02 Nov 2001

"The machine the spacecraft is sitting on moves up and down like a po-go," explains Don McCoy, assembly, integration and verification manager from ESTEC, who accompanied the spacecraft during recent tests at Intespace, the company responsible for testing the Mars Express spacecraft, in Toulouse, France. The tests were designed to check that Mars Express will be able to withstand the incredible noise and vibration generated during launch.

The po-going machine starts off at very low frequency, six cycles per second or 6 Hz, and gradually increases to 100 Hz. At first, the frequency is too low to be heard, but it quickly increases until the spacecraft begins to resonate, and make a lot of noise, at about 45 Hz. As the frequency increases further, the spacecraft begins to vibrate so rapidly that it appears stationary. At 73 Hz there's a second resonance and the noise builds to a crescendo. Then from 75-100 Hz it quietens down. In the background, throughout the piece, is the rhythmical swish of a valve opening and closing periodically, like the brush of cymbals keeping the beat.

During resonances, the acceleration of the spacecraft walls is considerable. "A fly on a panel of the spacecraft would find it hard to keep its footing. An acceleration many times that of gravity would be pushing it off," says McCoy.

The Mars Express spacecraft has now completed its phase of mechanical testing at Intespace and has returned to Alenia in Torino, Italy, for integration of the flight models of the instruments and the spacecraft electrical equipment.