A colour photograph of Uranus, taken by Voyager 2 in 1986 as it headed towards the planet Neptune

The exploration of Uranus has, to date, been through telescopes and a lone probe by NASA's Voyager 2 spacecraft, which made its closest approach to Uranus on January 24, 1986. Voyager 2 discovered 10 moons, studied the planet's cold atmosphere, and examined its ring system, discovering two new rings. It also imaged Uranus' five large moons, revealing that their surfaces are covered with impact craters and canyons.

A number of dedicated exploratory missions to Uranus have been proposed,[1][2] but as of 2023 none have been approved.[3][4]

Voyager 2

Voyager 2 made its closest approach to Uranus on January 24, 1986, coming within 81,500 km (50,600 miles) of the planet's cloud tops. This was the probe's first solo planetary flyby, since Voyager 1 ended its tour of the outer planets at Saturn's moon Titan.

The Uranian moon Miranda, imaged by Voyager 2

Uranus is the third-largest and fourth most massive planet in the Solar System. It orbits the Sun at a distance of about 2.8 billion kilometers (1.7 billion miles) and completes one orbit every 84 years. The length of a day on Uranus as measured by Voyager 2 is 17 hours and 14 minutes. Uranus is distinguished by the fact that it is tipped on its side. Its unusual position is thought to be the result of a collision with a planet-sized body early in the Solar System's history. Given its odd orientation, with its polar regions exposed to sunlight or darkness for long periods and Voyager 2 set to arrive around the time of Uranus's solstice, scientists were not sure what to expect at Uranus.

The presence of a magnetic field at Uranus was not known until Voyager 2's arrival. The intensity of the field is roughly comparable to that of Earth's, though it varies much more from point to point because of its large offset from the center of Uranus. The peculiar orientation of the magnetic field suggests that the field is generated at an intermediate depth in the interior where the pressure is high enough for water to become electrically conductive. Voyager 2 found that one of the most striking influences of the sideways position of the planet is its effect on the tail of the magnetic field, which is itself tilted 60 degrees from the planet's axis of rotation. The magnetotail was shown to be twisted by the planet's rotation into a long corkscrew shape behind the planet.

Radiation belts at Uranus were found to be of an intensity similar to those at Saturn. The intensity of radiation within the belts is such that irradiation would quickly darken (within 100,000 years) any methane trapped in the icy surfaces of the inner moons and ring particles. This may have contributed to the darkened surfaces of the moons and ring particles, which are almost uniformly gray in color.

A Voyager 2 image of the Uranian dark rings

A high layer of haze was detected around the sunlit pole, which also was found to radiate large amounts of ultraviolet light, a phenomenon dubbed "electroglow". The average temperature of the atmosphere of the planet is about 59 K (−214.2 °C). Surprisingly, the illuminated and dark poles, and most of the planet, show nearly the same temperature at the cloud tops.

Voyager 2 found 10 new moons, bringing the total number to 15 at the time. Most of the new moons are small, with the largest measuring about 150 km (93 mi) in diameter.

The moon Miranda, innermost of the five large moons, was revealed to be one of the strangest bodies yet seen in the Solar System. Detailed images from Voyager 2's flyby of the moon showed huge oval structures termed coronae flanked by faults as deep as 20 km (12 mi), terraced layers, and a mixture of old and young surfaces. One theory holds that Miranda may be a reaggregation of material from an earlier time when the moon was fractured by a violent impact.

The five large moons appear to be ice–rock conglomerates like the satellites of Saturn. Titania is marked by huge fault systems and canyons indicating some degree of geologic, probably tectonic, activity in its history. Ariel has the brightest and possibly youngest surface of all the Uranian moons and also appears to have undergone geologic activity that led to many fault valleys and what seem to be extensive flows of icy material. Little geologic activity has occurred on Umbriel or Oberon, judging by their old and dark surfaces.

