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Emily LakdawallaDecember 9, 2015

Akatsuki's new orbit, first images, and science plans

JAXA held a press briefing today to confirm the successful arrival of Akatsuki into Venus orbit -- an even lower orbit than they'd hoped to achieve. Akatsuki is Japan's first successful planetary orbiter. It's been a long time coming: today's announcement came twelve years to the day after Japan had to abandon efforts to put Nozomi into Mars orbit. The presenters were project manager Masato Nakamura (中村), trajectory engineer Chikako Hirose (廣瀬), and project scientist Takeshi Imamura (今村). There were three main topics: Akatsuki's orbital trajectory, new images, and science plans.


Kita Mitsunari

Celebrating "VCO" success
Project manager Masato Nakamura (中村), project scientist Takeshi Imamura (今村), and trajectory engineer Chikako Hirose (廣瀬), celebrate the successful arrival of Akatsuki, also known as Planet-C and Venus Climate Orbiter.

For The Planetary Society, this achievement is additionally meaningful, because all of our members' names (as of January 2010) are inscribed on a plate that is bolted to the side of Akatsuki, and are consequently now orbiting Venus.

AKATSUKI Message Plate

Japan Aerospace Exploration Agency (JAXA)

AKATSUKI Message Plate
The message plate attached to the Venus Climate Orbiter Akatsuki (PLANET-C)

Here is a video recording of the briefing, and here are briefing materials (PDF). Everything was, of course, in Japanese, but with help from Twitter I have put together a decent transcript of the briefing in English, which I reproduce in full at the bottom of this post. In particular, I would like to thank Shinya Torishima for his detailed live-tweeting in Japanese, and 5thstar for thorough English translations of Torishima's tweets. More official JAXA photos from the Akatsuki mission are available here, with English captions. Here's a summary of the high points.

Akatsuki's orbit

Chikako Hirose announced the shape of the new orbit: it has a periapsis of 400 kilometers, an apoapsis of 440,000 kilometers, a period of 13 days 14 hours, and an inclination of 3 degrees. For context: Originally, Akatsuki was planned to have an orbit with an apoapsis of 79,000 kilometers and a period of 30 hours. The 30-hour period was chosen to synchronize Akatsuki's orbital motion with the flow of Venus' upper level winds for roughly 20 hours of each orbit, allowing the spacecraft to do cloud tracking and determine the differential motions of different parts of the atmosphere. In February, JAXA announced that the new target orbit, with a much higher apoapsis of between 300,000 and 400,000 kilometers, would have a much longer period of 8 or 9 days. The plan as announced a few days ago was to put Akatsuki into an orbit with an apoapsis of about 490,000 kilometers with the orbit insertion burn, which would be reduced to 320,000 after a three-month checkout period.

Note that the 440,000-kilometer apoapsis actually achieved by Akatsuki upon arrival is substantially lower than planned for Akatsuki's initial checkout orbit. Hirose attributed the lower apoapsis to better performance of the engines than predicted. In general this is good news, but there is one thing to be slightly concerned about: the equatorial orbit necessarily has Akatsuki in Venus' shadow for some part of each day, which robs it of solar power and requires it to rely on batteries. Hirose said that the spacecraft cannot be permitted to be in shadow for longer than 90 minutes. Small changes in shape and orientation of the orbit can cause large differences in time spent in eclipse. They didn't discuss this in any further detail so I will assume unless told otherwise that the orbit that they are currently in poses no current danger to the spacecraft. Over the two-year nominal mission, the orbit inclination will increase to 25 degrees.

Images from orbit

They released three images, each taken by a different one of three of Akatsuki's six science instruments: the longwave infrared camera, the ultraviolet imager, and the 1-micron camera. (For a summary of Akatsuki's scientific instruments, read this English-language press kit PDF.) The three photos were taken about five hours after closest approach, when Akatsuki was at a distance of about 70,000 kilometers. They are lovely, and hint at great data to come.

Akatsuki's first UVI image of Venus after orbit insertion


Akatsuki's first UVI image of Venus after orbit insertion
Akatsuki's ultraviolet imager (UVI) sees shorter-wavelength ultraviolet than any other previous Venus imager, at 283 nanometers. Akatsuki took this photo about 5 hours after orbit insertion (December 7, 2015, at 05:19 UT) from a distance of 72,000 kilometers.
Akatsuki's first LIR image after orbit insertion

JAXA / National Institute of Advanced Industrial Science and Technology

Akatsuki's first LIR image after orbit insertion
Akatsuki's longwave infrared imager (LIR) takes images at a wavelength of 10 microns, studying cloud-top temperatures. This is the first photo of its kind taken in Venus orbit. It was acquired by Akatsuki about 5 hours after Venus arrival (December 7, 2015 at 05:19 UT) from a distance of 72,000 kilometers.
Akatsuki's first IR1 camera image after orbit insertion


Akatsuki's first IR1 camera image after orbit insertion
Akatsuki took this photo on December 7, 2015 at 4:50 UT, not quite five hours after orbit insertion, from an altitude of 68,000 kilometers. The camera is sensitive to infrared radiation at a wavelength of 1 micron. When looking at Venus' nightside, it will be able to detect deep clouds from their radiated heat.

