Spacecraft Reentry Basics
Many satellites do not remain in their orbits indefinitely, but gradually return to Earth. This is because Earth’s atmosphere does not end abruptly, but becomes progressively thinner at higher altitudes. In fact, there is still some atmosphere several hundred kilometers up, where some satellites orbit Earth. Because the atmosphere is so thin at those high altitudes, satellites can take a long time to come down.
For satellites in low Earth orbit (hundreds of kilometers in altitude), it may take years or tens of years to return to Earth. Higher-altitude satellites are of less concern with regard to reentry hazards because they can stay in orbit much longer — hundreds or even thousands of years.
As a satellite loses altitude it enters denser regions of the atmosphere, where compression and friction generates a great deal of heat. This is due to the high velocity of orbiting satellites, which can be more than 29,000 km/hr. The tremendous amount of heat generated can melt or vaporize the entire satellite or portions of the satellite. A similar effect occurs during a meteor shower, where streaks of light (meteors or “shooting stars”) are generated by bits of natural materials (meteoroids) as they burn up in the atmosphere.
Although many people believe that satellites burn up completely during atmospheric reentry, some satellite components can and do survive the reentry heating (of course, satellites designed to reenter, like the space shuttle or Soyuz, survive reentry entirely because they are protected by specially designed heat shields). Component survival on an unprotected satellite can occur if the component’s melting temperature is sufficiently high or if its shape enables it to lose heat fast enough to keep the temperature below the melting point.
This video shows the reentry and breakup of NASA’s Compton Gamma Ray Observatory. Spacecraft weight at the reentry point is approximately 12,000 kg (26,000 pounds). The spacecraft was launched in 1991 and reentered in June 2000.
During reentry, the object is decelerating quickly and the loads on the structure can exceed 10 g’s (10 times the acceleration of gravity). These loads combine with the high temperature to cause the structure to break apart.
When the satellite components lose enough speed, the heating rate is reduced, the temperature decreases, and the objects begin to cool. By this time, the objects have fallen to even denser regions of the atmosphere and fall virtually straight down from the sky. They impact the ground at relatively low speeds, but still represent a hazard to people and property on the ground.
It is very difficult to predict where debris from a randomly reentering satellite will hit Earth, primarily because drag on the object is directly proportional to atmospheric density, and atmospheric density varies greatly at high altitudes. In general, we can predict the time that reentry will begin to within 10 percent of the actual time. Unfortunately, reentering objects travel so fast that a minute of error in the time is equivalent to hundreds of miles on the ground.
If a satellite or rocket body has propulsive capability, it can use its rocket motor to target the reentry into a desired area, such as the ocean. This technique was used by NASA to ensure that debris from the 14,000-kg Compton Gamma Ray Observatory impacted in the ocean.
Reportedly, only one person has ever been struck by debris from a reentering satellite. Fortunately, this person was hit by a lightweight object and was not injured.
The risk that an individual will be hit and injured is estimated to be less than one in one trillion. To put this into context, the risk that an individual in the U.S. will be struck by lightning is about one in 1.4 million.
Reentry risk estimates are supported by the fact that, over the last 50 years, more than 5,400 metric tons of materials are believed to have survived reentry with no reported casualties (of course, it is possible that casualties have occurred somewhere in the world, but have not been reported). The largest object to reenter was the Russian Mir Space Station, which weighed 120,000 kg.
Why do satellites fall from orbit?
Objects in orbit are exposed to atmospheric drag, just as aircraft and automobiles are near the ground. In space, of course, the atmospheric drag is much less than that experienced closer to the ground but, over time, even a small amount of drag can result in a satellite’s reentry into the denser atmosphere.
Objects orbiting at low altitudes may be removed from orbit by atmospheric drag within weeks, months, or years depending on the object and its altitude. Objects at higher altitudes may remain in orbit for hundreds or thousands of years.
Of course, some satellites and launch hardware have propulsive capability that can be used to deorbit these objects more quickly.
Why does space hardware come apart during reentry?
On reentering the atmosphere, a large debris object will be subjected to extreme heating and loads caused by the interaction of the fast-moving object with the atmosphere (at the reentry point, the object is travelling more than 20 times faster than a bullet). At some point, the temperature of the object reaches a critical point, aerodynamic loads increase, and the object will break up. Breakup could be caused by the failure of critical structural components as their temperatures exceed their melting points or, in a more extreme case, by an explosion of fuel or pressurized gas remaining in the object’s tanks.
