Al sphere diameter: 1.2cm
Al sphere mass: about 1.7 g
Impact crater diameter: 9.0 cm
Impact crater depth: 5.3 cm

In such an impact, the pressure and temperature can exceed those found at the centre of the Earth, e.g. greater than 365 GPa and more than 6000 K.

Credits: European Space Agency, ESA

The most difficult problem to overcome was how to ensure that Giotto survived long enough to snap its close-up pictures of the comet nucleus when the spacecraft and the comet were heading towards each other at a combined speed of 245 000 km/hour (equivalent to crossing the Atlantic Ocean in 11 minutes!). At this speed, a 0.1-gram dust particle would be able to penetrate 8 cm of solid aluminium. Since it was out of the question to equip Giotto with a 600-kg aluminium shield, engineers turned to a more subtle, sandwich design first proposed by American astronomer Fred Whipple back in 1947 - long before the beginning of the Space Age.

The spacecraft's dust shield consisted of two protective sheets, 23cm apart. At the front was a sheet of aluminium (1-mm thick), which would vapourise all but the largest of the incoming dust particles. A 12-mm thick sheet of Kevlar at the rear would absorb any debris which pierced the front barrier. Together they could withstand impacts from particles up to 1 gram in mass and travelling 50 times faster than a bullet.

Credits: European Space Agency, ESA