"Putting on weight at Christmas?"

Checking Mars Express is not too heavy
8 January 2003

Mars Express launch diary 3

Don McCoy, Mars Express Assembly Integration and Verification Engineer, reports.

Once the Mars Express spacecraft is in orbit around Mars, everyone will want it to perform the scientific investigations. Scientists will want to know exactly where their instruments are looking on Mars for them to map the subsurface, surface, atmosphere, and particle environment around the planet.

In the case of the stereo camera (whose resolutions can achieve 2-3 metres), they will want to know very precisely where the spacecraft is looking during its frequent close approaches to the planet as it follows its eccentric orbit.

"Can we be sure the spacecraft will be up to the task? Yes! Mars Express has undergone a series of alignment checks for instruments and other critical equipment on board, such as the attitude-control thrusters and the main engine. The main engine, for example, is vital to place the spacecraft in the capture orbit around Mars.

"As for measurements, we use the same well-established principles of optical alignment used daily on Earth for measuring things from highway construction to defining property boundaries. For example, note the familiar tripod and theodolyte that you see at major construction sites. We did these measurements prior to the mechanical tests of vibration and acoustic excitation and then did them again to determine the spacecraft's stability during simulated vibrations of a launch.

Testing instruments alignments

"After the testing proved successful, we were 'rewarded' with a Christmas present in a way. We carried out the final mechanical test (the all-important mass measurement). A rocket can only lift so much mass from the surface of Earth. Our limit to accomplish the mission was 1200 kilograms, including our adaptor to the Soyuz-Fregat rocket.

"Obviously you make every effort to account for each gram that makes up a satellite, but there is always some doubt until everything is built and integrated on the spacecraft. We had been worried about this since the beginning of the project because it directly impacts our mission. Finally, just before Christmas, the measurements of the spacecraft became available. Following first indications, it appears that we will meet our mass requirements. Being the right weight is very good news and it certainly filled us with Christmas cheer!"

"Last month we visited the launch site in Kazakhstan to see how preparations were proceeding to receive the Mars Express spacecraft in February. We left the site confident that all will be ready on time. It also proved to be a test of the team's capability to survive the weather. It was cold, cold, cold in Kazakhstan (about -20°C). However, we will remember the warm welcome we had there.

What comes next? We go on with tests to ensure Mars Express will not be affected by the electrical environment in which it must operate. We are currently testing the spacecraft to ensure the electromagnetic compatibility of its equipment and instruments. We'll round things off with an extensive set of tests to verify the full operational capabilities of the spacecraft. These will include the overall system operations with the ground controllers in the European Space Operations Centre (ESOC) in Darmstadt, Germany."

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