Akatsuki's new orbit, first images, and science plans
Posted by Emily Lakdawalla
09-12-2015 18:30 CST
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.
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.
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.
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.
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:
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:
(My daughters and I are still working on coloring Go Miyazaki's drawing.)
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.
- Project Manager Mr. Nakamura: Akatsuki is now in orbit circling Venus. The burn on Monday was a success. Akatsuki's status is healthy. We will release images taken by 3 cameras.It will take 3 months to test the instruments. Science data acquisition will start in April. (1, 2, 3)
- Hirose: The orbit insertion was on December 7. The orbit period is 13 days and 14 hours. Orbit inclination is 3 degrees. Due to perturbations from the Sun, orbit inclination it will change up to 25 degrees. Periapsis is about 400 kilometers, apoapsis is 440,000 kilometers.Akatsuki is operating nominally. (1, 2, 3)
- Assoc. Prof. Imamura is explaining the images taken by Akatsuki. No tweets, sorry. (1)
- Imamura: We consider Akatsuki to be a weather satellite (of Venus). It can continuously take images. We canobserve the 3D motion of Venus' atmosphere. (1, 2)
- Imamura: It is now approaching apoapsis (of the Venus). It will go into Venus' shadow. We will now start checkouts. We think we can begin acquiring scientific data in April. (1, 2)
- Nakamura: There may be some tough days to come, we really appreciate your support. (1)
- Q: To 3 of you. How did you react when you first saw the images? Nakamura: I saw the images this morning and thought "what the heck?". They are the first of their kind. And I thought "There's a hope. Nice!" (1, 2, 3)
- Imamura: After 5 years from the launch, I thought it was beautiful. It was really really up close to Venus.I was anxious as I calculated the attitude of the spacecraft when these images were taken.When I first saw it, I screamed and made high fives. After all 10+ years in preparation, I was so grateful, and firmly realized it actually arrived (at Venus), by seeing how Venus entirely occupies the field of view. (1, 2, 3, 4)
- Q: It almost seemed like a trick. What was the key to the success? Hirose: We prepared as many Plan-Bs as we could, in case of anomalies. We even prepared additional [contingency] burn VOI-R1cjust in case all the Plan-Bs failed. All members of the team concentrated so as not to miss any signs (of anomalies). (1, 2, 3)
- Nakamura: All 4 thrusters had to burn equally. Precise manufacturing of the spacecraft contributed to its success. (1)
- Hirose: It was built with the craftsmanship, and the team operated it in full consideration, with continuous attention. I think it helped the spacecraft in good condition and has contributed to the success. (1, 2)
- Q: To 3 of you. There may be many people behind the scenes. Who are they and how would you like to thank them?Nakamura: First of all, the person who passed away before this maneuver. He did the original calculation of theorbit before Hirose. He died before the launch. I think it was his will which made it a success. And those who gave us encouragement after the failure. We could not stand it without your support. Thank you. (1, 2, 3, 4)
- Hirose: To my family. And to the team. The objectives of the team are to observe and publish. They supported me.I am really grateful to them. (1, 2)
- Imamura: Not to mention family and the team, but all engineering team members. And so many who workedat the corporations. I can never thank you more. And to those space fans. "Grateful" is my only word. (1, 2)
- Nakamura: Also to MEXT and the Space Development Committee members who supported us. Some told ISAS's Prof. Inatanito "please save Akatsuki". That comment roused me. I really thank them. (1, 2)
- Hirose: My husband made me soy milk (tounyu), pun intended (for successful orbit insertion "tounyu"). Also to NASA's help. Without it we could never achieve it. I am really grateful. (1, 2)
- Q: Tell us more specifics about the checkups after the insertion maneuver. Hirose: We checked the telemetry first, it was fine, then we checked the (orbital) velocity of Akatsukiwhich confirmed it was in the right orbit. (1, 2, 3)
- Q: Where is it now? Hirose: It is 310,000 kilometers from Venus now. Inclination is 2.5 degrees. [Torishima notes he will post images later.] (1, 2)
- Q: Why is the apoapsis a bit lower than the original ["original" as of a week prior] plan? Hirose: Efficiency and capacity of the thrusters were better than expected, and that helped to achieve lower apoapsis. (1, 2)
- Q: Am I correct the current orbit is bigger than the plan of 5 years ago? Hirose: That is correct. The apoapsis is higher than the intended orbit. It will end up around 310,000 to 340,000 km. (1, 2)
- Q: To Prof. Imamura about the image. What are those fuzzy objects? Clouds? Imamura: Indeed they are clouds. We have LIR reading and temperatures. UVI captured not only the distribution ofclouds but also visualizes the upward diffusion of SO2 by atmosphere circulation. (1, 2, 3)
- Imamura: We are relieved to learn that the instruments are healthy and show no signs of degradation. (1)
- Q: We understand the history of probes is a succession of failures. What is the value of failures? Nakamura: Sometimes we only learn from failures. It help us get the feeling of where to exploit our imagination. Japan is only at square one of that process. (1, 2, 3)
- Q: If you can go back by 5 years and given a choice of projects which fails and which does not, which one do you pick?Nakamura: Of course the latter. (1, 2)
- Q: To Hirose: Orbital calculation played a big role. How much did you calculate? How many parameters, tough cases?Hirose: We worked as a team of four. Each calculated tens of thousands of cases. Parameters were the times and locations of orbit insertions. Venus is spherical. There are infinite combinations of where would the closest approach be in 3D, what time...four of us calculated. It was not painful. But we had to consider the duration of Venus' shadow.In shadow, solar panels do not generate power, and the probe has to live on batteries. It must be within 90 minutes. The duration of shadow varies with orbits. One revolution might be under 90 minutes, but next rev would be 200 minutes, so we could not end the analysis there.We could not stop the analysis for 2.5 years. So we continued, asleep or awake. It was more difficult than painful. (1, 2, 3, 4, 5, 6, 7, 8)
