October 22, 2016

Japanese probe fires rockets to steer into orbit at Venus

Updated at 0315 GMT (10:15 p.m. EST)

Artist's concept of the Akatsuki spacecraft.
Artist’s concept of the Akatsuki spacecraft.

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.

“As a result of analyzing data transmitted from the orbiter, we confirmed that the thrust emission of the attitude control engine was conducted for about 20 minutes as scheduled,” the Japan Aerospace Exploration Agency said in a press release Monday.

“The orbiter is now in good health,” JAXA said. “We are currently measuring and calculating its orbit after the operation. It will take a few days to estimate the orbit, thus we will announce the operation result once it is determined.”

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.

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