Posted on 11/07/2017 by Marcello
Sitting on the dish
It was in March 1986 when I had the chance to witness the encounter of ESA’s “Giotto” probe with Halley’s comet during a mission to the Parkes Observatory, located in New South Wales, Australia, also informally known as “The Dish”. It was a young team of engineers, led by Rod Deeming and Boris Smeds, that had been sent to the place of (at that time) one of the largest radio telescopes in the world. Its impressive 64-metre movable dish, built in 1961, had already successfully monitored the Apollo 11 moon landing in 1969. The Parkes Observatory was selected to be the main part of ESA’s groundstation network, responsible for the reception of Giotto’s faint downlink signals as the probe approached and finally encountered the comet.
The local community were very excited about the event “their” dish was supporting, and the local newspapers were full of articles explaining every detail of the mission.
Our team was responsible for the reception, decoding, and recording of the telemetry as well as its transmission via leased lines to the control centre at ESOC. My personal task was the reception of telemetry messages from the PCM Preprocessor, their recording on big magnetic tapes, and their forwarding to the comms equipment. We were all very excited as the date of the encounter approached. As we had finished our preparatory installations well in time, there was not always work waiting to be done, so we also enjoyed some leisure time – mainly outside spacecraft passes – and in the evenings we were sometimes having good fun at the observatory quarters. We even made local friends in the vicinity, and I remember a big party with a spit-roasted suckling pig on a grill, where at some advanced hour Boris tought us the Swedish frogs dance (I *must* find a picture of that unforgotten event!).
One day before the encounter we had the great opportunity to take pictures of Halley’s comet with our cameras mounted on tripods standing on the big dish while it was tracking the spacecraft. We could keep the camera lens open for a few minutes, thus getting an undistorted picture of the comet with its tail.
Finally, when the day of the encounter had arrived, everybody was excitedly waiting for the predicted moment where Giotto would either hit the comet or pass by at a small distance. We were all assembled in the main antenna control room and were eagerly staring at the pen recorder showing the satellite’s downlink signal. Exactly at countdown time zero the signal dropped out, and everybody was sure the probe had crashed on the comet’s surface. But a few seconds later, like a phoenix, the signal recovered, showing however a strange pattern of several overlaying sine waves. It became clear that the collision had caused Giotto to tumble and spin around in a way that its antenna was no longer pointing towards the earth. This situation persisted during several minutes, with the signal becoming stronger and stronger, so that there was even hope that Giotto could have survived. But at 00:10:57 UTC on March 14 1986, seven minutes after the encounter, the signal finally dropped completely, and the Giotto mission was over.
Four years later, in 1992, the probe could successfully be reactivated during an earth fly-by, and sent to another comet, Grigg-Skjellerup. Its final decommissioning took place in 1992.
This mission has doubtlessly been one of the greatest and most exciting adventures of my life.