New Hazards for a New Age

Manufactured objects orbiting Earth. Red objects are debris; white objects are operating satellites. The outer ring is composed of satellites in geosynchronous equatorial orbit.

New Hazards for a New Age

William Ailor

The first satellite was launched in 1957. Since then, so many objects have been put into space that today more than 650 operating satellites and 9,000 objects are being tracked, not to mention more than 100,000 bits of debris too small to follow. Such debris includes pieces of aluminum chuffed from satellite boost stages, blobs of liquid metal coolant that leaked from discarded space reactors, debris resulting from satellite explosions, and lens covers and other hardware discarded during normal satellite operations. Some of this material will remain in Earth orbit for hundreds or thousands of years.

For many years, there was little concern about releasing material in orbit or simply leaving a satellite to drift in space at the end of its mission. Today there is heightened awareness that space debris poses a hazard to operating satellites because of high relative velocities at impact. For example, objects in low Earth orbit can collide at speeds exceeding 10 kilometers per second. At these speeds, even a small particle can cause serious damage.

As we enter the 21st century, it is becoming increasingly common for the Space Shuttle to have to dodge oncoming debris. Accordingly, the new Space Station will be outfitted with shields designed to protect astronauts from collisions with objects smaller than 2.5 centimeters.

The Delta Stage 2 before launch. After 9 months, the stage reentered the atmosphere. The large brown stainless steel section, one of the red pressurant tanks weighing 67 pounds, and a third, lightweight piece were recovered. Based on Aerospace analysis, it is likely that other components of the Delta Stage 2 survived the reentry. (NASA)

What Does the Future Hold?

Experts predict that in 10 years an additional 2,000 satellites could be operating above us—a four-fold increase over the current number. Many of these, like the 68-satellite Iridium constellation, will be operating in low Earth orbit. What will happen to the debris count as we increase our use of space?

Efforts are under way to develop international guidelines and policies designed to limit the growth of debris. Venting propellant tanks and discharging batteries at the end of a mission will eliminate the possibility of explosions. Tethering lens covers and other debris from normal operations will prevent these items from floating off and becoming a hazard. And policies requiring that all spacecraft and stages be deorbited or placed into disposal orbits at the end of their missions will prevent them from endangering future satellites.

Deorbiting space hardware ends the hazard for other satellites, but presents a new problem—the possibility of debris surviving reentry and hitting people or property on the ground or in the air.

Aerospace Center for Debris Studies

Recognizing the growing importance of these issues, The Aerospace Corporation established the Center for Orbital and Reentry Debris Studies in 1997. The center has focused internal research on developing techniques to predict and avoid collisions of satellites with oncoming objects and on improving our understanding of orbit decay and reentry breakup.

One product of this research is CollisionVision, a software suite designed to predict close approaches to launch vehicles and satellites and estimate the probability of collision. Developed to increase launch and on-orbit safety, Collision Vision has undergone extensive testing and verification. A special parallel processing computer was recently installed in the Aerospace Colorado Springs office to enable faster computations of close approaches and research into relative hazards.

Demand is mounting from commercial companies for the information provided by CollisionVision. For example, operators of satellites in low Earth and in high-altitude geosynchronous equatorial orbits are recognizing that early warning of close approaches may help them avoid possible satellite loss and liability. The center has been working to increase the services available to commercial operators. Developments in this area will benefit all users of space.

In this close approach, CollisionVision predicts that the two ellipsoids representing the position uncertainty of each spacecraft will intersect. CollisionVision estimates the probability of the two spacecraft actually colliding and helps the operator decide if a maneuver, which depletes valuable propellant reserves, is desirable.

The center has also provided background information and technical advice to policy makers assessing the implications of alternative approaches for defining disposal orbits and is helping to show the hazard that space debris poses to tethers and that tethers pose to other space objects.

Reentry Breakup: Improving Predictions

The reentry of objects into the atmosphere exposes hardware to severe aerodynamic heating and loads. Accurate predictions of the response and breakup characteristics of hardware would enable good predictions of hazards to people and property on the ground.

Good predictions would also enable spacecraft designs to incorporate features to enhance breakup. For example, if it is shown that stainless steel fuel tanks will survive reentry, constructing tanks of lower melting point materials such as aluminum may ensure that large pieces don't hit the ground. Structural elements can be designed to fail at certain temperatures, exposing critical elements to direct heating early in the trajectory.

Improved knowledge may help ensure that critical elements designed to survive reentry work as planned. For example, radioactive materials that provide heat for temperature control and power generation on satellites designed for deep space missions are encased in special materials to ensure survival during reentry. Accurate models of the reentry environment will increase confidence that these systems will work as planned.

The impact of the Delta Stage 2 563-pound stainless steel fuel tank 50 yards from a farmer's home in Texas was ample demonstration that space hardware can and does survive reentry. (NASA)

Space Debris in Your Backyard?

A Delta Stage 2 rocket body reentered over the central portion of the United States in the early morning hours of January 22, 1997. As this stage reentered the atmosphere traveling south it broke up over Topeka, Kansas, at approximately 3:30 a.m. and released a debris stream that stretched 400 miles over Oklahoma and Texas. The recovered debris generated from this reentry included a small fragment weighing less than 1 pound, a helium tank weighing 67 pounds, and a fuel/oxidizer tank weighing 563 pounds. The small fragment reportedly struck a woman near Tulsa, Oklahoma, but caused no injury.

Detailed reconstruction of the trajectory of the Delta stage in the 1997 reentry is enhancing our knowledge of reentry dynamics and reentry breakup. As the reentering stage was broken apart by aerodynamic heating and loads, each piece followed a unique trajectory. The lightest piece slowed quickly and came down in Oklahoma; the heavier objects proceeded further. Total length of the debris footprint for this breakup exceeds 750 kilometers, characteristic of orbit decay reentries.

The trajectory analysis was supplemented by results of a metallurgical examination of the stainless steel fuel tank, which showed that the surface temperature of the tank exceeded 1200°C as it fell toward Earth. As an interesting sidelight, the metallurgical analysis also showed that the tank was hit by more than a dozen micrometeoroids or small space debris objects during its 9 months in space.

It is clear that the 21st century will pose new problems for the use of space. The Aerospace Corporation, through its new Center for Orbital and Reentry Debris Studies, is helping to develop the tools required to operate safely and effectively in this new environment.

Left: The ground track followed by the reentering Delta tank. Trajectory reconstruction showed that major breakup began at approximately 78 kilometers. Blowup: The ground track for the fuel tank. The "dogleg" occurred after the tank had slowed to a freefall velocity at approximately 21 kilometers and was blown off track by the wind during its final fall to the ground. The tank hit the ground at a speed of approximately 38 meters per second.