was not recognised as a planet due to its dimness and slow orbit. Uranus became the first planet discovered with the use of a telescope. Uranus is tipped over on its side with an axial tilt of 98 degrees.

Equatorial Diameter: 51,118 km

Polar Diameter: 49,946 km

Moons: 27 (Miranda, Titania, Ariel, Umbriel & Oberon)

Rings: 13

Orbit Distance: 2,870,658,186 km (19.19 AU)

Orbit Period :30,687 days (84.0 years)

Effective Temperature: -216 °C

Discovery Date: March 13th 1781

Discovered By: William Herschel

At first William Herschel thought it was a comet, but several years later it was confirmed as a planet. Herscal tried to have his discovery named “Georgian Sidus” after King George III. The name Uranus was suggested by astronomer Johann Bode. The name comes from the ancient Greek deity Ouranos.

The planet rotates in a retrograde direction, opposite to the way Earth and most other planets turn. Uranus makes one trip around the Sun every 84 Earth years. During some parts of its orbit one or the other of its poles point directly at the Sun and get about 42 years of direct sunlight. The rest of the time they are in darkness.

Uranus is often referred to as an “ice giant” planet. Like the other gas giants, it has a hydrogen upper layer, which has helium mixed in. Below that is an icy “mantle, which surrounds a rock and ice core. The upper atmosphere is made of water, ammonia and the methane ice crystals that give the planet its pale blue colour. Uranus hits the coldest temperatures of any planet. With minimum atmospheric temperature of -224°C Uranus is nearly coldest planet in the solar system. While Neptune doesn’t get as cold as Uranus it is on average colder. The upper atmosphere of Uranus is covered by a methane haze which hides the storms that take place in the cloud decks. Uranus’ moons are named after characters created by William Shakespeare and Alexander Pope. Only one spacecraft has flown by Uranus. In 1986, the Voyager 2 spacecraft swept past the planet at a distance of 81,500 km.

Significant Dates:

• 13 Mar 1781: British astronomer William Herschel discovers Uranus - the first new planet discovered since ancient times - while searching for faint stars.
• 1787-1851: Four Uranian moons are discovered and named Titania, Oberon, Ariel and Umbriel..
• 1948: Another moon, Miranda, is discovered.
• 10 Mar 1977: While observing Uranus' passing in front of a distant star (SAO 158687), scientists at the Kuiper Airborne Observatory and the Perth Observatory in Australia were eager for a rare chance to observe the distant planet. Observations before and after the main event led to a major discovery: Uranus, like Saturn, is encircled with rings.
• 24 Jan 1986: NASA's Voyager 2 made the first - and so far the only - visit to Uranus. The spacecraft came within 50,600 miles (81,500 kilometers) of the planet's cloud tops. Voyager discovered 10 new moons, two new rings and a magnetic field stronger than that of Saturn.
• 22 Dec 2005: NASA announces the discovery of a new pair of rings around Uranus and two new, small moons (Mab and Cupid) orbiting the planet from photographs taken by the Hubble Space Telescope. The largest ring discovered by Hubble is twice the diameter of the planet's previously known rings.
• 2006: Observations made at the Keck Observatory and by the Hubble Space Telescope show that Uranus' outer ring is colored blue while the new inner ring is reddish.
• Dec 2007: Uranus reaches equinox. Equinox is when the planet is fully illuminated as the sun passes over its equator. Equinox also brings a ring-plane crossing, when Uranus' rings appear to get narrower as they pass through, appearing edge-on and then widen again as seen from Earth.
• 18 Mar 2011: New Horizons passes the orbit of Uranus on its way to Pluto, becoming the first spacecraft to journey beyond Uranus' orbit since Voyager 2. However, Uranus was not near the crossing point. The spacecraft is asleep during most of its eight-year interplanetary trek from Jupiter to Pluto. Mission controllers do wake up New Horizons for 50 days each year to perform necessary checkups on its instruments.

Credit: Karl Tate/Space.com

Reference: Karl Tate/spacedotcom/Nasa Solar System