Akatsuki (spacecraft)

Mission

Akatsuki was Japan's first planetary exploration mission since the failed Mars orbiter Nozomi probe launched in 1998. This larger orbit takes 10 days to complete instead of the originally planned 30 hours. Observations included cloud and surface imaging with cameras operating in infrared, visible, and ultraviolet wavelengths to investigate Venusian meteorology and the processes behind the atmospheric super-rotation, where the atmosphere spins at 300 km/h while the planet rotates at 6 km/h at the equator.

Other experiments aimed to confirm lightning and determine whether volcanism currently occurs on Venus. The main bus is a 1.45 m box with two solar arrays, each about 1.4 m², providing over 700 W of power in Venus orbit. Propulsion came from a 500 N bi-propellant orbital maneuvering engine and twelve mono-propellant hydrazine reaction control thrusters; it was the first spacecraft to use a ceramic (silicon nitride) retrofire thruster. Communication used an 8 GHz, 20-watt X-band transponder with a 1.6 m high-gain antenna, plus medium-gain and low-gain antennas.

Instruments

The spacecraft carried six science tools. Five cameras captured Venus from ultraviolet to mid-infrared. The Lightning and Airglow Camera looked for visible lightning flashes. An ultraviolet imager tracked sulfur dioxide and an unknown ultraviolet absorber.

A longwave infrared camera examined high-altitude cloud heat emissions at 10 micrometers. One infrared camera studied night-side surface heat and day-side cloud reflections until an electronics failure stopped it in December 2016. Another infrared camera observed night-side lower clouds and day-side carbon dioxide bands, also failing in December 2016. An Ultra-Stable Oscillator enabled radio occultation experiments.

Operations

Akatsuki left the Sagamihara Campus on 17 March 2010 and arrived at the Tanegashima Space Center on 19 March. On 4 May it was encapsulated inside the H-IIA payload fairing along with the IKAROS solar sail, and on 9 May the fairing was mated to the launch vehicle. The spacecraft launched on 20 May 2010 at 21:58:22 UTC from Tanegashima, delayed from 18 May due to weather. The burn started on time, but after expected occultation by Venus, communication did not recover as planned; the probe was in safe-hold mode, spin-stabilized at ten minutes per rotation.

JAXA stated on 8 December that the insertion had failed, and on 10 December reported the engines fired for less than three minutes. The likely cause was salt deposits jamming the helium pressurization valve, leading to oxidizer-rich combustion that damaged the chamber throat and nozzle. The probe entered a heliocentric orbit with a 203-day period, shorter than Venus's 225-day period, causing it to drift relative to Venus.

Science

Three hours after insertion in December 2015 and in a few glimmers in April and May 2016, the craft recorded a bow-shape feature stretching 6,000 miles almost pole to pole, termed a gravity wave in the planet's winds above the Aphrodite Terra region. The mission collected data from ultraviolet (280 nm) to mid-infrared (10 μm). Images revealed something similar to jet stream winds in the low and middle cloud region (45–60 km altitude), with wind speed maximizing near the equator; in September 2017 JAXA scientists named this the 'Venusian equatorial jet'.

In 2018, thick clouds of small particles appeared near the transition between upper and middle clouds, described as a new and puzzling morphology. By 2017 the science team published 3D maps of Venus's atmosphere, including pressure, temperature, water vapor density, and ionospheric electron density. In 2019, results on middle-cloud morphology, temporal changes, and winds were published, reporting unexpectedly high contrasts possibly indicating absorbers like water.

In March 2020 a flash was detected on the dark side, unlikely to be a bolide or cosmic ray, but not confirmed as lightning.

Did You Know?

• A public relations campaign allowed 260,214 people to send their names and messages on aluminium plates aboard Akatsuki.
• Around 90 aluminium plates were created for the spacecraft, including three with images of the Vocaloid Hatsune Miku and her super deformed-styled figure Hachune Miku.
• The probe's high-gain antenna was flat to prevent heat buildup.