ESA says Rosetta in good shape after 31-month snooze
BY STEPHEN CLARK
Posted: January 29, 2014
A first look at the European Space Agency's Rosetta spacecraft after its reactivation last week shows the probe endured an unprecedented power-saving hibernation with few problems, giving engineers confidence the mission can continue the final leg of its decade-long pursuit of a little-known comet thought to harbor the building blocks of life.
The verdict: Rosetta is healthy and ready for its high-stakes approach to comet Churyumov-Gerasimenko. In August, the $1.7 billion mission will arrive in the icy world's neighborhood, becoming the first spacecraft to enter orbit around a comet.
In November, Rosetta will eject a small German-built lander to latch on to the comet to collect panoramic imagery and drill into its mysterious surface -- another first in space exploration.
Rosetta's only blemish while asleep was found in the spacecraft's software. The glitch caused the probe's on-board computer to reboot at the start of the activation sequence Jan. 20, giving tense ground controllers at the Rosetta operations center in Germany a few extra minutes to wait before receiving the first signal from the spacecraft.
"We indeed had some issues in the software during the hibernation mode," said Andrea Accomazzo, Rosetta's spacecraft operations manager. "We are analyzing them, [but] they do not affect at all the current and future spacecraft activities. We had never seen them before."
Accomazzo said engineers switched on Rosetta's X-band transmitter to hasten the probe's download of telemetry to the ground, giving the control team a more detailed look at the spacecraft's condition.
Rosetta's solar panels appeared healthy, generating plenty of electricity for the spacecraft's busy year ahead. Temperatures inside Rosetta's propellant tank were running a bit cooler than expected, but well within specifications, according to a post on ESA's Rosetta mission blog.
Engineers continued their methodical checkout of Rosetta over the weekend, spinning up three of the craft's four reaction wheels and transitioning responsibility for pointing and attitude control to the wheels from the probe's rocket thrusters, Accomazzo told Spaceflight Now.
Accomazzo said one of the three wheels now controlling Rosetta's orientation continued to show "non-nominal behavior" first observed before Rosetta entered hibernation in June 2011.
The other problematic reaction wheel will be held in reserve. Rosetta was designed to operate on three reaction wheels, but engineers devised new software to employ two wheels to keep the spacecraft pointed properly, ensuring its solar panels remain aimed at the sun, the antenna turned toward Earth and its science instruments trained on the comet once the probe arrives.
Paolo Ferri, ESA's head of mission operations, told reporters in December that two of Rosetta's reaction wheels showed "noise" before the craft went to sleep in 2011.
Ferri said engineers also found a way around a leak in Rosetta's helium pressurization system. The controllers will operate Rosetta's propulsion system at lower pressures than planned, sacrificing fuel efficiency to ensure the spacecraft does not run out of helium gas required to push propellant into Rosetta's engines.
Ground teams adjusted Rosetta's trajectory and tested the probe's thrusters at low pressure before it went into hibernation, and Ferri said the results were positive.
"We do not expect a problem," Ferri said. "We have enough fuel."
Over the next few weeks, engineers will configure Rosetta's on-board solid-state mass memory for the storage of science and operations data before downloading the telemetry to Earth, according to ESA.
Accomazzo said controllers will not know the status of Rosetta's 11 science instruments, which include a suite of cameras, spectrometers and dust monitors, until they are scheduled to be powered up in late March.
Rosetta's Philae lander, riding piggyback on the orbiter mothership, will also be switched on in late March to verify its condition after Rosetta's multi-year cruise.
Engineers expect Rosetta to be fully checked out in time for a critical engine burn in May to bend the probe's path toward Churyumov-Gerasimenko and set up for the craft's arrival in the comet's vicinity this summer.
When Rosetta enters orbit around the comet in August, it will begin more than a year of up-close observation as Churyumov-Gerasimenko speeds closer to the sun, activating plumes of icy vapors as solar heating builds up in 2015.
The primordial world was locked into the inner solar system by the immense gravity of Jupiter during a close encounter with the gas giant a half-century ago. With a nucleus measuring about two-and-a-half miles across, it originated in the farthest reaches of the solar system and, like all comets, was knocked toward the sun after billions of years in the frozen distant Oort cloud.
"[Comets] are time capsules," said Mark McCaughrean, senior scientific advisor in ESA's science and robotic exploration directorate. "They are remnants of the birth of the solar system. They go back to the beginning of the solar system more than 4.6 billion years ago."
Rosetta's mission follows a series of European and NASA robotic probes dispatched for fleeting flybys of comets. ESA's Giotto spacecraft flew past Halley's comet in 1986, and the U.S. space agency sent several missions to visit comets in the 2000s.
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