Akatsuki (spacecraft)

Akatsuki (あかつき, 暁; "Dawn"), 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.

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 300 to 80,000 km (190 to 49,710 mi) in altitude, but its alternate orbit had to be highly elliptical ranging between 1,000 and 10,000 kilometres (620 and 6,210 mi) at its nearest point and about 360,000 kilometres (220,000 mi) 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 (US$174 million) 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 6 kilometres per hour (3.7 mph) at the equator, the atmosphere spins around the planet at 300 kilometres per hour (190 mph). Other experiments were designed to confirm the presence of lightning and to determine whether volcanism occurs currently on Venus.

The main bus is a 1.45 × 1.04 × 1.44 m (4.8 × 3.4 × 4.7 ft) box with two solar arrays, each with an area of about 1.4 m² (15 sq ft). The solar arrays provided over 700 W of power while in Venus orbit. The total mass of the spacecraft at launch was 517.6 kg (1,141 lb). The mass of the science payload was 34 kg (75 lb).

Propulsion was provided by a 500-newton (110 lb_f) bi-propellant, hydrazine-dinitrogen tetroxide orbital maneuvering engine and twelve mono-propellant hydrazine reaction control thrusters, eight with 23 N (5.2 lb_f) of thrust and four with 3 N (0.67 lb_f). It was the first spacecraft to use a ceramic (silicon nitride) retrofire thruster. The total propellant mass at launch was 196.3 kg (433 lb).

Communication was handled via an 8 GHz, 20-watt X-band transponder using the 1.6 m (5 ft 3 in) high-gain antenna. The high-gain antenna was flat to prevent heat from building up in it. Akatsuki also had a pair of medium-gain horn antennas mounted on turntables and two low-gain antennas for command uplink. The medium-gain horn antennas were used for housekeeping data downlink when the high-gain antenna was not facing Earth.