[meteorite-list] Japaneses Probe Fires Rockets to Steer Into Orbit At Venus (Akatsuki)

[Ron Baalke]

http://spaceflightnow.com/2015/12/06/japanese-space-probe-to-steer-into-orbit-around-venus/

Japanese probe fires rockets to steer into orbit at Venus
by Stephen Clark
Spaceflight Now
December 6, 2015

Five years after missing a shot to enter orbit at Venus, Japan's Akatsuki 
spacecraft completed a critical rocket burn late Sunday in a bid to salvage 
the research mission and become the only space probe operating around 
Earth's nearest planetary neighbor.

Four maneuvering thrusters were scheduled to ignite at 2351 GMT (6:51 
p.m. EST) Sunday for approximately 20 minutes and 30 seconds to slow down 
the Akatsuki probe enough for Venus' gravity to capture it into an elongated, 
high-altitude orbit.

Akatsuki was never designed to fire its secondary attitude control rocket 
jets for such a long time, but the thrusters were required to steer the 
craft into orbit after its main engine failed during the mission's first 
encounter with Venus exactly five years ago.

Officials confirmed the burn went as planned early Monday.

"It is in orbit!" wrote Sanjay Limaye, a planetary scientist based 
at the University of Wisconsin in Madison, in an email to Spaceflight 
Now.

"They were cautiously optimistic before the burn, but confident. Now 
smiling!" reported Limaye from Akatsuki's mission control center in 
Sagamihara, Japan. He is is a NASA-sponsored participating scientist on 
the Akatsuki mission.

It could take a few days to precisely measure Akatsuki's trajectory 
to verify it is in the proper orbit around Venus, officials said.

The Japan Aerospace Exploration Agency plans a press conference around 
0300 GMT (10 p.m. EST) to update the status of the mission.

The reaction control thrusters, originally designed to help point the 
spacecraft, were not rated for such a hefty propulsive maneuver.

Venus was 149.5 million kilometers, or nearly 93 million miles, from Earth 
at the time of Akatsuki's arrival Sunday. It took radio signals more 
than 8 minutes to travel at the speed of light between the two planets.

The spacecraft's guidance system targeted an orbit with a high point 
stretching up to 475,000 kilometers (295,000 miles) from Venus, farther 
than the distance of the moon from Earth, according to Takeshi Imamura, 
Akatsuki's project scientist at JAXA's Institute of Space and Astronautical 
Science.

The smaller thrusters aboard Akatsuki generate just 5 pounds of thrust, 
a fraction of the power provided by the probe's main engine. Even with 
four of the rocket jets operating - there are two sets of four pointing 
forward and aft from Akatsuki's main body - the secondary thrusters 
did not have the energy to put the spacecraft into its originally planned 
orbit.

At the low end of its looping path around Venus, Akatsuki should pass 
as close as 1,000 kilometers (621 miles) from the planet's scorched 
surface hidden beneath a blanket of thick clouds driven around Venus, 
Imamura told Spaceflight Now in an email. The pull of the sun's gravity, 
which is stronger at Venus that at Earth, will gradually perturb Akatsuki's 
orbit.

A follow-up rocket burn tentatively slated for March 26 will tweak Akatsuki's 
trajectory around Venus, lowering the peak altitude of its orbit to about 
330,000 kilometers (205,000 miles).

Instead of taking 30 hours to complete a lap around Venus under Akatsuki's 
original flight plan, the probe was expected to take 15 days to orbit 
the planet after arriving Monday. That will be changed to a nine-day orbit 
with the March adjustment.

Assuming all the maneuvers go well, then Akatsuki's science mission 
will begin.

"We expect two Earth years or more, but the estimate of the remaining 
fuel has a large uncertainty," Imamura wrote in response to questions 
from Spaceflight Now. "We cannot present a precise estimate."

Imamura told a meeting of Venus scientists in October that the plan to 
drive into orbit using Akatsuki's reaction control thrusters was risky, 
but ground controllers tested the rocket jets in a 10-minute firing  
half the duration of the orbit insertion maneuver - giving officials 
some confidence going into the make-or-break burn.

Engineers programmed Akatsuki's software to flip the spacecraft around 
and fire a separate set of four thrusters if the primary rocket jets run 
into trouble during the 20-minute burn.

Akatsuki's main engine, designed for 112 pounds of thrust, was unavailable 
after a failed burn during the mission's first encounter with Venus 
five years ago. The engine cut off less than three minutes into a 12 minute 
burn, providing an insufficient impulse for the craft to be captured in 
orbit.

Engineers believe a salt formation in a check valve inside the spacecraft's 
propulsion system restricted the flow of fuel to the main engine, starving 
it of fuel and creating an oxidizer-rich combustion condition, raising 
temperatures inside the engine before it failed.

The probe continued on in an orbit around the sun following the failed 
insertion maneuver in 2010, and ground controllers searched for a new 
way to steer Akatsuki into Venus' orbit during its next flyby of the 
planet in 2015.

Controllers commanded Akatsuki to dump extra propellant to lighten the 
mass of the spacecraft, allowing its thrusters greater control of the 
probe.

Akatsuki is Japan's first mission to Venus, and it blasted off aboard 
a Japanese H-2A rocket in May 2010 for a planned six-month cruise.

It cost 24.4 billion yen, or about $200 million at today's currency 
exchange rates, to build and launch the Akatsuki mission, which carries 
a suite of five cameras to observe Venus' atmosphere.

Engineers instructed Akatsuki to turn its cameras toward Venus immediately 
after the insertion burn in a bid to collect "contingency" imagery 
of the planet in case the maneuver did not work.

Assuming the arrival was a total success, Akatsuki is now the only spacecraft 
currently operating at Venus. The European Space Agency's Venus Express 
probe ended its mission there in late 2014.

"The orbit around Venus in the new plan will be a very long elliptical 
one," Imamura said before Akatsuki approached Venus. "From far distances, 
we continually monitor the global-scale dynamics of the atmosphere and 
clouds, and of course, from close distances, we take close-up images of 
the atmosphere, the surface, and we also observe lightning and airglow 
when the spacecraft is in the shadow of Venus."

Also named the Venus Climate Orbiter, Akatsuki is primarily designed to 
study the Venusian atmosphere.

The mission will observe climate and weather conditions on Venus with 
a suite of five cameras to look at 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 through Venus' global clouds, but scientists 
say an infrared imager aboard Akatsuki can resolve the surface.

A long-wavelength infrared camera and an ultraviolet instrument aboard 
Akatsuki will study the super-rotating cloud structures in the upper atmosphere. 
The ultraviolet camera will also track sulfur dioxide, a precursor to 
cloud formation at Venus.

Researchers also plan to measure radio waves transmitted through the planet's 
atmosphere to measure its profile.

Imamura said engineers have uploaded new software to Akatsuki to better 
see Venus from the spacecraft's higher-than-planned orbit, reducing 
the data volume coming back to Earth to streamline the mission's scientific 
return.

"By combining this information, we can model the three-dimensional structure 
of the atmosphere and the clouds," Imamura said.