|Mission type||Venus orbiter|
|Mission duration||~2 years
elapsed: 4 years, 1 month and 21 days
|Launch mass||517.6 kilograms (1,141 lb)|
|Power||>700 watts at 0.7 AU|
|Start of mission|
|Launch date||20 May 2010, 21:58:22UTC|
|Launch site||Tanegashima Yoshinobu 1|
|Pericytherion||300 kilometers (190 mi)|
|Apocytherion||79,000 kilometers (49,000 mi)|
|Flyby of Venus (failed insertion)|
|Closest approach||6 December 2010, 23:49:00 UTC|
Akatsuki (あかつき, 暁?, literally "dawn"), formerly known as the Venus Climate Orbiter (VCO) and Planet-C, is a Japanese space probe which was intended to explore Venus. It was launched aboard an H-IIA 202 rocket on 20 May 2010.
The total launch mass of the spacecraft including propellant was 517.6 kg (1,141 lb), with scientific instruments accounting for 34 kg (75 lb) of its total weight. The mission reached Venus on 7 December 2010 (JST) but failed to enter orbit around the planet, and thus remains in a heliocentric orbit. It had been 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. Plans to attempt an entry into an alternative Venerian orbit 5,000 to 300,000 km (3,100 to 186,400 mi) during November 2015 are being attempted.
Akatsuki is a Japanese space mission to the planet Venus. Planned observations include cloud and surface imaging from an orbit around the planet with an infrared camera, which are aimed at investigation of the complex Venerian meteorology. Other experiments are designed to confirm the presence of lightning and to determine whether volcanism occurs currently on Venus.
The main bus is a 1,450 × 1,040 × 1,440 mm (4.76 × 3.41 × 4.72 ft) box with two solar arrays, each with an area of about 1.4 m2 (15 sq ft). The solar array panels provide over 700 watts (0.94 hp) of power in Venus orbit. The total mass of the spacecraft at launch was 517.6 kg (1,141 lb). The mass of the science payload is 34 kg (75 lb).
Propulsion is provided by a 500-newton (110 lbf) bi-propellant, hydrazine / nitrogen tetroxide orbital maneuvering engine and twelve mono-propellant hydrazine reaction control thrusters, eight with 23 N (5.2 lbf) of thrust and four with 3 N (0.67 lbf). The total propellant mass at launch was 196.3 kg (433 lb).
Communications is via an 8 GHz, 20-watt X-band transponder using the 1.6 m (5 ft 3 in) slot array high-gain dish antenna used for most telemetry data. 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 will be used for housekeeping data downlink when the high-gain antenna is not facing Earth.
The scientific payload consists of six instruments including a Lightning and airglow camera (LAC), an ultraviolet imager (UVI), a longwave infrared camera (LIR), a 1 μm camera (IR1), a 2 μm camera (IR2), and the radio science (RS) experiment. The five imaging cameras will explore Venus in wavelengths from ultraviolet to the mid-infrared.
The LAC will look for lightning in the visible wavelengths of 552 to 777 nanometers. The LIR will study the structure of high-altitude clouds at a wavelength where they emit heat (10 μm). The UVI will study the distribution of specific atmospheric gases such as sulfur dioxide in ultraviolet wavelengths (293–365 nm). The IR1 will peer through semi-transparent windows in Venus' atmosphere to see heat radiation emitted from Venus' surface rocks (0.90–1.01 μm) and will help researchers to spot active volcanoes, if they exist. The IR2 will detect heat radiation emitted from the lower reaches of the atmosphere (1.65—2.32 μm).
There was a public relations campaign held between October 2009 and January 2010 by The Planetary Society and JAXA, to allow individuals to send their name and a message aboard Akatsuki. Names and messages were printed in fine letters on an aluminum plate and placed aboard Akatsuki. 260,214 people submitted names and messages for the mission. Around 90 aluminum plates were created for the spacecraft, including three aluminum plates in which the images of the Vocaloid Hatsune Miku and her super deformed figure Hachune Miku were printed.
