Dawn launch sends Japanese orbiter on the way to Venus
BY STEPHEN CLARK
Posted: May 20, 2010
An H-2A rocket successfully launched from an island spaceport in southern Japan Thursday, beginning a $275 million mission to explore the hellish atmosphere of Venus and set sail on the power of sunlight.
Liftoff occurred at 2158:22 GMT (5:58:22 p.m. EDT), the precise instant when Earth's rotation aligned the Tanegashima launch pad with the trajectory toward Venus.
The rocket disappeared into mostly cloudy skies moments later, piercing the picturesque pinkish haze of dawn in southern Japan, where it was just after sunrise.
A few minutes later, the rocket jettisoned its twin boosters and the nose cone enshrouding the Akatsuki orbiter, Ikaros solar sail and an array of other small satellites built by Japanese university students.
The H-2A's second stage engine fired twice to place the payloads on a high-energy trajectory to escape the bonds of Earth and enter interplanetary space. The launch was the first time the H-2A rocket has dispatched payloads beyond the moon.
The 1,100-pound Akatsuki probe was deployed at T+plus 27 minutes, 27 seconds. The Ikaros solar sail separated a few minutes later, after the rocket cut loose an attach fitting to expose the secondary payload.
A NASA tracking site in Goldstone, Calif., received the first signals from Akatsuki around 0100 GMT Friday (9 p.m. EDT Thursday), confirming the probe was healthy following launch.
Thick clouds blanketing Tanegashima prevented the H-2A rocket from launching on its first try Monday night. Japanese officials retargeted launch for Thursday to give time for the weather to improve and for engineers to prepare the rocket for another attempt.
The conditions improved enough by Thursday to permit the launch.
After sojourning through the inner solar system for more than six months, Akatsuki will ignite its main engine again Dec. 7 to guide the probe into an elongated orbit around Venus. The car-sized spacecraft's operational orbit will take it from just over the Venusian cloud tops to a maximum altitude of nearly 50,000 miles during its two-year mission.
The orbit will offer Akatsuki's five cameras unparalleled views of the sweltering Venusian atmosphere, which is composed of thick carbon dioxide, sulfuric acid clouds and super-rotating jet stream winds reaching 225 mph.
Venus has a runaway greenhouse effect that drives surface temperatures to nearly 900 degrees Fahrenheit, hot enough to melt lead. The atmospheric pressure at the surface is about 90 times greater than at sea level on Earth.
"Although Venus is believed to have formed under similar conditions to Earth, it is a completely different world from our planet, with extremely high temperatures due to the greenhouse effect of carbon dioxide and a super-rotating atmosphere blanketed by thick clouds of sulfuric acid," said Takeshi Imamura, Akatsuki's project scientist.
Akatsuki, which is also called Planet-C and Venus Climate Orbiter, is the first interplanetary weather satellite, according to Japanese space officials.
Each of the probe's cameras is designed to study a slice of the Venusian atmosphere, ranging from surface imagery to observations of the planet's sulfur cloud tops at an altitude of 60 miles.
Two short-wavelength infrared imagers will observe low-altitude cloud patterns, chart the distribution of water vapor and carbon monoxide, and map the surface of Venus with a goal of finding active volcanoes. Thick clouds prevent visible cameras from seeing the surface.
Data from Europe's Venus Express orbiter recently showed evidence of fresh lava flows around three mountains, giving scientists new hope of catching a volcano in the act.
A long-wavelength infrared camera and an ultraviolet instrument will look at the super-rotating cloud structures in the upper atmosphere. The ultraviolet camera will also track sulfur dioxide, a precursor to cloud formation at Venus.
"On most planets like Earth and Mars, the atmosphere usually rotates very slowly as compared to the planetary rotation," said Masato Nakamura, Akatsuki's project manager. "On Venus, however, the atmosphere rotates at a very fast rate approximately 60 times that of the planet's rotation. In fluid mechanics, this very unusual behavior is called super rotation."
Another camera will snap frames 30,000 times per second with a wide field-of-view to hunt for lightning at Venus.
Results from Akatsuki will give scientists more tools to better understand how Earth and Venus differ.
Scientists say Venus formed much like Earth and probably enjoyed calmer times in its ancient past. But something went wrong long ago, leading a potentially once-temperate Venus on an evolutionary course much different than Earth.
Before researchers can pinpoint what caused Venus to become hellishly inhospitable, they must dispatch robotic missions to unravel the planet's conditions and how they link together.
"Using [Akatsuki] to investigate the atmosphere of Venus and comparing it with that of Earth, we hope to learn more about the factors determining planetary environments," Imamura said. "From this viewpoint, we will be able to understand more about the reason why Earth is as it is now, and how it might change in the future."
If the sail successfully deploys, scientists will monitor the probe's trajectory to measure the small acceleration caused by propulsive force from photons, particles of sunlight that exert tiny amounts of pressure on the ultra-thin membrane.
Officials hope Ikaros will prove the solar sailing concept, which would provide a renewable resource for future deep space propulsion.
Read more about the Ikaros mission here.