Everything that goes up must come down — eventually. For objects in orbit, “coming down” means slamming into Earth’s atmosphere at speeds exceeding 17,000 mph and generating temperatures hotter than the surface of the Sun. Most objects disintegrate entirely. Some don’t. And predicting which pieces survive and where they’ll land is one of the hardest problems in orbital mechanics.

The Technical Bits

Reentry is the process by which an object in orbit descends into Earth’s atmosphere and either burns up or reaches the surface. It begins when atmospheric drag — the friction of air molecules against the object — slows it enough that it can no longer maintain its orbit. This typically starts at around 120 kilometers (75 miles) altitude, where the atmosphere becomes dense enough to meaningfully affect an object’s trajectory.

There are two fundamental types of reentry:

• Controlled reentry: The spacecraft uses its engines to deliberately deorbit at a specific time and place, targeting an uninhabited area — usually the South Pacific Ocean Uninhabited Area (SPOUA), informally known as the “spacecraft cemetery” or “Point Nemo.” This is how the ISS disposes of cargo spacecraft and how large rocket stages are brought down safely.
• Uncontrolled reentry: The object has no propulsion capability (or has run out of fuel) and reenters whenever atmospheric drag naturally brings it down. The timing and location of uncontrolled reentries are extremely difficult to predict with precision.

During reentry, objects experience extreme heating — surface temperatures can exceed 1,650°C (3,000°F) for large objects. This causes most materials to melt, vaporize, or ablate away. However, components made from titanium, stainless steel, or certain ceramics can survive the thermal onslaught and reach the ground.

Why It Matters

Reentry matters for several interconnected reasons:

• Public safety: While the odds of being hit by falling space debris are astronomically small (roughly 1 in several trillion for any individual), the cumulative risk across the entire population is non-trivial as the number of objects in orbit grows. In 2024, a piece of hardware from the ISS survived reentry and crashed through a home in Naples, Florida — a vivid reminder that surviving debris is real.
• Environmental impact: As recently highlighted by research linking Falcon 9 reentries to upper-atmosphere lithium plumes, reentering objects deposit metals and chemicals at high altitudes where their long-term effects on atmospheric chemistry are not yet well understood.
• Regulatory compliance: International guidelines require that operators either deorbit satellites within 25 years of end of life or move them to a graveyard orbit. Demonstrating compliance requires accurate reentry prediction and planning.
• Tracking and awareness: Predicting reentries is a core function of Space Domain Awareness. Organizations like the 18th Space Defense Squadron issue reentry predictions that are updated as objects descend.

You can monitor reentry predictions for tracked objects using KeepTrack.

What Most People Mix Up

The most common misconception is that objects “burn up” completely during reentry. While most small satellites and debris fragments do disintegrate entirely, larger objects routinely survive in part. Studies estimate that 10 to 40 percent of a large satellite’s mass can reach the ground, depending on its materials and construction. Fuel tanks, reaction wheel assemblies, and structural titanium components are among the most common survivors.

Another widespread confusion is between reentry and “falling out of orbit.” Objects in low Earth orbit don’t suddenly “fall” — they experience a gradual decay as atmospheric drag slowly lowers their orbit over weeks, months, or years. The final plunge from about 200 kilometers altitude to the surface happens relatively quickly (within one orbit), but the process leading up to it is slow and measurable. That’s what makes reentry prediction possible, even if the precise timing remains uncertain until the final hours.

Fun Fact Space Nerds Might Not Know

The uncertainty in reentry predictions is staggeringly large until the very end. A typical uncontrolled reentry prediction might have a timing uncertainty of plus or minus 20% of the remaining orbital lifetime. If an object is predicted to reenter in 10 days, the actual window is 8 to 12 days. Since a satellite in a decaying orbit circles Earth roughly every 90 minutes and covers different ground tracks each revolution, even a timing error of one orbit (90 minutes) shifts the predicted impact point by thousands of kilometers. This is why reentry warnings often cover entire continents rather than specific cities — and why a precise prediction is only possible in the final few hours.

The most dramatic uncontrolled reentry in history was Skylab in 1979. Despite NASA’s best efforts to target the Indian Ocean, pieces of the 77-ton space station scattered across Western Australia. The Shire of Esperance famously issued NASA a $400 littering fine. It went unpaid for over 30 years until a radio DJ raised the money in 2009.

The Physics of Survival

What determines whether a piece of debris survives reentry? Three main factors:

• Material: Titanium (melting point 1,668°C) and stainless steel (1,400°C) survive far more readily than aluminum (660°C). Carbon fiber composites can either burn away or remain surprisingly intact depending on the resin system used.
• Shape and mass: Dense, compact objects shed heat more slowly than thin, flat ones. A solid titanium pressure vessel will survive; a thin aluminum panel will not.
• Shielding: Objects nested inside a spacecraft’s structure benefit from the outer layers burning away first, effectively shielding the interior components during the hottest phase of reentry. This is why internal components like batteries and reaction wheels frequently survive while the spacecraft’s skin does not.

Think of reentry as the universe’s recycling program — messy, unpredictable, and increasingly in need of better management as we put more and more objects into orbit.