During the press briefing, project manager Nakamura mentioned that these three cameras had been shielded from excessive solar heating by keeping other parts of the spacecraft rotated toward the Sun, including the high-gain antenna and other instruments. He is quoted as saying that the other instruments were "stressed" and that the instruments so stressed "saved" these three cameras. According to Imamura, the three camera instruments from which we do have images are healthy and "show no signs of degradation." The other three instruments from which we have not yet seen data are the 2-micron infrared camera (IR2), the lightning and airglow camera (LCA), and the ultrastable oscillator, which is part of a radio science experiment. I am guessing (but am not sure) that the ultrastable oscillator is what Nakamura was referring to when he mentioned that the "radio transmitter has been suspended as well".

The images are interesting because they were taken from a distance similar to the spacecraft's originally intended apoapsis distance, which is to say that they show the cameras' capabilities at the distance from Venus at which they were designed to be operated. They show what the lowest-resolution Venus images would have looked like, had the spacecraft successfully entered orbit in 2010. From the new orbit apoapsis of 440,000 kilometers, Venus will appear about one-seventh as large to these cameras.

Akatsuki's future plans

Nakamura said that the team will take three months to check out the spacecraft, its instruments, and operations in Venus orbit; science data acquisition will start in April. Project scientist Imamura said that we can consider Akatsuki to be a weather satellite for Venus, continuously imaging the planet for many days at a time, observing the 3D motions of the atmosphere. Although the new data set will mostly be lower in spatial resolution than hoped, it will actually be superior in temporal resolution to the original plan. And of course they will still get quite high-resolution data near periapsis, just less frequently. They will target specific phenomena for the rarer periapsis approaches. The shift toward a mission that studies how Venus changes over time means that Akatsuki's mission will be a better one, the longer and more continuously the spacecraft manages to operate in orbit; a two-year mission is the nominal plan, but Nakamura and Imamura expressed hope that it could be extended beyond that.

Capturing the public imagination

All three of the panelists agreed that Akatsuki wouldn't be a success without lots of support. Imamura singled out "space fans," and it does seem that Akatsuki's direct engagement with the public has been quite different from previous JAXA missions. Here is how Akatsuki's official Twitter account shared those new images:

【twitter班より】12月7日にあかつき搭載の3つのカメラ(IR1, UVI, LIR)で高度約7万kmから撮像した金星画像も掲載されています。

— 「あかつき」チーム (@Akatsuki_JAXA) December 9, 2015

Fans have also given back to the mission directly. When we were watching the orbit insertion, the JAXA team was making use of a real-time visualization of the spacecraft produced by a science book editor, science writer, and space fan named Isana Kashiwai, who goes by @lizard_isana on Twitter. His visualization makes use of orbital data predictions from JAXA. (Another of Kashiwai's simulators, called GoogleSatTrack, was used by NASA in their broadcast of the final shuttle mission -- more information on that, in Japanese, here and here.)

There were poignant moments, too. The trajectories of Akatsuki and its predecessors Hayabusa and Nozomi had all been developed by an engineer named Masafumi Kimura. Kimura died suddenly in June 2009 at the age of 49, before Akatsuki launched. Kimura's friends and family sent wishes to Venus on his behalf aboard the Akatsuki message plates; they count Akatsuki's safe arrival one of his successes. (Here is a Yahoo Japan article about Kimura.)

And, as always with Japanese missions, there was fan art aplenty. I'll single out two here:

改めて!おめでとうございます…!! 職場(※定時後)で思わず拍手してました。これからもご安全に…! #あかつき応援

— 小森雨太 (@comori_uta) December 9, 2015

今日はいよいよ「あかつき」の軌道投入の結果がわかる日ですね。成功のお祈りを込めて「あかつきさん塗り絵」を作りました。自由に塗ってね。 ダウンロードはこちら #あかつき応援

— モフ子 (@moffmiyazaki) December 8, 2015

(My daughters and I are still working on coloring Go Miyazaki's drawing.)

@moffmiyazaki my daughters had to go to bed but maybe they will finish in the morning

— Emily Lakdawalla (@elakdawalla) December 9, 2015

Last but not least: An Akatsuki song! (Lyrics posted here.) Thanks to Toshi Hasegawa for the tip.

Please accept marketing-cookies to watch this video.

Following is the transcript of the press briefing, concatenated from 5thstar's translations of Shinya Torishima's tweets. In most cases, it took 5thstar three English tweets to transcribe one of Torishima's Japanese tweets; I provide links to 5thstar's tweets after each statement.

Read more: mission status, Akatsuki (Planet-C), Venus

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
Read more articles by Emily Lakdawalla

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