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Russia Mission Control adjusts ISS orbit by almost 2 km

December 01, 2011, 13:51 UTC+3
MCC conducted a test operation to adjust the orbit of the ISS
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MOSCOW, December 1 (Itar-Tass) — The Russian Mission Control Centre (MCC) on Thursday conducted a test operation to adjust the orbit of the International Space Station (ISS) aimed at creating favourable conditions for docking with the next manned Soyuz spacecraft.

The MCC told Itar-Tass that the “orbit correction passed nominally.” The manoeuvre was performed in automatic mode with the help of two powerful vernier engines of the Zvezda service module. For the first time accelerometers (meters of linear acceleration) installed on the American segment of the station were used in the operation, the MCC said.

The engines ignited at 03:11, Moscow time and worked for 63 seconds. During this time, the average altitude of the ISS orbit, according to expert estimates, has increased by about 1.8 kilometres and reached some 392.2 kilometres. The average altitude is a virtual value used by ballisticians. It is an imaginary circular orbit with the period of revolution around the Earth equal to the station’s real elliptical orbital period.

The MCC said that now, after US space shuttle flight program’s termination, the ISS orbit can be raised to nearly 400 kilometres (earlier, its height at the request of the US side was maintained at a level no higher than 360 kilometres) that gives considerable fuel saving.

The ISS Expedition 30 crew of three people, who since November 16 are on the orbital mission at the station, had been warned in advance of the manoeuvre conducted on command from the Earth. As astronauts’ participation in orbit correction operations is not required, they were sleeping at the time (they were to rise at 10 a.m. Thursday).

The ISS orbit adjustment manoeuvres are usually carried out in order to bring the station to the desired orbit for docking with cargo or manned spacecraft, to create conditions for a successful landing, as well as to avoid collision with space debris - small meteorites, fragments of old satellites and ships. For example, it so happened less than two months ago, on September 29, when the station had to be quickly moved from the dangerous proximity to a fragment of a carrier rocket.

The Thursday orbit adjustment is the second of three planned manoeuvres aimed at creating the conditions for the landing of the ISS crew in a favourable area, as well as to prepare the station for docking with the next Soyuz craft that in late December will take into orbit a new ISS expedition. The first operation to raise the orbit was held on November 18, before landing of the Soyuz TMA-02M spaceship, which returned to Earth three astronauts from the ISS Expedition 28/29 crew. Another similar manoeuvre is scheduled for December 7.

At the low altitudes at which the ISS orbits there is a variety of space debris, consisting of many different objects including entire spent rocket stages, dead satellites, explosion fragments—including materials from anti-satellite weapon tests, paint flakes, slag from solid rocket motors, coolant released by RORSAT nuclear powered satellites and some of the 750,000,000 small needles from the American military Project West Ford. These objects, in addition to natural micrometeoroids, are a significant threat. Large objects can destroy the station, but are less of a threat as their orbits can be predicted. Objects too small to be detected by optical and radar instruments, from approximately 1cm down to microscopic size, number in the trillions. Despite their small size, some of these objects are a still a threat because of their kinetic energy and direction in relation to the station. Spacesuits of spacewalking crew could puncture, causing exposure to vacuum.

Space debris objects are tracked remotely from the ground, and the station crew can be notified. This allows for a Debris Avoidance Manoeuvre (DAM) to be conducted, which uses thrusters on the Russian Orbital Segment to alter the station’s orbital altitude, avoiding the debris. DAMs are not uncommon, taking place if computational models show the debris will approach within a certain threat distance. Eight DAMs had been performed prior to March 2009, the first seven between October 1999 and May 2003. Usually the orbit is raised by one or two kilometres by means of an increase in orbital velocity of the order of 1 m/s. Unusually there was a lowering of 1.7 km on 27 August 2008, the first such lowering for 8 years. There were two DAMs in 2009, on 22 March and 17 July. If a threat from orbital debris is identified too late for a DAM to be safely conducted, the station crew close all the hatches aboard the station and retreat into their Soyuz spacecraft, so that they would be able to evacuate in the event it was damaged by the debris. This partial station evacuation has occurred twice, on 13 March 2009 and 28 June 2011. Ballistic panels, also called micrometeorite shielding, is incorporated into the station to protect pressurized sections and critical systems. The type and thickness of these panels varies depending upon their predicted exposure to damage.

The orbital outpost descends several dozen metres daily.

The launch of the next Soyuz spacecraft the ISS Expedition 30/31 crew is scheduled for December 21, and its docking with the ISS - for December 23.

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