All nine previously known rings were studied by the spacecraft and showed the Uranian rings to be distinctly different from those at Jupiter and Saturn. The ring system may be relatively young and did not form at the same time as Uranus. Particles that make up the rings may be remnants of a moon that was broken by a high-velocity impact or torn up by gravitational effects. Voyager 2 also discovered two new rings.

In March 2020, NASA astronomers reported the detection of a large atmospheric magnetic bubble, also known as a plasmoid, released into outer space from the planet Uranus, after reevaluating old data recorded by the Voyager 2 space probe during a flyby of the planet in 1986.[5][6]

Proposed missions

Mission concepts
to Uranus		Agency/countryType
MUSEESAorbiter and
atmospheric probe
OceanusNASA/JPLorbiter
ODINUSESAtwin orbiters around Uranus and Neptune
NASA Uranus orbiter and probeNASAorbiter and
atmospheric probe
Uranus PathfinderUnited Kingdomorbiter
Tianwen-4CNSAflyby probe

A number of missions to Uranus have been proposed. Scientists from the Mullard Space Science Laboratory in the United Kingdom have proposed the joint NASA–ESA Uranus Pathfinder mission to Uranus. A call for a medium-class (M-class) mission to the planet to be launched in 2022 was submitted to the ESA in December 2010 with the signatures of 120 scientists from across the globe. The ESA caps the cost of M-class missions at €470 million.[7][3][8]

In 2009, a team of planetary scientists from NASA's Jet Propulsion Laboratory advanced possible designs for a solar-powered Uranus orbiter. The most favorable launch window for such a probe would have been in August 2018, with arrival at Uranus in September 2030. The science package would have included magnetometers, particle detectors and, possibly, an imaging camera.[9]

In 2011, the United States National Research Council recommended a Uranus orbiter and probe as the third priority for a NASA Flagship mission by the NASA Planetary Science Decadal Survey. However, this mission was considered to be lower-priority than future missions to Mars and the Jovian System, which would later become Mars 2020 and Europa Clipper.[4][10][11]

A mission to Uranus is one of several proposed uses under consideration for the unmanned variant of NASA's heavy-lift Space Launch System (SLS) currently in development. The SLS would reportedly be capable of launching up to 1.7 metric tons to Uranus.[12]

In 2013, it was proposed to use an electric sail (E-Sail) to send an atmospheric entry probe to Uranus.[13]

In 2015, NASA announced it had begun a feasibility study into the possibility of orbital missions to Uranus and Neptune, within a budget of $2 billion in 2015 dollars. According to NASA's planetary science director Jim Green, who initiated the study, such missions would launch in the late 2020s at the earliest, and would be contingent upon their endorsement by the planetary science community, as well as NASA's ability to provide nuclear power sources for the spacecraft.[14] Conceptual designs for such a mission are currently being analyzed.[15]

MUSE, conceived in 2012 and proposed in 2015, is a European concept for a dedicated mission to the planet Uranus to study its atmosphere, interior, moons, rings, and magnetosphere.[16] It is suggested to be launched with an Ariane 5 rocket in 2026, arriving at Uranus in 2044, and operating until 2050.[16]

In 2016, another mission concept was conceived, called Origins and Composition of the Exoplanet Analog Uranus System (OCEANUS), and it was presented in 2017 as a potential contestant for a future New Frontiers program mission.[17]

In 2022, the Uranus orbiter and probe mission was placed as the highest priority for a NASA Flagship mission by the 2023–2032 Planetary Science Decadal Survey, ahead of the Enceladus Orbilander and the ongoing Mars Sample Return program, due to the lack of knowledge about ice giants.[18]

Future launch windows are available between 2030 and 2034.[19]

China plans to send its first exploration mission to Uranus in 2045 as part of Tianwen-4.[20][21][22]