Akatsuki left the Sagamihara Campus on 17 March 2010, and arrived at the Tanegashima Space Center's Spacecraft Test and Assembly Building 2 on 19 March. On 4 May, Akatsuki was encapsulated inside the large payload fairing of the H-IIA rocket that launched the spacecraft, along with the IKAROS solar sail, on a 6-month journey to Venus. On 9 May, the payload fairing was transported to the Tanegashima Space Center's Vehicle Assembly Building, where the fairing was mated to the H-IIA launch vehicle itself.
Orbit insertion failure
Akatsuki was planned to initiate orbit insertion operations by igniting the orbital maneuvering engine at 23:49:00 on 6 December 2010 UTC. The burn was supposed to continue for 12 minutes, to an initial orbit of 180,000 to 200,000 km (110,000 to 120,000 mi) apoapsis / 550 km (340 mi) periapsis / 4 days orbital period around Venus.
The orbit insertion maneuver was confirmed to have started on time. But after the expected blackout due to occultation by Venus, the communication with the probe did not recover as planned. The probe was found to be in safe-hold mode, spin-stabilized state with 10 minutes per rotation. Due to the low communication speed through low-gain antenna, it took a while to determine the state of probe. JAXA stated on 8 December, that the probe's orbital insertion maneuver had failed. At a press conference on 10 December, officials reported that Akatsuki's engines fired for less than 3 minutes, far short of what was required to enter into Venus orbit. Further research have found what the likely reason for probe malfunction was the salt deposits jamming the valve between helium pressurization tank and fuel tank. As result, engine combustion have become oxidizer-rich, with resulting high combustion temperatures damaging combustion chamber throat and nozzle. The similar vapor leakage problem has also destroyed the Mars Observer probe back in 1993.
JAXA is developing plans to attempt another orbital-insertion burn when the probe returns to Venus in 2015. This requires placing the probe into a hibernation state to prolong its life beyond the original 4.5-year design. JAXA expressed some confidence in keeping the probe operational, pointing to reduced battery wear, since the probe is orbiting the Sun instead of its intended Venerian orbit.
It was suggested by telemetry data from the original failure that the throat of the orbit maneuver engine (OME) was still largely intact, and trial jet thrusts of the probe's onboard OME were performed twice, on 7 and 14 September 2011. However, the thrust was only about 40 newtons (9.0 lbf)—10% of expectations. Following these results, it was determined that insufficient specific impulse will be available for orbital maneuvering by the OME. It was concluded that the remaining combustion chamber throat was completely destroyed by transient ignition of the engine. As a result, the strategy of using only the monopropellant (hydrazine) reaction control system (RCS) thruster was instead adopted. Because the RCS thrusters do not need oxidiser, the remaining 65 kg of oxidiser (MON) was vented overboard in October 2011 to lighten the spacecraft.
Three peri-Venus orbital maneuvers were executed on 1 November, 10 November and 21 November 2011 using the reaction control system (RCS) thruster. A total delta-V of 243.8 m/s was imparted to the spacecraft. Following this, the spacecraft will make a rendezvous with Venus in November 2015. Because the RCS engines' specific impulse is low compared to the specific impulse of the OME, the previously planned insertion into low Venerian orbit has become impossible. Instead, the probe can (depending on chosen deceleration method) reach a final orbit with an apoapsis of hundred thousand kilometers and a periapsis of a few thousand kilometers from Venus. The final orbit is expected to be prograde (in the direction of the super-rotation) and lie in the orbital plane of Venus. The exact orbital insertion method and target orbit was under consideration by JAXA as of 2013 and was expected to be decided in late 2013.[dated info]
Also, it is unclear if the spacecraft will survive on its transfer orbit, with some perihelia as close as 0.6 AU from sun, without damage. Temperatures within the spacecraft may rise 30–40 degrees above values expected before launch, damaging batteries and sensitive electronics. By 14 April 2014 Akatsuki had been through six of the projected nine close solar approaches and was still functioning, despite exceeding the maximum design heating by 30%. The probe reached its current orbit's most distant point from Venus on 3 October 2013 and has been approaching the planet since.
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