Whatever the cause, the first major breakup event generally occurs at an altitude between 74 and 83 km. At this point, the object breaks into several smaller objects, and each continues to fragment or melt as long as sufficient heating and loads exist. When surviving objects have slowed sufficiently, the heat rate drops and a cloud of debris remains to fall and impact the ground.
Are there examples of objects that have survived reentry?
More than 50 debris objects have been recovered and documented over the years (see recovered debris), and there are several noteworthy examples.
On Jan. 22, 1997, a Delta second stage reentered and four objects were recovered: a 250-kg stainless steel tank, a 30-kg pressure sphere, a 45-kg thrust chamber, and a lightweight piece that struck but did not injure a woman.
On April 27, 2000, a Delta second stage used in the launch of a Global Positioning System (GPS) satellite reentered and debris impacted near Cape Town, South Africa. The recovered debris was nearly identical to that recovered in Texas.
In general, components made of aluminum and similar materials with low melting temperatures do not survive reentry, while pieces or components made of materials with high melting temperatures, such as stainless steel, titanium, and glass, often do survive. Large pieces with moderate melting temperatures can also survive reentry, radiating heat over their large surface areas. Pieces that survive reentry tend to be large and in some cases heavy, posing a significant hazard to any people and property within the bounds of the object’s reentry debris footprint.
It is interesting to note that even low melting temperature materials can survive under the right circumstances. For example, a lightweight piece that comes off of the parent body early in the reentry may not experience sufficient heating to melt, and another object contained within the body of the satellite and protected by surrounding structure through most of the reentry may also survive.
How much material from a satellite will survive reentry?
Generally, about 10-40 percent of a satellite’s mass will survive reentry. The actual percentage for a specific object depends on the materials used in the object’s construction and on shape, size, and weight of the reentering object. For example, if the object consists of empty fuel tanks made of stainless steel or titanium, both of which have high melting temperatures, much of this material will survive. If much of the structure is made of aluminum, which has a low melting temperature, a smaller percentage will survive.
What is a “debris footprint?”
Debris that survives reentry will impact within a “debris” or “impact” footprint, the area on the ground which contains all of the debris pieces. It is possible to estimate the size of the footprint, but very difficult to predict where the footprint will be on Earth’s surface or where specific pieces of debris will land.
The size of the footprint is determined by estimating the breakup altitude of the satellite or space hardware and then estimating the mass and aerodynamic properties of surviving debris. The heavy debris will generally travel farther downrange to the toe of the footprint; lighter material will generally be near the heel. Footprint lengths can vary from 185 km to perhaps 2,000 km, depending on the characteristics and complexity of the object.
The footprint width is generally determined by the effects of wind on the falling debris objects, with heavy objects affected less, and lightest the most. The width of the footprint may also be affected by the breakup process itself. For example, if the object should explode during reentry, fragments will be spread out across the footprint. A footprint width of perhaps 20-40 km is typical, with the most pronounced effects near the heel of the footprint.
Can we predict where debris will land?
It is very difficult to predict where debris will actually land. A one-minute error in predicting the reentry time will change the debris’ location by nearly 300 miles. The few surviving pieces of the object will be spread over a long footprint, so that two nearly identical fragments may impact many miles apart. For example, there were a total of four spheres on the Delta stages that left debris in Texas and South Africa, but only one sphere was found in both cases. The others most likely survived, but impacted some distance away and were not recovered.
Since predicting where specific pieces will land is very difficult, analysts generally predict the location of the debris footprint to indicate the general area where debris will land. Unfortunately, predicting where the debris footprint will be on Earth’s surface for a specific object’s reentry is also difficult.
The primary difficulty with predicting the footprint location relates to the uncertainties in predicting the object’s lifetime in orbit. Given sufficient tracking of the object during the orbit decay combined with measurements of the sun’s activity over extended time (solar activity can have dramatic effects on the upper atmosphere), it is possible to obtain a fairly good approximation of the date and time (but not location) of an object’s final reentry.
In general, there is about a ±10 percent uncertainty of the time of the final reentry. Considering that orbiting objects are travelling at more than 7 km/sec, a prediction made at the beginning of the object’s last orbit, which could take about 90 minutes to complete, could be off by as much as ±9 minutes, equivalent to more than 7,000 km on the ground. This means that the debris footprint could be located anywhere along this 7,000-km-long path.
