We live on a wet planet, and when objects beyond the atmosphere come crashing down to the surface, whatever doesn’t burn up in the atmosphere tends to do so with a splash. So it was with the Tiangong-1, a Chinese prototype space station launched into orbit in 2011 and crashed into the Pacific Ocean April 1. The risk to people on the planet’s surface was infinitesimally small, theoretically possible to calculate but beyond irrelevant to label as a danger, and apart from the thrill of tracking an object crash from the heavens, the conclusion to the whole episode is one of understated calm.
That wasn’t the case in 2008, when the Bush administration decided to shoot down USA 193, a falling spy satellite. The rationale for the shoot-down, offered publicly and debated afterwards, was an assumed risk to people on the ground below from a tank of frozen hydrazine propellant. As satellite communications company Iridium describes it, hydrazine is “toxic for humans, but satellites love it.”
That sounds absolutely awful. And in 2008 the specific unpleasantness of hydrazine-human contact was used as a pretext to shoot the satellite down. Here’s how Gen. James E. Cartwright, then vice-chairman of the Joint Chiefs of Staff, described that need in the New York Times:
Ask the Pentagon to solve the problem of a dangerous inanimate mass, and odds are it will find a kinetic answer. So why didn’t any nation attempt to shoot down the Tiangong-1, which carried the same fuel?
As debated in 2008, it’s not all that clear that a hydrazine tank will invariably survive reentry and pose a risk. Skepticism of the hydrazine rationale was particularly strong in the arms control community, where the interception and destruction of USA 193 was seen as part of a larger context. Just the year before, China shot down one of its own weather satellites. Many nations maintain anti-satellite capabilities, and one of the clearest ways to prove that the nation can, in fact, threaten objects in space is simply to just shoot an object down.
Here’s how Geoffrey Forden, then part of MIT’s Science, Technology and Global Security Working Group, described the security and political trade-offs at the time:
That space is a peaceful domain is not set in stone. The present balance has nations putting satellites with military uses, like surveillance or communications, in orbit but keeping weapons out of the picture, and that is an arrangement mutually beneficial to every nation with objects in orbit and to people on Earth’s surface below. Mixing space and weapons threatens that balance, and anti-satellite missile tests are definitely a case of mixing space and weapons. Even if the intercept itself is brief, the debris in orbit is a persistence problem that threatens every future satellite put into space.
If hydrazine was the central concern with a tumbling reentry in 2008, where was it in 2018? The hydrazine risk did not go entirely uncovered, with the substance occasionally handled calmly though it was mostly written about with scary fonts and hyperbolic headlines. And, so far, nations have only publicly acknowledged shoot downs of their own satellites, a likely concession to the already geopolitically fraught nature of anti-satellite operations.
Which leaves us where we started. The Tiangong-1 crashed uneventfully into the Pacific Ocean, without any real panic or attempt at armed intervention. It is entirely possible that, left alone, the same fate would have befallen whatever parts of USA 193 didn’t burn up during reentry.
It’s also possible, in an increasingly contested environment, that space powers such as China and the United States feel they have significantly demonstrated their anti-satellite capabilities.