Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a supercritical phase of matter, which in astronomy is called 'ice' or volatiles. The planet's atmosphere has a complex layered cloud structure and has the lowest minimum temperature of 49 K (−224 °C; −371 °F) out of all the Solar System's planets. It has a marked axial tilt of 82.23° with a retrograde rotation period of 17 hours and 14 minutes.
Uranus
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| Uranus in true colour, as captured by Voyager 2. Uranus's pale, muted appearance is due to a shroud of haze above its clouds |
Discovery
Discovered by William Herschel
Designations
Named after
the Latin form Ūranus of the Greek god Οὐρανός Ouranos
Adjectives
Uranian (/jʊˈreɪniən/)
Uranus has the third-largest diameter and fourth-largest mass among the Solar System's planets. Based on current models, inside its volatile mantle layer is a rocky core, and surrounding it is a thick hydrogen and helium atmosphere. Trace amounts of hydrocarbons (thought to be produced via hydrolysis) and carbon monoxide along with carbon dioxide (thought to have been originated from comets) have been detected in the upper atmosphere. There are many unexplained climate phenomena in Uranus's atmosphere, such as its peak wind speed of 900 km/h (560 mph), variations in its polar cap, and its erratic cloud formation. The planet also has very low internal heat compared to other giant planets, the cause of which remains unclear.
Uranus has a ring system, a magnetosphere, and many natural satellites. Its ring system is extremely dark, with only about 2% of the incoming light reflected. Uranus's 28 natural satellites include 18 known regular moons, of which 13 are small inner moons. Further out are the larger five major moons of the planet: Miranda, Ariel, Umbriel, Titania, and Oberon. Orbiting at a much greater distance from Uranus are the nine known irregular moons. The planet's magnetosphere is highly asymmetric and has many charged particles, which may be the cause of the darkening of its rings and moons.
Uranus is visible to the naked eye, but it is very dim and was not classified as a planet until 1781, when it was first observed by William Herschel. About seven decades after its discovery, consensus was reached that the planet be named after the Greek god Uranus (Ouranos), one of the Greek primordial deities. As of 2024, it had been visited up close only once when in 1986 the Voyager 2 probe flew by the planet. Though nowadays it can be resolved and observed by telescopes, there is much desire to revisit the planet, as shown by Planetary Science Decadal Survey's decision to make the proposed Uranus Orbiter and Probe mission a top priority in the 2023–2032 survey.
History
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| Position of Uranus (marked with a cross) on 13 March 1781, the date of its discovery |
Discovery
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| William Herschel, discoverer of Uranus |
Sir William Herschel observed Uranus on 13 March 1781 from the garden of his house at 19 New King Street in England (now the Herschel Museum of Astronomy), and initially reported it (on 26 April 1781) as a comet. With a homemade 6.2-inch reflecting telescope, Herschel "engaged in a series of observations on the parallax of the fixed stars."
Herschel recorded in his journal: "In the quartile near ζ Tauri ... either [a] Nebulous star or perhaps a comet." On 17 March he noted: "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place." When he presented his discovery to the Royal Society, he continued to assert that he had found a comet, but also implicitly compared it to a planet:
Herschel notified the Astronomer Royal Nevil Maskelyne of his discovery and received this flummoxed reply from him on 23 April 1781: "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it."
The object was soon universally accepted as a new planet. By 1783, Herschel acknowledged this to Royal Society president Joseph Banks: "By the observation of the most eminent Astronomers in Europe it appears that the new star, which I had the honour of pointing out to them in March 1781, is a Primary Planet of our Solar System." In recognition of his achievement, King George III gave Herschel an annual stipend of £200 (equivalent to £30,000 in 2023) on condition that he moved to Windsor so that the Royal Family could look through his telescopes.
