Uranus Orbiter and Probe

Voyager 2 is the only space probe to have visited the Uranus system, completing a flyby on January 24, 1986. The 2011–2022 Planetary Science Decadal Survey recommended a Flagship-class orbiter mission to an ice giant with priority behind what would become the Mars 2020 rover and the Europa Clipper.^[5][6][7] Ice giants are now appreciated as a common type of exoplanet, precipitating the need for further study of ice giants in the Solar System.^[8] The ice giants Uranus and Neptune were seen as unique yet equally compelling scientific targets, but a Uranus orbiter and atmospheric probe was given preference for logistical and cost reasons.^[5][7] A Uranus orbiter would logically follow Flagship-class orbiter missions undertaken at Jupiter and Saturn (Galileo and Cassini, respectively).

In 2017, prior to the 2023–2032 survey, a committee narrowed twenty mission concepts to three scenarios for Uranus and a fourth for Neptune.^{[8][9][10][11]} A mission to Neptune is viewed by some to be of greater scientific merit^[12] because Triton, likely a captured Kuiper belt object and ocean world, is a more compelling astrobiology target than the moons of Uranus (though Ariel and Miranda in particular are possible ocean worlds).^[13] There was also a study that considered a New Frontiers-level Uranus orbiter mission concept if a Flagship-class mission to Neptune were favored.^[14] Nevertheless, again due to cost and logistical considerations including launch vehicle availability and available launch windows, the 2023–2032 Planetary Science Decadal Survey recommended the Uranus Orbiter and Probe instead of an analogous proposal for Neptune, Neptune Odyssey.^[3][4]

The orbiter paired with an atmospheric probe will address a variety of scientific questions across all aspects of the Uranus system:^[3]

The atmospheric probe element of this mission would study the vertical distribution of cloud-forming molecules, thermal stratification, and wind speed as a function of depth. The 2010 mission design envisioned a probe of 127 kg (280 lb), less than half that of the Galileo atmospheric probe.^[7] A later design study suggested results could be significantly enhanced by adding a second probe which could be as small as 30 kg (66 lb) in mass and about 0.5 m (20 in) in diameter.^[15]

A study by Amy Simon et. al. identified several challenges and difficulties that the probe might encounter. The first challenge is slowing down when the probe finally arrives at Uranus. Because Uranus is the least massive and least dense gaseous planet in the Solar System, it also has the lowest gravity. This makes braking using Uranus's gravity less efficient and more challenging. The study suggested that the probe should use multiple flybys of Uranus's largest moon Titania to slow down because this moon has the largest mass and strongest gravity among all Uranian moons, making it the most effective moon to use when slowing down.^[16]

The next challenge identified in the article is the possibility of collisions with ring particles due to uncertainties in the structure and composition of Uranus's rings. The study suggested that the probe should enter orbit by passing between the rings and the planet. It also proposed using Uranus's atmosphere for aerobraking to minimize the use of fuel. However, the risk of striking a ring particle during this maneuver was recognized as a significant concern.^[16]

The study also identified the extreme temperature variations caused by the changing intensity of sunlight at different distances from the Sun as a potential challenge. If the Uranus Orbiter and Probe performs a flyby of Venus (at 0.7 AU), it would be exposed to nearly twice the solar heat received by Earth. In contrast, once it reaches Uranus (at 20 AU), it would receive only about 1/400 of the sunlight that Earth receives. These dramatic fluctuations in temperature could potentially damage the probe's systems.^[16]

The study also identified the extreme tilt of Uranus and its system of regular moons as another major challenge. Because the spacecraft would travel through the Solar System along a trajectory that is nearly aligned with the ecliptic plane, changing its course to match Uranus's highly inclined equatorial plane would require a substantial amount of fuel or carefully planned gravity assists. This orbital plane change — from a nearly horizontal trajectory to a highly inclined one — would be one of the mission's most demanding maneuvers.^[16]

Lastly, because there is a significant possibility that the Uranus Orbiter and Probe will not be ready to launch during the 2031–2032 window, when Jupiter and Uranus are favorably aligned for a gravity assist, the study also explored alternative propulsion technologies. These technologies could compensate for the missed launch opportunity by allowing the spacecraft to travel directly to Uranus while minimizing or eliminating the need for planetary gravity assists.^[16]

• Atmosphere of Uranus
• Exploration of Uranus
• Moons of Uranus

Uranus mission proposals

• MUSE
• Oceanus (New Frontiers-class proposal of the 2010s)
• ODINUS
• Uranus Pathfinder (ESA M-Class proposal of the 2010s)