How Was China's Tiangong-1 Space Station Crash Tracked So Accurately?
China's Tiangong-1 space lab burned up in the atmosphere over the southern Pacific Ocean late last night (April 1), falling right in the middle of the window predicted by a number of organizations, including the U.S. Strategic Command's Joint Force Space Component Command (JFSCC), the U.S.-based analysis group Aerospace Corp., the China National Space Administration and the Inter-Agency Space Debris Coordination Committee. The last organization is an international group that consisted of NASA and the European Space Agency (ESA), as well as the space agencies of 11 other nations.
Space.com talked with Harvard-Smithsonian's Jonathan McDowell, who has been commenting on the school-bus-size station's uncontrolled descent and predictions throughout the deorbiting process, to understand how agencies tracked the tumbling space lab.
"When we track Earth satellites, there are really three main ways to do it," McDowell told Space.com. "With radars on the ground, with telescopes, also on the ground, and with telescopes on satellites. For the most part, low-Earth orbit tracking is done with radars on the ground." [See China's Falling Space Station in These Radar Images]
In the U.S., for instance, there is a network of radars in the Northern Hemisphere that tracks objects in space as the world turns, he said. As satellites pass through the radar beams, scientists can get a sense of its distance from the radar source and its speed in the direction the radar is looking from. By combining observations from several radars, researchers can piece together a satellite's path.
"Of course, part of the fun is, if you saw a satellite 10 minutes ago with one radar, and you see a satellite now with another radar, how can you be sure it's the same satellite? It's actually quite a difficult problem â¦ when you have 18,000 satellites going around, to disentangle them all," McDowell said. To do so, researchers have to model a rough orbit for the satellite, using radar data points to fine-tune their understanding of its location and speed.
To forecast where a satellite like the falling Tiangong-1 is going next, researchers have to take into account the drag of Earth's upper atmosphere â" a challenging feat, because that changes as the spacecraft tumbles at different angles and also as the atmosphere expands and contracts with temperature changes, altering from day to day and based on how active the sun is.
It's very difficult to precisely predict how an object like Tiangong-1 is tumbling. Researchers have to make do with an average impact of wind as it hits the object's different dimensions, and, though the actions of the atmosphere can be calculated based on what other satellites are doing, that method doesn't give instantaneous details.
Despite these limitations, the different organizations did a good job of predicting the window for Tiangong-1's descent, McDowell said. "For example, on the ESA site, which has this little graphic of how the window narrowed over time, it was never horribly wrong," he said. "It had big uncertainties to start with, and then it shrunk and shrunk until early on Sunday it was 6 hours, and later on Sunday it was 2 hours â¦ and then we got re-entry, and it was pretty much in the middle of that 2-hour window.