- What does "re-entry" mean on this page?
- We use Space-Track's definition. Its predicted time is when an object is expected to reach 10 km altitude on the way down, and the location it gives is, in its words, "the location above which an object is predicted to arrive at 10km altitude during its descent, not the predicted Earth impact location." For an uncontrolled re-entry, nobody can say in advance where any surviving pieces reach the ground, so we never show a landing spot.
- Why is the window hours wide?
- Air drag at the edge of space is hard to forecast. ESA's re-entry guide applies an uncertainty of plus or minus 20% of the object's remaining time in orbit, and attributes prediction errors roughly to the atmosphere (70%), the object's ballistic coefficient (20%) and orbit determination (10%). The window narrows as the day approaches: Swift's first Space-Track message, about four days ahead, carried plus or minus 18 hours.
- Why can nobody say exactly where it will come down?
- An object this low goes around the Earth in about an hour and a half, so a few hours of uncertainty in time means several laps of its orbit. ESA's guide says re-entries "cannot be predicted with a high degree of confidence until the last few passes prior to re-entry." That is why we show the latitudes it passes over and, when the data supports it, its possible paths: never a single point.
- Is a re-entry dangerous?
- Space-Track issues these messages for objects "which have the potential for generating debris which could pose a hazard to people or property on Earth." Whether anything survives depends on the object. We only say that parts are expected to survive when the operator says so, and we never estimate risk ourselves. Follow your national authority for any safety advice.
- Where do these predictions come from, and how often do they change?