AKATSUKI returns to Venus

Abstract

AKATSUKI is the Japanese Venus Climate Orbiter that was designed to investigate the climate system of Venus. The orbiter was launched on May 21, 2010, and it reached Venus on December 7, 2010. Thrust was applied by the orbital maneuver engine in an attempt to put AKATSUKI into a westward equatorial orbit around Venus with a 30-h orbital period. However, this operation failed because of a malfunction in the propulsion system. After this failure, the spacecraft orbited the Sun for 5 years. On December 7, 2015, AKATSUKI once again approached Venus and the Venus orbit insertion was successful, whereby a westward equatorial orbit with apoapsis of ~440,000 km and orbital period of 14 days was initiated. Now that AKATSUKI's long journey to Venus has ended, it will provide scientific data on the Venusian climate system for two or more years. For the purpose of both decreasing the apoapsis altitude and avoiding a long eclipse during the orbit, a trim maneuver was performed at the first periapsis. The apoapsis altitude is now ~360,000 km with a periapsis altitude of 1000-8000 km, and the period is 10 days and 12 h. In this paper, we describe the details of the Venus orbit insertion-revenge 1 (VOI-R1) and the new orbit, the expected scientific information to be obtained at this orbit, and the Venus images captured by the onboard 1-µm infrared camera, ultraviolet imager, and long-wave infrared camera 2 h after the successful initiation of the VOI-R1.

Venus Atmospheric Thermal Structure and Radiative Balance

“…The image on the right was obtained at a later time and longitude showing the cold collar in the north and the lower cloud tops (warmer brightness temperatures) in the polar region and very low thermal contrasts (< 2 K) over low and mid latitudes. Akatsuki images are from JAXA The Akatsuki orbiter launched by Japan in May 2010 (Nakamura et al 2007(Nakamura et al , 2016 is beginning to investigate this relationship after a successful orbital insertion around Venus on 7 December 2015, with routine observations beginning in April 2016. The first image of Venus obtained from orbit from the LIR camera (Fukuhara et al 2011) in the 8-12 μm wavelengths shows small scale temperature detail (Fig.…”

“…Akatsuki orbiter succeeded on its second attempt to enter into orbit around Venus on 7 December 2015 and has been observing the planet routinely since beginning of April 2016 (Nakamura et al 2007(Nakamura et al , 2016. From its low inclination orbit, Akatsuki provides more low latitude radio occultation profiles (but fewer due to its 10.5-day orbit) compared to the high latitude coverage from Venus Express from its 24 hour polar orbit.…”

Initial performance of the radio occultation experiment in the Venus orbiter mission Akatsuki

“…The first attempt on Venus orbit insertion has failed, and the second attempt conducted in December 2015, after 5-years of interplanetary cruise, was successful (Nakamura et al 2016). Regular observations of the Venusian atmosphere with radio occultation technique have started in March 2016.…”

Absolute calibration of brightness temperature of the Venus disk observed by the Longwave Infrared Camera onboard Akatsuki

“…An observation sequence with n = 32 was prepared for the original orbit plan in which the spacecraft constantly observes Venus except near periapsis in a 31-h elliptical orbit (Nakamura et al 2007). However, the current orbit with a period of 10 days is much more elongated than the original orbit plan (Nakamura et al 2016). Distance from the spacecraft to Venus at the apoapsis, which was originally estimated at 79,000 km, increased to 370,000 km.…”

“…Subsequently, five cameras and an ultra-stable oscillator for a radio occultation experiment onboard Akatsuki started observation of the Venus atmosphere (Nakamura et al 2016). The Longwave Infrared Camera (LIR), which is one of the five cameras onboard Akatsuki, maps of brightness temperature of the Venus disk on both day and night hemispheres by detecting emissions at wavelengths from 8 to 12 μm (Fukuhara et al 2011).…”