[meteorite-list] 20 Years Ago: Giotto's Comet Halley Encounter

[Ron Baalke]

http://www.esa.int/esaCP/SEMSZ0NVGJE_index_0.html

Giotto's brief encounter
European Space Agency
10 March 2006

Twenty years ago, in the night between 13 and 14 March 1986, ESA's Giotto 
spacecraft encountered Comet Halley. It was ESA's first deep space 
mission, and part of an ambitious international effort to solve the 
riddles surrounding this mysterious object.

The adventure began when Giotto was launched by an Ariane 1 rocket (flight 
V14) on 2 July 1985. After three revolutions around the Earth, the 
on-board motor was fired to inject it into an interplanetary orbit.

After a cruise of eight months and almost 150 million kilometres, the 
spacecraft's instruments first detected hydrogen ions from Halley at a 
distance of 7.8 million kilometres from the comet on 12 March 1986.

Giotto encountered Comet Halley about one day later, when it crossed the 
bow shock of the solar wind (the region where a shock wave is created as 
the supersonic solar particles slow to subsonic speed). When Giotto 
entered the densest part of the dusty coma, the camera began tracking the 
brightest object (the nucleus) in its field of view.

Excitement rose at the European Space Operations Centre in Darmstadt, 
Germany, as the first fuzzy images and data came in. The ten experiment 
teams scrutinised the latest information and struggled to come up with a 
preliminary analysis.

The first of 12 000 dust impacts was recorded 122 minutes before closest 
approach. Images were transmitted as Giotto closed in to within a distance 
of approximately 2000 kilometres, as the rate of dust impacts rose sharply 
and the spacecraft passed through a jet of material that streamed away 
from the nucleus.

The spacecraft was travelling at a speed of 68 kilometres per second 
relative to the comet. At 7.6 seconds before closest approach, the 
spacecraft was sent spinning by an impact from a 'large' (one gram) 
particle. Monitor screens went blank as contact with Earth was temporarily 
lost.

TV audiences and anxious Giotto team members feared the worst but, to 
everyone's amazement, occasional bursts of information began to come 
through. Giotto was still alive.

Over the next 32 minutes, the sturdy spacecraft's thrusters stabilised its 
motion and contact was fully restored. By then, Giotto had passed within 
596 kilometres of the nucleus and was heading back into interplanetary 
space.

The remarkably resilient little spacecraft continued to return scientific 
data for another 24 hours on the outward journey. The last dust impact was 
detected 49 minutes after closest approach. The historic encounter ended 
15 March when Giotto's experiments were turned off.

For more information:

Gerhard Schwehm, ESA Rosetta Project Scientist
E-mail: gerhard.schwehm @ esa.int