Can we control where debris will land?
In some cases, we can control the location of the debris footprint by performing a deorbit maneuver. If the satellite or rocket stage has propulsive capability, it can be commanded to execute one burn or a series of burns designed to lower the orbit perigee so that the object will reenter at a specific location. This same type of maneuver is used to deorbit manned spacecraft at the end of a mission. Very few satellites and rocket stages have sufficient propulsion capability to perform a controlled deorbit, however.
For larger objects, which pose a hazard to people and property on the ground, a controlled deorbit is most desirable since this technique assures that the debris impacts in the ocean. A good example is NASA’s deorbit of the 14,000-kg Compton Gamma Ray Observatory (CGRO) into the south Pacific Ocean. More than 35 percent of CGRO’s mass was expected to survive reentry, and the falling debris would be a hazard to humans if it fell in the wrong place. After the failure of one of its three gyroscopes in December 1999, the decision was made to deorbit CGRO in a controlled manner before its remaining gyroscopes could fail. CGRO was safely deorbited in the Pacific Ocean on June 4, 2000, via four thruster burns.
Current U.S. government standards state that the risk from any reentry will not exceed 1 in 10,000. If we do not or cannot control where it will land, we must minimize the risk by ensuring that the spacecraft breaks into small pieces during reentry. Techniques for designing spacecraft for eventual disposal are part of a strategy called “design for demise” (see AIAA’s Aerospace America, “Design for Demise,” February 2012).
Has anyone been hit by falling debris?
Reportedly, only one individual has been struck by debris from a reentering spacecraft. Lottie Williams of Tulsa, Oklahoma, reported that she was struck on the shoulder while walking. The timing and location were consistent with debris from the Delta second stage reentry from which debris was recovered several hundred miles away in Texas. CORDS analyzed the piece and confirmed it to be part of the fuel tank of a Delta II rocket that launched a satellite in 1996.
There have been several noteworthy reentries affecting populated areas. For example, the Feb. 7, 1991, reentry of the USSR’s Salyut 7/Kosmos 1686 space station, with a mass of 36,700 kg, occurred over a populated region. Soviet ground controllers attempted to control the space station’s reentry for impact in the Atlantic Ocean by setting it into a tumble, thereby altering the atmosphere’s drag on the vehicle. However, their efforts were unsuccessful. Salyut 7 reentered over Argentina, scattering much of its debris over the town of Capitan Bermudez, 400 km from Buenos Aires. The townspeople observed the reentry of Salyut 7’s debris in their night sky as incandescent meteors, traveling from the southwest to the northeast. The next day, metal fragments were found dispersed throughout town. Luckily, no one was hit.
The 1979 reentry of the U.S. vehicle Skylab was similar. NASA controllers modulated the attitude of Skylab during the final orbits to encourage reentry in an orbit that passed over as few populated areas as possible. The final reentry of Skylab rained small bits of debris over a town on the southern Australian coast. Heavier debris landed inland. Again, no one was hurt.
How fast will debris be moving when it lands?
In general, debris from satellites and rocket bodies will be falling straight down and will impact the ground at relatively low velocities. Just as air will slow a piece of paper more than it will a lead ball, a light piece of debris (such as a bit of insulation) will slow rapidly and hit the ground at a slower speed than a heavier piece of debris. In general, impact velocities will range from about 30 km/hr for lightweight debris to 300 km/hr or more for heavier objects. You might also expect that local winds will blow a light piece of debris more off-track than a heavier chunk. The wind does spread out the falling debris, making it more difficult to find surviving material on the ground.
What is the overall risk from reentry debris?
The overall risk to an individual from reentering debris is extremely small compared to the hazards we face daily. For example, the risk that an individual will be hit and injured by a piece of debris is estimated to be less than one in one trillion. To help understand this, the risk that an individual in the U.S. will be hit by lightning is one in 1.4 million. The chance that an individual in the U.S. will be killed in a hurricane is about one in six million.
The reentry risk predictions are supported by the fact that over the last 40 years, more than 5,400 metric tons of materials are predicted to have survived reentry, with no reported casualties. Of course, it is possible that casualties have occurred somewhere in the world, but have not been reported. The largest object to reenter was the Russian Mir Space Station, which weighed 120,000 kg.
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