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Spektr-R makes critical orbit correction
Russia's most important and prolific scientific satellite has changed its flight path on July 16, 2017, to continue its invaluable mission into 2018 and beyond.
The Spektr-R control room at NPO Lavochkin circa 2016.
By January 2017, more than 100 requests from different countries for observation time on the Spektr-R orbital radio telescope had been received for the 5th round of the so-called open scientific program which was to begin on July 1, 2017. According to Yuri Kovalev, the head of Spektr-R's scientific program, demand for the observatory is increasing as more scientists are realizing its capabilities. In the meantime, the spacecraft needs a crucial orbit correction.
Since its last major orbit correction in March 2012, Spektr-R has been orbiting the Earth in very dynamic but safe orbit. Original calculations indicated that its solar panels would be receiving enough sunlight until the onset of prolonged eclipses of the Sun by the Earth on Jan. 21, 2017. The spacecraft was also originally projected to be safe from falling back to Earth until July 18, 2021.
However by the middle of 2016, the Spektr-R descended to within 7,030-7,230 kilometers from the center of the Earth, or close enough for our planet's gravitational influence to begin disturbing the satellite's orbit with effects that could not be precisely anticipated back in 2012.
The new trajectory measurements made in the second half of 2016 showed that Spektr-R would plunge back to Earth on May 21, 2018, though its unacceptable lighting situation was now pushed back from January 2017 to January 2018. Therefore, urgent correction in 2016 was no longer deemed necessary.
However after Jan. 6, 2018, the eclipses onboard the spacecraft would begin exceeding the two-hour limit for the observatory's solar panels not being exposed to the Sun. In fact, the orbital night would last up to 6.4 hours, including complete darkness for no less than 4.5 hours.
A ballistic experts team led by Grigory Zaslavsky, a veteran of the Soviet space program at the Keldysh Applied Mathematics Institute, IMP, then planned four opportunities for a maneuver from May to August 2017 which would put the Spektr-R back on a safe path. All engine firings were planned to take place within range of two Russian deep-space tracking stations in Medvezhi Ozera near Moscow and Ussuriisk in the Russian Far East. Each firing window was spaced to be at least two orbits or more apart from the next opportunity to give ground control enough time to deal with unforeseen problems.
The main planning of the orbit correction was completed in the first half of November 2016, so that the new prognosis for its trajectory would be available by the start of bids for the 5th round of scientific observations in December of the same year.
According to the final plan, on July 16, 2017, Spektr-R was scheduled to conduct an engine firing which will put it on a safe path. Should the first attempt fail, mission control would have another opportunity around Aug. 5, 2017.
If maneuvers go as planned, the Spektr-R will be guaranteed a reliable power input until late 2020.
Cis-lunar alternatives considered
Behind the scene, the decision to conduct the 2017 orbit correction came after a long debate of possible scenarios to preserve the satellite and support its scientific observations.
Flight planners also considered a much more radical change of orbit, taking advantage of Spektr-R's first close encounters with the Moon beginning in March 2017.
No less than five flybys of the Moon would take place by April 2018, including closest encounters in January and February 2018.
Using gravity assisted maneuver near the Moon, the spacecraft could fly as far as 500,000 or even 700,000 kilometers away from Earth crossing the orbit of the Moon. The new orbit could dramatically extend the base of interferometry with ground based telescopes and thus increase the sensitivity of observations to another record-breaking level.
Scientists considered keeping Spektr-R in this new orbit from several months but no more than a year, before bringing it back to its routine location in 2018, again using gravity-assisted maneuvers near the Moon.
Unfortunately, a more detailed analysis of the risky operation showed that with the available margin of error, the spacecraft could end up too close to the Moon and, as a result, crash into the lunar surface.
As of the beginning of 2017, Spektr-R's Navigator service module still had between 80 and 85 percent of original propellant cache. Some of the considered orbital maneuvers would spend more than 50 percent of available propellant, but the ultimately approved correction requires only 5 or 1o percent.
In preparation for the orbit correction, Spektr-R conducted a test maneuver on June 16, 2017, to ensure that its main engines were functioning properly and that there was no need to employ backup engines, Roskosmos State Corporation said. The trial firing, which had no significant effect on the observatory's orbit, was two orders of magnitude shorter than the planned primary maneuver, according to the State Corporation.
The main maneuver lasting 290.3 seconds began on July 16, 2017, at 9:00 Moscow Time (2 a.m. EDT) and was conducted successfully, Roskosmos announced on the same day.
The Spektr-R radio observatory. Credit: NPO Lavochkin