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Targetting Giotto
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Targetting Giotto

October 15, 2009
This is slide 19 of 20 from the first Giotto slide set. The following caption is the one originally provided with the slide set.

About 2 days before the encounter a final decision will have to be made where to target the Giotto spacecraft. Unfortunately, the position of the comet nucleus can only be estimated because the nucleus is disguised by the coma dust and gas. Based on observations from the Earth the nucleus position is known with an accuracy of 500 km (1Ã). If the spacecraft is targeted at 500 km the actual flyby distance could be anywhere between 0 and 1000 km or more. This is not a very satisfactory situation.

The two Soviet Vega spacecraft (only one shown) encounter Halley's Comet 7 and 4 days before Giotto and their onboard cameras will observe the nucleus from a distance of 10 000 km. The Soviets have agreed to provide ESA with the nucleus position as derived from their camera images to enable Giotto to be targeted more precisely. NASA supports this so- called 'Pathfinder Concept' by tracking the Vega spacecraft using precise L-band Very Long Baseline Interferometry (VLBI) navigation techniques. This reduces the Vega orbit uncertainty from a few hundred to about 40 km. Thanks to the Three-Agency Pathfinder-Agreement the Giotto targeting uncertainty is reduced from 500 to 130 km (1σ).


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