Name
The name Uranus references the ancient Greek deity of the sky Uranus (Ancient Greek: Οὐρανός), known as Caelus in Roman mythology, the father of Cronus (Saturn), grandfather of Zeus (Jupiter) and the great-grandfather of Ares (Mars), which was rendered as Uranus in Latin (IPA: [ˈuːranʊs]). It is the only one of the eight planets whose English name derives from a figure of Greek mythology. The pronunciation of the name Uranus preferred among astronomers is /ˈjʊərənəs/ YOOR-ə-nəs, with the long "u" of English and stress on the first syllable as in Latin Uranus, in contrast to /jʊˈreɪnəs/ yoo-RAY-nəs, with stress on the second syllable and a long a, though both are considered acceptable.
Getting There
It’s possible to traverse the realm of the ice giants quickly—the New Horizons spacecraft reached even more distant Pluto in just under a decade. However, that spacecraft was intentionally kept as light as possible to allow a high velocity launch. While New Horizon’s was a bantam-weight scout, scientists want a highly capable orbiter for what likely would be a once-in-several-generations mission to orbit one of these worlds.
Even if the launch vehicle existed to directly fling the heavier ice giant orbiter to its destination, a New Horizon style mad dash wouldn’t be possible. When it reached Pluto, that spacecraft was going so fast that it would have been impractical to carry enough fuel to insert itself into orbit. The mission design for an ice giant mission becomes a trade off between speed and the mass of the fuel needed to brake from that velocity into orbit. Using the a mid-range commercial launch vehicle such as the Atlas V, a reasonable balance results.
Using a more expensive Delta IV Heavy, a year to a year and half can be cut from the transit time. Adding a solar electric propulsion unit to provide a boost in flight could cut the flight time a year. Any combination of these latter options comes with the trade off of a higher, and possibly much higher, overall mission cost. (When it becomes available, the Falcon Heavy will provide an additional option.)
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| EXAMPLE TRAJECTORY FOR A MISSION TO ORBIT URANUS (Option 5 from the report) Image: Ice Giants Pre-Decadal Study Final Report |
Challenge for exploring these worlds is that the sun is too faint for solar power so radioisotope power supplies would be required. Over time, the components of these supplies degrade, reducing power to the spacecraft. (The radioisotopes also decay, but that loss is slower.) An enabling technology for these missions is an enhancement already under development that boosts power delivery late in a mission’s life by incorporating longer-lived components (for those of you who follow these technologies, this is the enhanced Multi-Mission Radioisotope Thermal Generator, or eMMRTG). The proposed designs would carry either four or five eMMRTGs. The Curiosity rover, by comparison, carries just one of the current generation MMRTGs. At the projected end of these missions, the combined output from the multiple eMMRTGs will be less than either four or five 100 Watt light bulbs, depending on the number carried.
The report notes that the expected power will require turning instruments on and off because not all can operate at the same time. That act stresses the electrical components such as solder joints. Instruments for missions to Uranus or Neptune, the report notes, may require additional levels of redundancy be built in to ensure they can operate for the full length of the missions.
The Next Steps
As discussed in the introduction to this post, planning missions for the outer solar system is a long game. A key decision will come in the rankings of priorities of missions in the next Decadal Survey. Then, assuming a high ranking, the size of NASA’s planetary science budget will determine its capabilities, and whether just one or two of these worlds will be explored. A mission to either of these worlds would have similar costs—ranging from around $2 billion to $2.3 billion depending on options—to other large planetary missions such as NASA’s Curiosity rover.
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| COST-BENEFIT ANALYSIS The estimated relative science benefit (vertical axis) versus estimated cost of different mission options using the minimum three instrument payload recommended for the flyby or orbiter spacecraft. The study team concluded that the limited science from a flyby mission would not be worth the cost savings from the much more capable orbiter mission. Sending missions to both planets was judged to approximately double the science return but at substantially higher costs. Image: Ice Giants Pre-Decadal Study Final Report |
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| CLOUDS ON URANUS False color images of both hemispheres of Uranus reveal the differing altitudes of clouds. The color balance was chosen to make the highest-altitude clouds white, middle-altitude clouds greenish, and lowest-altitude clouds deep blue. This color balance choice is responsible for the bright red color of the rings, which are actually gray, not red. Image: Lawrence Sromovsky / W. M. Keck Observatory |
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