See also

References

  1. ↑ "Revisiting the ice giants: NASA study considers Uranus and Neptune missions". Planetary Society. 21 June 2017. Retrieved 24 June 2017.
  2. ↑ "Ice Giant Mission Study Final Report". NASA / Lunar and Planetary Institute. June 2017. Retrieved 25 June 2017.
  3. 1 2 Sutherland, Paul (January 7, 2011). "Scientists plan Uranus probe". The Christian Science Monitor. Retrieved January 16, 2011.
  4. 1 2 Deborah Zabarenko (March 7, 2011). "Lean U.S. missions to Mars, Jupiter moon recommended". Reuters. Retrieved March 13, 2011.
  5. ↑ Hatfield, Mike (25 March 2020). "Revisiting Decades-Old Voyager 2 Data, Scientists Find One More Secret – Eight and a half years into its grand tour of the solar system, NASA's Voyager 2 spacecraft was ready for another encounter. It was Jan. 24, 1986, and soon it would meet the mysterious seventh planet, icy-cold Uranus". NASA. Retrieved 27 March 2020.
  6. ↑ Andrews, Robin George (27 March 2020). "Uranus Ejected a Giant Plasma Bubble During Voyager 2's Visit – The planet is shedding its atmosphere into the void, a signal that was recorded but overlooked in 1986 when the robotic spacecraft flew past". The New York Times. Retrieved 27 March 2020.
  7. ↑ Arridge, Chris (2010). "Uranus Pathfinder". Retrieved January 10, 2011.
  8. ↑ ESA official website: "Call for a Medium-size mission opportunity for a launch in 2022". January 16, 2011. Retrieved January 16, 2011.
  9. ↑ Hofstadter, Mark (2009). "The Case for a Uranus Orbiter and How it Addresses Satellite Science" (PDF). Archived from the original (PDF) on May 16, 2018. Retrieved May 26, 2012. See also a draft.
  10. ↑ "Vision and Voyages for Planetary Science in the Decade 2013–2022" (PDF) (Press release). National Academies. 2011. Retrieved March 7, 2011.
  11. ↑ Mark Hofstadter, "Ice Giant Science: The Case for a Uranus Orbiter", Jet Propulsion Laboratory/California Institute of Technology, Report to the Decadal Survey Giant Planets Panel, 24 August 2009
  12. ↑ Gebhardt, Chris (November 20, 2013). "New SLS mission options explored via new Large Upper Stage". NASAspaceflight.com.
  13. ↑ Fast E-sail Uranus entry probe mission
  14. ↑ Leone, Dan (August 25, 2015). "NASA To Study Uranus, Neptune Orbiters". Space News.
  15. ↑ Stephen Clark "Uranus, Neptune in NASA's sights for new robotic mission", Spaceflight Now, August 25, 2015
  16. 1 2 Bocanegra-Bahamón, Tatiana (2015). "MUSE Mission to the Uranian System: Unveiling the evolution and formation of ice giants" (PDF). Advances in Space Research. 55 (9): 2190–2216. Bibcode:2015AdSpR..55.2190B. doi:10.1016/j.asr.2015.01.037.
  17. ↑ New Frontiers-Class Missions to the Ice Giants. C. M. Elder, A. M. Bramson, L. W. Blum, H. T. Chilton, A. Chopra, C. Chu6, A. Das, A. Davis, A. Delgado, J. Fulton, L. Jozwiak, A. Khayat, M. E. Landis, J. L. Molaro, M. Slipski, S. Valencia11, J. Watkins, C. L. Young, C. J. Budney, K. L. Mitchell. Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989).
  18. ↑ "Planetary Science and Astrobiology Decadal Survey 2023–2032". National Academies. Retrieved 17 May 2022.
  19. ↑ Davis, Jason (21 June 2017). "Revisiting the ice giants: NASA study considers Uranus and Neptune missions". Planetary Society. Retrieved 31 July 2020.
  20. ↑ Jones, Andrew (September 22, 2022). "China wants to probe Uranus and Jupiter with 2 spacecraft on one rocket". space.com. Retrieved February 5, 2023.
  21. ↑ "China's Mars mission 'going smoothly', chief designer says – China". Chinadaily.com.cn. Retrieved 2022-05-26.
  22. ↑ Andrew Jones (2023-12-21). "China's plans for outer Solar System exploration". The Planetary Society. Retrieved 2023-12-27.

Bibliography

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