Traveling at 4.26 miles per second, a tiny aluminum sphere representing a piece of orbital debris, or "space junk," can be seen crashing into a spacecraft protective shield. This test and others like it were conducted to verify that this protective micrometeoroid and orbital debris shield is able to stop sufficiently large impacts. The impact generates what appears to be fire, but the assessment was conducted in a space-like vacuum, so there is no air or fuel. The brilliant, yellow flash is due to the momentary increase in material temperatures and associated glow.

Why is NASA's Johnson Space Center throwing everything, with the exception of the kitchen sink, at protective shields with the express purpose of impacting them? Well, space junk poses a serious risk to all spacecraft traveling near Earth. The International Space Station, located in low-Earth orbit, shares its small bit of space with approximately 2,700 tons of space junk.

The problem only increases at higher altitudes, with an additional 3,600 tons of space junk in geostationary orbit. Large enough to be tracked, there are more than 22,000 pieces of debris traveling around Earth right now. Even smaller debris, of which there is about 100,000,000 fragments larger than 0.04 inches, can create significant damage to a spacecraft due to their incredible speeds. The sequence of graphics on the right show the growing quantity of tracked space junk and operating spacecraft orbiting Earth over time.

The average impact of space junk on the space station is about 6 miles per second, but can reach speeds of up to 10 miles per second. In addition to space junk, the orbiting laboratory is impacted continuously by micrometeoroids traveling an average of 14 miles per second. This material can even reach speeds of up to 45 miles per second-a sobering prospect.

Impacts from space junk and micrometeoroids pose a risk to the International Space Station, as well as other space vehicles. Shield testing has helped produce and verify the performance of numerous protective shields on space station and other spacecraft.

The testing conducted by the Johnson's Hypervelocity Impact Technology (HVIT) team is essential to protecting both the spacecraft and crew. HVIT is located in Houston and coordinates hypervelocity testing performed primarily at the White Sands Test Facility in Las Cruces, New Mexico. Testing will continue through the month due to manufacturing and installation requirements.

Even small particle impacts can cause significant damage, so current tests that began late October can help verify new shield designs and improve the associated technology. For example, earlier testing and analysis demonstrated that multi-layered shields perform better than single-layer shields of the same areal mass.

This latest series of investigations were conducted on multi-layered shields, but the materials were arranged in a configuration not previously tried before. The tested shield (or a similar design) will be built, launched and installed on one of station's Passive Mating Adapters (PMAs) early next year. The PMA acts as one of the docking ports that allow visiting vehicles to dock to the space station. The shield will protect an area at the forward-base area of the adaptor, which recent risk analyses has shown to have higher than acceptable risk.

There is an entire science devoted to accessing the potential risk to spacecraft by micrometeoroids and orbital debris. Developing and testing spacecraft shield designs to reduce risk will be an ongoing process as the quantity of orbital debris around Earth increases.

This team of shield testers was founded more than 35 years ago. Since their inception, they have analyzed well over 10,000 impact experiments. These tests, which include computer simulation, have contributed to shield ballistic limit equations-or, the velocity needed for a particular projectile to penetrate a particular piece of material.

The International Space Station isn't the only beneficiary of this research. As we begin to explore deeper into the solar system, having high-impact shields for longer, more daunting trips will be at the top of the list of NASA's must-haves. Small teams-like Johnson's HVIT group-will help us punch through the barriers on the way to Mars.