| programme = PLANET series

| previous_mission = Nozomi

| next_mission =

, also known as the Venus Climate Orbiter (VCO) and Planet-C, was a Japan Aerospace Exploration Agency (JAXA) space probe tasked with studying the atmosphere of Venus. It was launched aboard an H-IIA 202 rocket on 20 May 2010, but failed to enter orbit around Venus on 6 December 2010. After the craft orbited the Sun for five years, engineers successfully placed it into an alternate Venusian elliptic orbit on 7 December 2015 by firing its attitude control thrusters for 20 minutes, making it the first Japanese satellite to orbit Venus.

By using five different cameras working at several wavelengths, Akatsuki studied the stratification of the atmosphere, atmospheric dynamics, and cloud physics. Astronomers working on the mission reported detecting a possible gravity wave (not to be confused with gravitational waves) in Venus's atmosphere in December 2015.

JAXA lost contact with the probe in late April 2024. Operation was terminated officially on 18 September 2025.

Mission

Akatsuki was Japan's first planetary exploration mission since the failed Mars orbiter Nozomi probe which was launched in 1998. Akatsuki was originally intended to conduct scientific research for two or more years from an elliptical orbit around Venus ranging from in altitude, The budget for this mission is ¥14.6 billion () for the satellite and ¥9.8 billion (US$116 million) for the launch.

Observations included cloud and surface imaging from an orbit around the planet with cameras operating in the infrared, visible and UV wavelengths to investigate the complex Venusian meteorology and elucidate the processes behind the mysterious atmospheric super-rotation. On Venus, while the planet rotates at at the equator, the atmosphere spins around the planet at . Other experiments were designed to confirm the presence of lightning and to determine whether volcanism occurs currently on Venus. The mass of the science payload was .

Propulsion was provided by a bi-propellant, hydrazine-dinitrogen tetroxide orbital maneuvering engine and twelve mono-propellant hydrazine reaction control thrusters, eight with of thrust and four with . The total propellant mass at launch was .

  1. the Lightning and Airglow Camera (LAC) searched for lightning in the visible spectrum (552–777 nm)
  2. the ultraviolet imager (UVI) studied the distribution of specific atmospheric gases such as sulfur dioxide and the famous unknown absorber at ultraviolet wavelengths (283–365 nm)
  3. the longwave infrared camera (LIR) studied the structure of high-altitude clouds at a wavelength where they emit heat (10 μm)
  4. the infrared 1 μm camera (IR1) imaged the night side heat radiation (0.90–1.01 μm) emitted from Venus's surface and help researchers to search for active volcanoes. While on the day side, it sensed the solar near-infrared radiation (0.90 μm) reflected by the middle clouds. Operation of the instrument was terminated in December 2016 due to an electronic failure.

The spacecraft was launched on 20 May 2010 at 21:58:22 (UTC) from the Tanegashima Space Center,

Orbit insertion failure

[[File:Akatsuki original orbit.jpg|thumb|Movement of the spacecraft orbit and Venus in a Sun-Earth line fixed rotating coordinate in the original plan. Numerals in the figure indicate days after VOI. JAXA stated on 8 December that the probe's orbital insertion maneuver had failed. At a press conference on 10 December, officials reported that Akatsukis engines fired for less than three minutes, far less than what was required to enter into Venus orbit. Instead of taking about 30 hours to complete an orbit around Venus—as was originally planned—the new orbit targeted would place Akatsuki in a nine-day orbit after an adjustment in March 2016.

After JAXA engineers measured and calculated its orbit following the 7 December orbital insertion, JAXA announced on 9 December that Akatsuki had successfully entered the intended elliptical orbit, as far as from Venus, and as close as from Venus's surface with an orbital period of 13 days and 14 hours.

A follow-up thruster burn on 26 March 2016 lowered Akatsukis apoapsis to about , periapsis altitude periodically changing from , and shortened its orbital period from 13 to about 10 days. On 9 December 2016, the near-infrared 1-μm and 2-μm cameras became unavailable for observations due to an electronic failure. Its long-wave infrared camera, ultraviolet imager, and lightning and airglow camera continued normal operation. Extended operations were approved until the end of 2020, with further mission extensions to be considered based on the spacecraft's condition at that time. As of November 2019, it was estimated that Akatsuki had enough fuel to continue operating for at least 2 more years.

, operation was planned to continue through FY2028.

<gallery>

File:Venus - October 24 2018.png|An image of Venus with AKATSUKI Ultraviolet Imager (UVI)

283 365 UVI 2018 06 03 08 03.jpg|An image of Venus with a crescent shaped area that is illuminated by sunlight. The image was taken with UVI.

174 226 232 date 2016 07 11 14 10.jpg|An image of the night side of Venus with the AKATSUKI 2-μm Camera (IR2). In the dark areas the light is absorbed by Carbon dioxide| clouds.

097 101 surface date 2016 07 12 04 05.jpg|An image of the surface of Venus with the AKATSUKI 1-μm Camera (IR1)

</gallery>

See also

  • List of Venus probes

References

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  • Venus Climate Orbiter Akatsuki official website
  • JAXA Akatsuki Planet-C page
  • JAXA Akatsuki Special Site
  • DARTS (ISAS JAXA) Akatsuki Science Archive
  • Planet-C page (Solar Terrestrial Physics Group)
  • Detailed Space Review article about Akatsuki and its recovery
  • Venus Climate Orbiter Akatsuki (PDF, 1.72 Mb)
  • Akatsuki – List of scientific publications
  • A new look at Venus with Akatsuki by the Planetary Society