- Q: Were there another chances of orbital insertion? Hirose: The current orbit was the only chance to achieve an apoapsis lower than 310,000 km. It we tried it earlier, it would crash into Venus. If we tried it later, the apoapsis would move higher. (1, 2, 3)
- Q: You cannot lower the orbit because of the remaining fuel? Hirose: We could have achieved an apogee of 80,000 km if the main engine was not broken. With RCS engines, 310,000 kilometers is the limit, considering the remaining fuel. (1, 2, 3)
- Q: Could you elaborate a bit more about "at the square one" of planetary exploration? Nakamura: We are at the beginning phase of our planetary exploration. We can now distribute the exploration datato the world. So far Japan could not do that. It is a big step forward now we can do that. (1, 2, 3)
- Q: How Japan should move ahead in planetary exploration in your opinion? Nakamura: Prof. Kawaguchi used to say "It's important to be the first". [See, for instance, this article.] I think we should do even if someone has done it before. There should be something we learn from, only if we do it by ourselves. (1, 2, 3)
- Q: Could you elaborate a bit more about the protection of the cameras? Nakamura: The cameras were carefully kept in relatively benign conditions. They were shielded from direct sunlight, which therefore stressed the other instruments. The cameras were saved by them.(explanation with figures and models) (1, 2, 3, 4)
- Q: About Japanese technologies on board. How did they contribute to industry? Nakamura: Cameras are all Japanese. Batteries, power supply, solar panels, ceramic thrusters are also. The spacecraft as a whole was also assembled in Japan. Some are imported, but Japanese companies made it. That's important. I'm not sure if there were direct impacts to Japanese industry, but for example, how to make solar panels work more efficiently, is just like participating an F1 Grand Prix, and we can learn from the process. And that process will educate the engineers who participated. That will contribute. (1, 2, 3, 4, 5, 6)
- Q: To Imamura, what kind of observations would you like to do? Imamura: Planets change on many different time scales. We wish to take data longer than 2 or 3 years if possible, but two points stand out in a given time frame. One is to take homogeneous and continuous data, which helps us understand any hidden climate changes. We would also like to focus on some specific phenomena as a campaign. There are some interesting thing going on at the surface directly facing the Sun. We'd understand how that will affect the atmospheric circulation, by [looking at it from] changing camera angles. For a thunderstorm, we'd employ other cameras together with the Lightning and Airglow Camera. We will plan it by considering the relative positions of the Earth, Sun and Venus. (1, 2, 3, 4, 5, 6, 7, 8)
- Q: There has been Magellan, Venus Express, and now Akatsuki. Do you expect a growing interest in Venus exploration? Nakamura: Yes, we expect a lot. Past explorations observed what is there, on the surface, or in the atmosphere. Akatsuki's main interest is change over time. Planetary scientists around the world will discover how interesting Akatsuki's data could be. I hope it will also deepen understanding of the environment of Earth, by comparing the differences among planetary atmospheres. (1, 2, 3, 4, 5)
- Q: Is it the world first attempt to enter the planetary orbit by using RCS thrusters only? Nakamura: Yes, it is. NASA was groaning about this plan, but I did not want it either. Those RCSs were tested on the ground for much longer period of burn, but never before under Akatsuki's situation. (1, 2, 3)
- Q: To 3 of you. You say it's tough, what are the specific issues and challenges? Nakamura: There were some instruments not being used in the past 5 years, such as high gain antenna. It's been pointed towards the Sun. We need to check whether it's healthy. Radio transmitter has been suspended as well. We hope it works. There will be some orbital changes, and we hope to succeed as well. We really hope to take as much data as possible. (1, 2, 3, 4, 5) Hirose: Surface characteristics of the probe might have changed. Different surfaces are now subject to the Sun light in the orbit. That may be an issue. We also need to save fuel consumptions. That's another issue. (1, 2) Imamura: We'd like to take as much data as possible. It would be difficult if the lifetime is too short. The observation period has to be long, and should work continuously. How long would be the scientific issue. (1, 2)
- Q: What are the impacts to the observation with this orbit? Imamura: We already knew we would be in this orbit 5 years ago. So we switched our mindsets and prepared for it. We considered the plan under this condition. The original period was 30 hours. High-res images could have been taken. With the current orbit, the resolution would be lower, but we can take homogeneous and continuous data [over time]. And that's a bright side. Better data will be taken in terms of time evolution. We'd like to combine this with the high res images near the perigee. (1, 2, 3, 4, 5, 6)
- Q: To Imamura, how did you keep your motivation during the past 5 years? Imamura: Two things. One is we are given 5 years. With that 5 years, we were not doing nothing, but rather participating in other missions, learning techniques. You'd say, "sharpening the fangs". Also we've been watching the probe for these 5 years, and it was indeed hard to keep the focus without data, but in the end our efforts have been rewarded with this successful insertion into orbit. Nakamura: In addition, I'd say you are the leader and everyone trusted you, and followed you, and that was the key. (1, 2, 3, 4, 5, 6)
- Q: What are the impacts to the observation of [Venus' atmospheric] superrotation with this orbit that is more elliptical than planned? Imamura: Most important point is to take continuous data. It can be achieved with this orbit. Our instruments are some of the best among other probes. We know from our simulated clouds that we can deduce the wind vector and distribution. We don't know yet how precisely we can observe with the other wave lengths. It will be a long challenge and I hope you understand. (1, 2, 3, 4, 5)