NASA Orbital Debris Program Office - Reentry

Reentry

Because of the increasing number of objects in space, NASA has adopted guidelines and assessment procedures to reduce the number of non-operational spacecraft and spent rocket upper stages orbiting the Earth. One method of postmission disposal is to allow reentry of these spacecraft, either from orbital decay (uncontrolled entry) or with a controlled entry. Orbital decay may be achieved by firing engines to lower the perigee altitude so that atmospheric drag will eventually cause the spacecraft to enter. However, the surviving debris impact footprint cannot be guaranteed to avoid inhabited landmasses. Controlled entry normally occurs by using a larger amount of propellant with a larger propulsion system to drive the spacecraft to enter the atmosphere at a steeper flight path angle. It will then enter at a more precise latitude, longitude, and footprint in a nearly uninhabited impact region, generally located in the ocean.

Spacecraft that re-enter from either orbital decay or controlled entry usually breakup at altitudes between 84-72 km due to aerodynamic forces causing the allowable structural loads to be exceeded. The nominal breakup altitude for spacecraft is considered to be 78 km. Larger, sturdier, and denser satellites generally breakup at lower altitudes. Solar arrays frequently break off the spacecraft parent body around 90-95 km because of the aerodynamic forces causing the allowable bending moment to be exceeded at the array/spacecraft attach point.

After spacecraft (or parent body) breakup, individual components or fragments will continue to lose altitude and receive aeroheating until they either demise or survive to impact the Earth. Many spacecraft components are made of aluminum, which has a low melting point. As a result, these components usually demise at a higher altitude. On the other hand, if an object is made of a material with a high melting point, (e.g., titanium, stainless steel, beryllium, carbon-carbon), the object will demise at a lower altitude and in many cases will survive. Also, if an object is contained inside a housing, the housing must demise before the internal object receives significant aeroheating. Some objects may have a very high melt temperature such that they can never demise, but are so light (e.g., tungsten shims) that they impact with a very low velocity. As a result, the kinetic energy at impact is sometimes under 15 J, a threshold below which the probability of human casualty is very low. Thus, the debris casualty areas computed for these objects do not figure into the total debris casualty area in a reentry survivability analysis.

The reentry survivability of spacecraft components is computed by either of two NASA methods. One is the Debris Assessment Software (DAS), a conservative, lower-fidelity software tool found under the "Mitigation" section and the second is a more accurate and higher-fidelity software tool called the Object Reentry Survival Analysis Tool (ORSAT).