Protection

Measures to protect satellites can be taken to mitigate the effects of an impact with debris: specific or intrinsic shielding can be used.

Specific shielding comprises multi-layer shielding (Kevlar or Nextel), with protection surfaces added around the item to be protected. They are only effective against debris of up to 2 cm in size.

Intrinsic shielding consists in using the walls of the satellite itself as shields, or in opting for specific attitudes (case of the Space Shuttle for example).

Intelligent satellite architecture design enables fragile equipment to be protected by placing it either on surfaces that will be relatively little exposed, for example inside the satellite, or behind less sensitive equipment whenever possible.

All these measures will often lead not only to extra mass but also to extra cost. These measures will of course only have a positive effect if the debris is smaller than about 2 cm.

This solution therefore only covers a part of the risk.

A good example of specific shielding is the ATV-5’s shield.

 Figure 1.- Result of a hyper-velocity impact test on the ATV-5 shield

 (Credits: ESA–S. Laagland)

In Figure 1 we can see the result of a hyper-velocity impact test on a replica of the ATV-5 shield. During this test, an aluminium projectile 7.5 mm in diameter was fired at the shield by the gas gun in the Ernst Mach Institute (EMI) in Fribourg. The projectile reached a speed of 7 km/s.

Under the impact, the top layers of the ATV-5 shield were completely pierced (see Figure 1) but that was expected! That is what they are there for, to be expendable and cause the projectile to break up into multiple fragments and vapour. They thus attenuated the impact on the innermost layer, consisting of 3 mm of aluminium, which shows only a large number of small scratches (see Figure 2). This multi-layer shield called Whipple (after the name of its inventor) was conceived in the 1940s and gives greater protection than a single layer of aluminium of comparable thickness. “Adding an absorbent pad between the two plates of aluminium helps increase the protective effect. This is called a ‘reinforced Whipple shield’”, explains Christian Durin, a “materials and processes” expert at CNES in Toulouse. An added bonus is reduced mass, a point that cannot be ignored when calculating launch costs.

 

 Figure 2. - Result of a hyper-velocity impact test on the inner layer of the ATV-5 shield

 (Credits: ESA–S. Laagland)