March 14, 2016 1:00 pm JST

Venus finally in sight for JAXA's once-wayward Akatsuki probe

SHOJI YANO, Nikkei staff writer

A rendering of the Akatsuki orbiter near Venus.(Courtesy of JAXA, Akihiro Ikeshita)

TOKYO -- After a five-year delay, Japan's Akatsuki space probe is set to begin full-scale observations of Venus next month.

     Akatsuki was launched by the Japan Aerospace Exploration Agency, commonly known as JAXA, and entered orbit around Venus last December on its second attempt. The first try, in 2010, failed due to mechanical problems. The new orbit is significantly longer and will take the probe far further from Venus than originally planned.

     Venus, the second planet from the sun, is quite similar to our own planet in size, mass, gravity and composition. For this reason, Venus is often called the Earth's twin or sister.

     But there are also stark differences. Temperatures on Venus, for example, can reach a blistering 460 C, which experts believe is due to the greenhouse gas effect, as carbon dioxide makes up most of the planet's atmosphere. Scientists hope that unlocking the secrets of the Venusian climate will help deepen our understanding of the meteorological phenomena and global warming here on Earth.

Take two

Akatsuki's journey began in May 2010, when it was launched from JAXA's space center on Tanegashima, a small island in southern Japan. Initially, JAXA estimated the spacecraft would orbit Venus once every 30 hours, traveling on an ellipse that would pass 300km from the planet at its closest point and 80,000km at its farthest.

     The probe attempted to enter this orbit in December of that year, but it experienced trouble with its major engines.

     Its present orbit is less than ideal, passing 440,000km from the planet at its farthest point. That is roughly five times greater than initially planned and means the orbit time is now nine days.

    The change in orbit has affected the probe's observation plan. Of its seven planned missions, Akatsuki will be able to complete only one: taking serial images of clouds. Unfortunately, the probe's five cameras, each capturing images in different wavelengths, including infrared and ultraviolet, will not be able to provide the same resolution at this greater distance. Observing volcanic eruptions on the Venusian surface may also be difficult.

     There is an upside to the situation, however. Takeshi Imamura, an associate professor at JAXA's Institute of Space and Astronautical Science, says the longer orbital period means the probe will be able to collect longer continuous stretches of data.    

     On a nine-day orbit, Akatsuki should be able to shoot a vast amount of images over a wider field and for more hours, with the result almost like a movie. "The images may not be as sharp as they should be," Imamura said, "but in terms of time continuity, it's helpful."

     Scientists are particularly interested in clues that might help explain an atmospheric phenomenon called super-rotation. Venus takes 243 days to rotate on its axis. Because winds on the planet travel in the same direction at speeds of up to 100 meters per second, the atmosphere actually turns much more quickly, circling the planet in just four days. On the Earth, it is impossible for the atmosphere to move faster than the planet rotates. Understanding this unusual phenomenon could help scientists understand meteorological mechanism of other planets, including our own.

     The success of Akatsuki's mission will depend on its devices functioning properly. The new observation schedule is expected to be completed in two years -- but the spacecraft was originally designed to operate for only four and half years, and it has already been active for nearly six years.

     Some spacecraft can operate longer than originally designed. Suzaku, Japan's X-ray astronomy satellite, was active for 10 years before its mission was ended in 2015, much longer than its intended lifetime of two years.

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