, also known as the Venus Climate Orbiter (VCO) and Planet-C, is 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 alternative Venusian elliptic orbit on 7 December 2015 by firing its attitude control thrusters for 20 minutes and made it the first Japanese satellite orbiting Venus.
By using five different cameras working at several wavelengths, Akatsuki is studying 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' atmosphere in December 2015.
Mission
Akatsuki is 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, but its alternate orbit had to be highly elliptical ranging between at its nearest point and about at its farthest. This larger orbit takes 10 days to complete instead of the originally planned 30 hours. The budget for this mission is ¥14.6 billion () for the satellite and ¥9.8 billion (US$116 million) for the launch.
Observations include 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 are designed to confirm the presence of lightning and to determine whether volcanism occurs currently on Venus.
Spacecraft design
The main bus is a box with two solar arrays, each with an area of about . The solar arrays provide over 700 W of power while in Venus orbit. The total mass of the spacecraft at launch was . The mass of the science payload is .
Propulsion is 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 . It is the first spacecraft to use a ceramic (silicon nitride) retrofire thruster. The total propellant mass at launch was .
Communication is via an 8 GHz, 20-watt X-band transponder using the high-gain antenna. The high-gain antenna is flat to prevent heat from building up in it. Akatsuki also has a pair of medium-gain horn antennas mounted on turntables and two low-gain antennas for command uplink. The medium-gain horn antennas are used for housekeeping data downlink when the high-gain antenna is not facing Earth.
Instruments
The scientific payload consists of six instruments. The five imaging cameras are exploring Venus in wavelengths from ultraviolet to the mid-infrared: the Lightning and Airglow Camera (LAC) is looking for lightning in the visible spectrum (552-777 nm) the ultraviolet imager (UVI) is studying the distribution of specific atmospheric gases such as sulfur dioxide and the famous unknown absorber at ultraviolet wavelengths (283–365 nm) the longwave infrared camera (LIR) is studying the structure of high-altitude clouds at a wavelength where they emit heat (10 μm) the infrared 1 μm camera (IR1) is imaging on the night side heat radiation (0.90–1.01 μm) emitted from Venus's surface and help researchers to spot active volcanoes, if they exist. While on the day side, it sensed the solar near-infrared radiation (0.90 μm) reflected by the middle clouds. Unavailable for observation after December 2016 due to an electronic failure. the infrared 2 μm camera (IR2) studied the night side lower clouds' opacity to the thermal emission from the surface and deeper atmosphere (1.74–2.32 μm). It also sensed on the day side the band at 2.02 μm, which can be used to infer the altitude of the top of the clouds. Finally, the 1.65-μm filter was used during the cruise phase to study the zodiacal light. Unavailable for observation after December 2016 due to an electronic failure. the Ultra-Stable Oscillator (USO) for performing radio occultation experiments.