Reentry

Ground-based optical observations of the orbital debris environment

Tracking the deorbit and reentry of orbital debris:

ORBITAL DEBRIS REENTRY FACTS
- In general a space object has to be over 1000 kg (1 ton) in mass to be likely to partially survive the reentry process. This includes derelict satellites, space stations and rocket bodies. Only small pieces of these will hit the ground.
- It has been estimated that of the 200 to 600 orbital reentries each year, about 20% may be large enough to have partially survived reentry and dropped at least some fragments on the Earth's surface.
- This equates to an average of about 80 per year. Most of these will be small pieces, and most will drop into the ocean.
- Two basic types of debris have been found to survive the reentry process. These are refractory materials with very high melting temperature (titanium, steel, ceramics and some glasses) and interior or protected components that are not exposed to the heat of reentry until a fairly late stage in the reentry process (insulation, compact components).
- Uncontrolled reentries are always at very low grazing angles (<1 deg). The reentering object is traveling at about 8 km/sec down to below 80 km altitude. 
Source: SPACE DEBRIS REENTRY HAZARDS , https://www.spaceacademy.net.au/watch/debris/reentryhaz.htm
- Location of uncontrolled reentries is unpredictable. 
- Major breakup at ~78 km. 
- 10 to 40% of mass survives reentry and impacts the Earth’s surface posing hazard to people and property (e.g. of the ATV-1 mass of 12.3 tons about 3.5 tons in 183 fragments survived re-entry, 28.4% of mass).
- Debris spread over long, thin ground footprint (e.g. for ATV ~ 817km by 30km).
- On average, there is one spacecraft or rocket body uncontrolled re-entry every week, with an average mass around 2000 kg.
- Currently, approximately 70% of re-entries of intact orbital objects are uncontrolled, corresponding to about 50% of the returning mass, (i.e. 100 metric tons per year). 
Source: Tommaso Sgobba, Space Debris Re-entries and Aviation Safety, International Association for the Advancement of Space Safety
- The mass of lithium, aluminum, copper, and lead from the reentry of spacecraft was found to exceed the cosmic dust influx of those metals. About 10% of stratospheric sulfuric acid particles larger than 120 nm in diameter contain aluminum and other elements from spacecraft reentry. Planned increases in the number of low earth orbit satellites within the next few decades could cause up to half of stratospheric sulfuric acid particles to contain metals from reentry.
- There are some significant differences between the ablation of meteors and spacecraft. Most of the meteoric mass is deposited at altitudes between 75 and 110 km by a very large number of submillimeter meteoroids. Reentering spacecraft, which are larger and moving more slowly, ablate between 40 and 70 km over a ~300 km long footprint. 
Source: Daniel M. Murphy et al. Metals from spacecraft reentry in stratospheric aerosol particles, https://doi.org/10.1073/pnas.2313374120
- By 2030 cumulative hazard to people on the ground due to reentries from a single constellation could be on the order of 0.1/year, or one casualty would be expected every 10 years. The probability of debris striking a commercial aircraft would be 0.001/year, and without emergency action by pilots, the maximum yearly casualty expectation for reentries of satellites disposed from a single large constellation for people in aircraft could be 0.3/year. 
Source: William H. Ailor, Hazards of reentry disposal of satellites from large constellations, July 2019 Journal of Space Safety Engineering 6(2) DOI:10.1016/j.jsse.2019.06.005
- Today, human-made (anthropogenic) objects material does make up about 2.8% of mass influx into Earth’s atmosphere compared to the annual injected mass of natural origin (meteoroids), but future satellite constellations may increase this fraction to nearly 40%.
- The large amount of aerosols injected by the ablation of anthropogenic material may have an effect on Earth’s climate as aerosols in the high-altitude atmosphere have a negative radiative forcing effect.
Schulz, L., Glassmeier, K.H., 2021. On the Anthropogenic and Natural Injection of Matter into Earth’s Atmosphere, Advances in Space Research, Volume 67, Issue 3, p. 1002-1025

- Some materials already exceeding in comparison to natural origin mass injection in Earth's atmosphere. For example, aluminum in today's anthropogenic mass injection contributes with 211 t/yr compared to 131 t/yr of natural origin injection.


Trajectories of reentry spacecraft and debris. Source: WU Ziniu, et al. Space Debris Reentry Analysis Methods and Tools, Chinese Journal of Aeronautics 24 (2011) 387-395.

Recovered orbital debris locations placed over population density map. Compiled by: Space Debris Research Lab - MIRCE Akademy 2023. GitHub

"Removing derelict space objects to reduce orbital debris hazards is merely a transferring of risk from space to the ground, which must also be managed."

Limiting Future Collision Risk to Spacecraft: An Assessment of NASA's Meteoroid and Orbital Debris Programs