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The Whipple shield or meteoroid bumper, invented by Fred Whipple, is a type of hypervelocity impact shield used to protect manned and unmanned spacecraft from collisions with micrometeoroids which impact at an average speed of 26 km/s in low Earth orbit and orbital debris whose impact speeds range between 3 and 18 km/s.

As opposed to monolithic shielding of early spacecraft, Whipple shields consist of a relatively thin outer bumper placed a certain distance off the wall of the spacecraft. This improves the shielding to spacecraft mass ratio, critical for spaceflight components, but also increases the thickness of the spacecraft walls, which is not ideal for fitting spacecraft into launch vehicle fairings. The advantage of a bumper placed at a standoff over a single thick shield is that the bumper shocks the incoming particle and cause it to fragment and melt. Furthermore, the collision with the bumper imparts a radial motion to the particle fragments which spreads the impulse from the particle/bumper fragments over a larger area of the spacecraft wall.

Applications
The Apollo service module and the lunar modules were the first US crewed spacecraft designed to use Whipple shields. One inch thick honeycomb skin panels protected the service module propellant tanks and a 0.101 mm thick bumper protected the lunar module ascent stage crew cabin and propellant tanks.

The Skylab Whipple shield was another early application and earned it's notoriety by nearly ending the mission. Hypervelocity impact testing in 1966 showed that a micrometeoroid perforating the Skylab wall would shoot a 2 foot long flame into a pure oxygen crew cabin atmosphere (what was then the current design, but later changed to a mixed gas atmosphere to reduce flammability). Four weeks after the Apollo 1 fire von Braun convened a meeting with the various NASA center meteoroid protection subject matter experts to resolve the issue. The team concluded that a Whipple shield was required to reduce the risk to an acceptable level. Concerned with schedule delays, von Braun required the shield to leave the Skylab outer mold line unchanged. This dictated a complicated mechanism to deploy the shield 13 cm from the orbital workshop when Skylab reached orbit. However the auxiliary tunnel Whipple shield had a design flaw which caused it to pop out into the air stream at maxq resulting in the whole shield and one of the solar arrays tearing loose during ascent. Thereafter, some NASA engineers referred to deployable mechanisms as "deplorable" mechanisms.

Robotic spacecraft designers typically accept the impact risk and forgo Whipple shields to save weight and cost. (As of June 2013 only 3 robotic spacecraft are thought to have had mission ending micrometeoroid or orbital debris impacts: Olympus in 1993, Cerise in 1996, Iridium 33 in 2009.) Spacecraft that are targeted to pass through a cometary coma are the exception. ESA's Giotto comet 1P/Halley flyby passed within 596 km of the nucleus and experienced numerous impacts. Around 12,000 dust particles, with a total mass of 2 g, impacted the shield and slowed the spacecraft by 23 cm/s. The dive through the coma required a robust shield. The shielding weighed 49.6 kg, used a 1 mm thick aluminum bumper standing off 23 cm from the 13.5 mm thick Kevlar reinforced plastic/foam sandwich and was designed to stop a 100 micron diameter dust particle impacting at 68 km/s. The Deep Impact bus was another comet flyby spacecraft that used a novel Whipple shield. The bumper was made of a CFRP honeycomb tilted with respect to the trajectories of the comet 9P/Tempel dust particles. The particle trajectories refracted at the bumper resulting in a grazing collision with the spacecraft bus primary structure.

Whipple shield variations
There are two variations on the Whipple shield in use today: enhanced Whipple shields and multi-shock shields. (There are over 400 shield configurations on the International Space Station alone, monolithic shields, Whipple shields and enhanced Whipple shield, with higher risk areas receiving the more robust enhanced Whipple shields.)

Enhanced Whipple shield
The enhanced Whipple shield has a Nextel/Kevlar blanket or panel between the Whipple shield's bumper and rear wall. (The assembly is sometimes referred to as a stuffed Whipple shield.) The Nextel 312 aluminoborosilicate ceramic fiber fabrics are included to further fragment and melt the meteoroid or orbital debris particle after impacting the bumper and the Kevlar is included to slow the fragments. The NASA International Space Station modules use Nextel/Kevlar fabrics sewed together into flexible blankets, while the ESA Columbus module uses Nextel/Kevlar fabrics bonded together with a plastic resin to form rigid panels for simplified handling.

Multi-shock shields
Multi-shock shields use multiple bumpers spaced apart to increase the shield's ability to protect the spacecraft.

The multi-shock shock shield was first applied to the Space Station Freedom propulsion modules. But with the space station restructure in 1993, the propulsion module was canceled and the design never completed.

The next application was to the Stardust robotic spacecraft.

The CONTOUR robotic spacecraft, launched in 2003 used a multi-shock shield with an overall thickness of 25 cm with five layers of Nextel fabric spaced at 5 cm intervals.

The Deep Impact robotic spacecraft bus launched in 2005 used a multi-shock shield composed of three copper sheets.

Spacesuit thermal micrometeoroid garments and the walls of the NASA TransHab and the Bigelow Genesis I and Genesis II inflatable modules could be classified as multi-shock shields, though of a more complicated construction than discussed here.