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JAXA Explores the Planets of the Solar System The World's First Planetary Meteorological Satellite: Exploring the Mystery of the Wind on Venus Takeshi Imamura, Project Scientist, AKATSUKI (Venus Climate Orbiter)

Satellite Built to Observe Wind

Q. What is AKATSUKI going to be observing on Venus? And what are its new and unique features compared with previous Venus exploration missions?

Venus enveloped by clouds. The atmosphere circulates at high speeds in the direction of the arrows.
Venus enveloped by clouds. The atmosphere circulates at high speeds in the direction of the arrows.

AKATSUKI's observation plan in Venus orbit
AKATSUKI's observation plan in Venus orbit

AKATSUKI is the world's first planetary probe that deserves to be called a meteorological satellite. On Venus, a high-speed wind called super-rotation is blowing all over the planet, in the direction of planetary rotation, with a velocity reaching 400 km per hour at an altitude of around 60 km from the surface. This wind blows 60 times faster than the planet's rotation, which is very slow (one Venusian day takes 243 Earth days). AKATSUKI will investigate why this mysterious phenomenon occurs. Another objective is to study the formation of the thick sulfuric acid clouds that envelop Venus, and to detect lightning on the planet.
The unique feature of this mission is that it will map the movement of the Venusian atmosphere in three dimensions, by taking continuous images of a broad swath all at once, using different wavelengths ranging from infrared to ultraviolet. This way, it will be possible to observe various phenomena occurring simultaneously at different altitudes in the atmosphere, and to obtain three-dimensional information. There have been satellites built to take 3D dynamic images of Earth's atmosphere - such as the geostationary meteorological satellite HIMAWARI - but no such satellite has ever been sent to observe another planet.
AKATSUKI is equipped with five cameras. One of them, a near-infrared camera, will be able to peer through the thick clouds of sulfuric acid and observe the surface of Venus, which is normally completely obscured by these clouds. In addition to studying meteorological phenomena, we might be able to see whether Venus has any active volcanoes.
AKATSUKI will have an equatorial elliptical orbit, with an altitude ranging from about 300 km to 80,000 km. From close up, we will observe the atmosphere and the clouds, and from far away, the global atmospheric dynamics. Also, when AKATSUKI is hidden behind Venus, as viewed from the Earth, radio waves transmitted from the probe will reach Earth after passing through the Venus atmosphere. By analyzing these radio waves, we will be able to investigate the atmosphere's temperatures and the layered structure of trace gasses. AKATSUKI is scheduled to be launched in 2010, and to arrive at Venus six months later. The observation period is planned to last at least two years.

AKATSUKI Raises Global Expectations

Q. What do we know about Venus from previous and current missions?

A CG image of Venus, based on radar observation (Courtesy of NASA)
A CG image of Venus, based on radar observation (Courtesy of NASA)

Russia and the United States have studied Venus in the past, and a European spacecraft is in operation today. These studies have shown us, for example, that:

The Venusian atmosphere is mainly comprised of carbon dioxide, and is full of sulfuric acid clouds
Venus is a world of high heat and high pressure, with temperatures over 400°C and atmospheric pressure at the surface about 90 times greater than on Earth
There are no oceans like on Earth, but there are relatively new volcanic terrains
There is no magnetic field.

The high heat on Venus is a result of a greenhouse effect caused by carbon dioxide. This can be seen as the ultimate result of runaway global warming.
An interesting finding from a recent study is the detection in Venus's highland regions of a mineral with a spectral feature similar to granite, which usually forms in an environment where there is water. This discovery was made by a Japanese researcher, through analysis of observation data acquired on previous missions. This may suggest that there were once oceans on Venus like there are on Earth. If this is true, what did they look like and how did they disappear? We cannot answer these questions without understanding the mechanism of Venus's atmospheric circulation and the history of its variation. The meteorological research we are trying to conduct with AKATSUKI is an important opportunity to solve such puzzles, and the mission is highly anticipated by scientists around the world who study planetary atmosphere.

Q. Europe's Venus Express arrived at Venus in April 2006, and is still in operation. How is it different from AKATSUKI?

Venus orbiter Venus Express (Courtesy of ESA)
Venus orbiter Venus Express (Courtesy of ESA)

The two probes have the same objective - to explain the environment of Venus. However, while Venus Express primarily studies the chemical composition of the planet's atmosphere and surface, AKATSUKI is going to observe the fluid motion of the atmosphere. Our objective is to understand the meteorological phenomena that take place on Venus, and their impact on the distribution of clouds and temperatures.
In Japan, many scientists have long been interested in planetary meteorology. When the AKATSUKI mission was proposed in 2001, it inspired the rest of the world to focus on Venus as well, and Europe started its own mission. Venus Express and AKATSUKI are like sister satellites, and a very good cooperative relationship has been built as we have progressed in our missions. Venus Express has the same basic structure as Europe's Mars Express orbiter, which was launched in 2003, and for that reason it had a shorter development period and was launched before AKATSUKI. When AKATSUKI joins it in space, we are planning to conduct joint observations.

Studying Venus to Learn About Earth

Q. What is the significance of exploring Venus?

Venus and Earth have taken different paths
Venus and Earth have taken different paths

Venus is known as Earth's twin planet. In our solar system, it's the only planet that is similar to Earth in size and mass, and its birth is thought to have been similar in timing and shape. So what made Venus and Earth take different paths? How is what is happening on Venus today different from what is happening on Earth? Once we can explain the structure of Venus, we will be able to better understand Earth. For example, we may discover the reasons that only Earth has been able to sustain oceans, and why only Earth is abundant in life. We will also learn how changes in atmospheric mass and rotation velocity impact meteorological phenomena. I think this is the major reason so many planetary scientists are interested in Venus. Besides, Venus is a very mysterious planet. From the inexplicable wind and lightning, to the presence of oceans in its past and the current status of volcanic activity on Venus, there are many interesting questions waiting to be solved. As scientists, we have no choice but to pursue the answers.

Final Tests Before Launch

Q. What is the status of AKATSUKI today, just months before the scheduled launch? And what was the greatest development challenge you have experienced so far?

AKATSUKI in vibration test
AKATSUKI in vibration test

The spacecraft is almost complete, and is now at the final test stage. For example, we are now testing vibration resistance of the spacecraft during launch, and using a vacuum chamber to make sure the instruments can function properly in a space environment.
We have had many challenges in development. At first, in the planning stage, we had to spend a lot of time convincing people of the value of this unique idea - a planetary meteorological observation satellite. Once development started, we had to deal with various troubles, such as onboard instruments that did not function as designed. In addition, it is essential to keep the five onboard cameras on AKATSUKI at stable temperatures, but this is quite challenging: Venus is so close to the Sun that the spacecraft faces intense heat on the day side, but when it enters the shadow of Venus it is plunged into equally intense cold. We struggled with the designs for heat flow and power inside the spacecraft.

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