Lastly, just this past March, a titanium pressure sphere from an Ariane booster upper stage struck a home in Kasambya, Uganda. No injuries were reported.

Consuming story

Why is that certain space gear avoids being consumed during the heat of reentry? Engineers are now diving into reasons why some hardware can survive a scorching fiery fall.

To help unravel a myriad of burning questions, one proposal is to equip spacecraft with tiny black boxes - devices akin to those installed on planes to help investigators sort out the what and why of mishaps and crashes.

For the last couple of years, experts at The Aerospace Corporation in El Segundo, California have been looking into fabricating a "black box" for satellites.

"We need good data on what's actually happening as a spacecraft breaks up during reentry," said William Ailor, director of The Aerospace Corporation's Center for Orbital and Reentry Debris Studies, or CORDS for short. "Reentries are beginning to be a fact of life to those of us in the space business," he said.

"We want to calibrate computer models to make sure they are accurate. To minimize hazards to people on the ground you want to make sure your data is right and your models are good. Having this type of black box on a spacecraft is about the best way to do that," Ailor told SPACE.com.

Ailor said a patent on their black box design has been filed.

Paperback rider

Aerospace engineers envision the black box about the size of a paperback book. Riding inside a satellite, the device is replete with its own reentry heat shield. That permits the black box to function while the surrounding spacecraft is eviscerated in the drop toward destruction.

The low-in-weight black box is self-contained. No need for telemetry or power from any outside source. Embedded within a satellite or rocket stage, a thermal switch would activate the device when temperatures start to climb at the start of reentry.

Loaded with ultra-small sensors, including accelerometers and a Global Positioning System navigation chip, the box would record the stresses and strains on the hardware during its fall from space.

The spacecraft black box is seen as cheap, totally passive and long-lived.

"It would sit there in orbit for years. Then when the reentry starts, it will come alive and make a phone call home," Ailor said.

Following an ultra blast furnace-like reentry, the black box has five to seven minutes of free-fall time to broadcast what it has learned about the breakup. This recorded data is dutifully zapped up to orbiting satellites, perhaps of the Iridium series, Ailor said.

First-hand data

"We're trying to get first-hand information, rather than make assumptions about how the heating goes," Ailor said.

Better knowledge about the reentry process should be a boon to space engineers.

For one, it could be cost-saver. "You may design a deorbit system into a spacecraft when you don't need one," Ailor said. Furthermore, the black box would tell the tale of when and where somebody's space hardware has come down. In addition, say for a planetary probe gone awry -- such as NASA's Mars Observer and the Mars Polar Orbiter -- the unit could relay back to Earth what actually took place, he said.

Ailor said the black box design for space vehicles has been funded mostly by way of his corporation. Some Air Force money has also helped move the idea forward. Additional support from other agencies is hoped for, he said.

"We don't see any showstoppers," Ailor said. One way to help calibrate the black box for future work is to mount it into a craft -- perhaps a Russian Progress vehicle -- then reenter that craft into a well-instrumented area, he said.

Melting-point

While black boxes can offer insight into complex reentry processes, they aren't likely to answer all questions. Considerable work is underway on spacecraft building materials too, and other techniques to deal with space debris.

Reactions of materials during hardware reentry is one area of important research, said Nicholas Johnson, NASA's Chief Scientist and Program Manager for Orbital Debris at the Johnson Space Center in Houston, Texas.

The space agency has a safety standard that requires all new NASA space projects to evaluate potential risks to people on Earth as a result of spacecraft reentries, that is, if long-term on-orbit disposal is not selected, Johnson told SPACE.com.

"First, the limitation of high melting-point materials, such as titanium, beryllium, or stainless steel, can reduce the amount of surviving debris. Of course, these materials are often chosen for specific properties needed for the spacecraft design," he said.

Any substituted material must still satisfy the needs of space vehicle builders, such as strength, thermal expansion, and other attributes, Johnson said.

Even high melting-point materials can demise during reentry if they are small enough or thin enough, so strict prohibitions on their use are not necessary. On the other hand, low melting-point materials -- like aluminum -- can survive reentry depending upon the shape and mass, Johnson said.

"Therefore, an avenue of research is to determine where the demise/survival limits are for these materials," the NASA space debris expert said.

No specific design standard

For example, NASA has begun conducting studies on just how thick the wall of a tank can be -- given its length, diameter, and material makeup -- before it is likely to survive reentry. Such studies also need to take into account the effect of tank insulation materials, such as graphite epoxy wrapping material.

Johnson said one way to promote hardware demise during reentry is by breaking the vehicle up into smaller components earlier in the reentry phase, around 93-miles (150-kilometers) altitude above Earth.

"This could be done conventionally, using small explosive charges, or with special materials that burn with high intensity. For many reasons, this solution is normally the least attractive," Johnson said.

Johnson said that a specific design standard is not envisioned due to the widely varying size of spacecraft and, to a lesser extent, due to mission requirements. The NASA and U.S. government philosophy, as well as that of foreign space agencies, is to set the desired safety standard and to grant spacecraft engineers the flexibility of how to meet that standard.