Japanese orbiter officially begins science mission at Venus

Artist’s concept of Japan’s Akatsuki spacecraft at Venus. Credit: JAXA

Five months since a belated arrival at Venus, Japan’s Akatsuki spacecraft has officially started a modified scientific survey of the sweltering, shrouded planet’s atmosphere and climate.

The probe’s science cameras are collecting regular images of Venus’s exotic clouds, and Japanese engineers are optimistic Akatsuki can remain operational for at least two years, and perhaps through 2020.

Akatsuki braked into orbit around Venus in early December, five years later than originally planned after it missed an arrival opportunity in 2010.

Scientists checked out the orbiter’s science instruments since the craft arrived at Venus, and declared Akatsuki operational in April, according to the Japan Aerospace Exploration Agency. One of the spacecraft’s instruments, the lightning and airglow camera, is still being calibrated before it shifts to regular observations, JAXA said.

Launched by a Japanese H-2A rocket in May 2010, Akatsuki survived an unplanned five-year cruise to Venus, passing closer to the sun and withstanding higher temperatures it was designed to endure after a propulsion failure thwarted an orbit insertion burn in December 2010.

A salt formation blocked an engine valve, starving Akatsuki’s main thruster of fuel during the critical firing to swing into orbit around Venus.

Five years later, when Akatsuki was again in the vicinity of Venus, the spacecraft fired its secondary attitude control thrusters to steer into orbit. But the smaller engines had less power than the probe’s primary thruster, driving Akatsuki into an orbit stretching nearly five times farther from Venus than originally intended.

A course correction burn April 4 slightly adjusted Akatsuki’s orbit to reach a peak altitude of 370,000 kilometers (about 230,000 miles), roughly the distance between the Earth and the moon. At the low end of its orbit, Akatsuki passes between 1,000 and 10,000 kilometers (620-6,200 miles) above Venus’s cloud tops.

The higher orbit will complicate the mission’s scientific observations, with Venus appearing much smaller to Akatsuki’s cameras. It also produces fewer opportunities for radio occultation measurements, which use radio signals passed between the spacecraft and Earth-based antennas to study the vertical structure of Venus’s atmosphere.

The orbiter’s lightning and airglow camera, conceived to take the first pictures of lightning flashes on Venus, can only collect images for about one hour during each 10.8-day orbit, when Akatsuki is in the shadow of the planet. The limited imaging windows for the lightning camera have extended the instrument’s calibration time, but scientists expect it to begin full science observations in June.

Takeshi Imamura, Akatsuki’s project scientist at JAXA’s Institute of Space and Astronautical Science, told a gathering of planetary scientists in Britain last month that the mission’s shortcomings could be overcome with more high-resolution imaging at the low point of Akatsuki’s orbit, and an extension of the probe’s operations beyond its original two-year lifetime through 2020.

Akatsuki, which means dawn in Japanese, is the only mission currently operating at Venus. It is also Japan’s first spacecraft to orbit another planet.

The craft is the first Venus mission dedicated to unraveling the planet’s complex atmosphere, with high-altitude super-rotating jet streams, thick clouds made of sulfuric acid and a blanket of heat-trapping carbon dioxide, driving surface temperatures to 900 degrees Fahrenheit (480 degrees Celsius), hot enough to melt lead.

One of Akatsuki's infrared cameras took this image of the night side of Venus, the most detailed view of the planet's nighttime clouds ever acquired. Credit: JAXA
One of Akatsuki’s infrared cameras took this image of the night side of Venus, the most detailed view of the planet’s nighttime clouds ever acquired. Credit: JAXA

Akatsuki carries three infrared cameras, each tuned to resolve a different layer in Venus’s atmosphere.

One of the imagers can see deep into the planet’s shroud of clouds and toxic haze to see Venus’s surface terrain. An early picture from the camera revealed Aphrodite Terra, a highland region near the planet’s equator so far only observed by radar surveys.

Another sensor in longwave infrared can detect the temperature of Venus’s cloud tops, while a two-micron imaging channel is suited for observations of the planet’s night side.

Akatsuki beamed back the most detailed global image ever captured of Venus’s night side clouds March 25. The six-second exposure, posted above, reveals previously unseen wave and stripe patterns.

“This is to date the most detailed global view of the night-side disk,” JAXA wrote in a description accompanying the image. “At 2.26 microns, spatially-inhomogeneous clouds appear as silhouette back-illuminated by thermal radiation from the hot lower atmosphere. Such images, combined with other cameras’ data, will be used to study the 3D structure and dynamics of Venus atmosphere.”

The longwave infrared camera spotted an unexpected bow-shaped cloud stretching thousands of kilometers running from the northern hemisphere to the southern hemisphere of the planet.

“This is the first time to learn (of) such a phenomenon,” JAXA said in a statement.

An ultraviolet camera aboard Akatsuki can help scientists study the origin of the planet’s clouds and track weather patterns as they move around the planet with jet stream winds moving at up to 400 kilometers per hour (250 mph).

Imamura said the processes responsible for the formation of Venus’s sulfuric acid clouds seem to be much more complicated than expected.

Scientists plan near-continous global imaging throughout Akatsuki’s mission, plus closeups when the spacecraft is near Venus, occasional searches for lightning and atmospheric profile measurements with the probe’s radio science platform.

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