What Is the Risk of Being Hit by Reentering Debris?
Humans have been placing objects in orbit for decades. Eventually, these objects must come back down to Earth. What is the probability that a person on the ground will be hit by a piece of space debris?
The Earth orbit environment is crowded with satellites, rocket bodies, and debris, all of which will eventually reenter the atmosphere. Most of this debris will burn up and not reach the ground, but some is resilient enough to survive to Earth’s surface, thereby posing risks to people and property.
There are more than 21,000 space debris objects in orbit that are large enough to track. Of those, more than 13,000 are in low Earth orbit (LEO). Atmospheric drag in LEO causes an object’s orbit to decay, lowering its altitude to a point approximately 120 kilometers above the ground where reentry begins. The LEO belt ranges from 160 to 2000 kilometers in altitude. The lifetime of an object in LEO varies considerably, because as an object’s orbit decreases, atmospheric drag exponentially increases. For objects in LEO above 1000 kilometers, time to reentry may be hundreds or even thousands of years. An object in a lower LEO orbit (less than 400 kilometers) will reenter Earth’s atmosphere in less than two years. Approximately 400 of these tracked objects reenter each year, 40 of which are rocket bodies or inactive satellites large enough for some fragments to impact the ground.
Awareness of the space debris problem has led to a series of recommendations and international guidelines to try to reduce its growth. Controlled reentry, where a satellite or rocket body is guided to a safe path over an ocean (typically the Pacific) is the preferred method for end-of-mission disposal. Uncontrolled reentries pose a risk to people and property, even if only a few large components survive and fall to the ground. Although there has been considerable international cooperation in adhering to these recommendations and guidelines, the majority of large, massive debris objects in orbit were launched before the guidelines were produced, and so no provision for their controlled reentry was ever made. In addition, a small number of satellites and rocket bodies fail before their missions end; hence any disposal procedures for them cannot be implemented. These objects too will eventually decay to an uncontrolled reentry.
Calculating the Risk
How is the risk to people on the ground from uncontrolled reentries calculated? When an object’s altitude decays to approximately 120 kilometers, reentry will usually occur within the next orbital revolution. Objects in near-circular orbits reenter the atmosphere at very shallow angles, typically at less than 1 degree. As an object descends through the atmosphere, increasing atmospheric density leads to higher forces and torques, and a rapid increase in temperature. At approximately 80 kilometers, the combined heat and physical stress causes a major breakup of the object. As components separate, each is subjected to different drag forces and heating because of their varying size, shape, and mass. Fragments that are large with low mass (such as empty propellant tanks) are most likely to survive reentry and fall to the surface. Approximately 10 to 40 percent of this dry mass survives and impacts Earth, posing a hazard to people and property on the ground.
To predict when and where a reentry might be most risky to humans, models are used for a given structure, along with predictions for the drag forces and aerodynamic heating that will occur. These models can provide an estimate of the number, size, and mass of major components that separate from the parent body during a breakup. Applying the model to each component helps researchers predict which objects will survive and reach the ground.
The risk of injury is measured by an object’s kinetic energy when it strikes, and assumes a hit to an individual’s head, chest, or abdomen. The amount of kinetic energy considered sufficient to cause serious injury or death is 15 Joules or greater, and assumes that the person hit is outdoors and unprotected. This is the equivalent of dropping a 15-pound bowling ball from a 9-inch height. Factors such as the availability of shelter or debris bouncing or rolling can also be applied to the risk assessment.
It is extremely difficult to predict precise locations and times of reentry. A reentering object will likely be tumbling, and that motion will be unknown and unpredictable. Consequently, the drag on the object will vary considerably as it descends. For example, the drag on a rocket body 5 meters in diameter and 15 meters long tumbling broadside through the atmosphere is subject to drag almost 4 times greater than when its long axis is parallel to its velocity.
For reentering satellites of asymmetrical shape, the difference in drag from various orientations can be even greater. Variations in the local atmospheric density, ballistic coefficient, and uncertainties in the initial orbit combine to produce an uncertainty in reentry time of plus-or-minus 20 percent of the “time to go.” For example, if the tracking network can generate an orbit 90 minutes (one revolution) before reentry, the uncertainty in the reentry time is plus-or-minus 18 minutes. In this example, the object has an orbital velocity of over 7.7 kilometers per second. This leads to a predicted location for reentry anywhere along a ground track 16,600 kilometers long. A difference of just one minute in the reentry time translates to a difference of almost 500 kilometers in the reentry location relative to the ground—the distance of Washington, D.C. to New Haven, CT, or Los Angeles to San Jose, CA.
The spread of fragments along and perpendicular to the ground track depends on where each fragment is released, its flight characteristics, and the upper-level winds encountered once an object has reached the lower atmosphere. Anyone located along the ground track has some degree of risk from the reentering debris. Fortunately, the region where the debris may fall is not very wide and there is a higher probability that the surviving components will fall near the predicted time and location rather than along the end points of the track. The risk along the track can be displayed as a color-coded band to help identify the most likely regions where surviving debris may land.
Once estimates of object number, size, and mass have been determined, along with similar estimates regarding time and location of ground impact, expectations of casualties are calculated. This requires gathering readily available data of the affected region’s population density. In this example, an individual is represented by a 0.4-square-meter area (a 2-foot square). The next step is to calculate the probability of a reentering object landing on a specific 0.4-meter square in the potential impact area. For U. S. territory, the risk is considered to be high when the casualty expectation exceeds one in ten thousand.
As an example, assume one of Phobos-Grunt’s tanks survives reentry to impact. The tank is 1.7 by 2.7 meters and will land flat (broadside to the ground, rather than end first), creating the largest impact area. The impact location with the highest probability along the Phobos-Grunt ground track peaks at 0.496, so there is an approximate 50-percent chance that the tank will land in the region of highest probability, a region 800 kilometers long and 50 kilometers wide. Further, assume that the population density in this region is 100 people per square kilometer (4 million people). If everyone were evenly distributed, each person would be 100 meters from their nearest neighbor. The tank hazard area is actually larger than the tank because it can land at any orientation around a person’s location, and if one part of the tank covers a person’s square, a casualty has occurred. Therefore, the area where the tank can land and still cause a casualty is 15.13 square meters. Dividing the casualty area of the tank by the total area and multiplying it by the total population, the casualty expectation for this 42,500-square-kilometer area is 1.5 for every thousand people.
This example has a risk significantly higher than the accepted threshold for the United States, but the population density assumed is extremely high for a corridor 800 kilometers long. Most regions of the country have a substantial amount of open space between higher density population centers. Ohio, for example, has a population density of 100 people per square kilometer, but it also has large areas of open farmland.
There is nothing unusual about the Phobos-Grunt tank example. Many rocket bodies have tanks this size that can survive reentry, so it can be quite easy for an uncontrolled reentry to produce casualty expectations above the one in ten thousand threshold. Of course, this is for reentries that occur over land. Since 71 percent of Earth’s surface is covered by water, 71 percent of uncontrolled reentries result in an ocean impact. The reality is that although there are a substantial number of reentries that can exceed the accepted casualty expectation, the risk to humans from each event is still very low.
Related article: Addressing the Dangers of Debris
Related publication: Crosslink, Fall 2015, Understanding Space Debris