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.”

Iridium continues:

Hydrazine will burn if given the opportunity. In fact, in the presence of oxygen (air), a catalyst (a spot of rust in a room, for example), and a small amount of heat, hydrazine will spontaneously explode.
These characteristics make it desirable as a fuel for satellites, but dangerous to the humans who are around it. Besides being highly and easily combustible, it is extremely toxic, caustic and probably carcinogenic. Humans exposed to hydrazine vapor will suffer burns in the eyes, nose, mouth, esophagus and respiratory tract. Severe burns can be fatal. Liquid hydrazine on the skin is quickly absorbed and acts as a neurotoxin. Burning hydrazine is extremely hot, but produces no visible flame (yes, it’s like invisible fire!), which can quickly spread to other combustible materials (clothes, skin, etc.).

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:

“If we fire at the satellite,” he said, “the worst is that we miss. And then we have a known situation, which is where we are today. If we graze the satellite, we’re still better off, because likely we’ll still bring it down sooner, and therefore more predictably. If we hit the hydrazine tank, then we’ve improved our potential to mitigate that threat. So the regret factor of not acting clearly outweighed the regret factors of acting.”

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:

NASA has a requirement on its controlled reentries that there be a 1 in 10,000 chance of killing somebody on the Earth. If the chances of the hydrazine tank surviving the reentry are greater than roughly 1 in 1,000, then this uncontrolled decent does not satisfy NASA’s requirements and it can be argued that other measures should be taken. On the other hand, there are certainly political considerations that would argue against that. It could be argued, for instance, that it increases the legitimacy of China’s ASAT test on 11 January 2007. Legitimating any kinetic kill anti-satellite weapon increases the chances that a space war might actually be fought; creating tremendous swarms of debris that could, as some experts on space debris argue, cause a catastrophic chain reaction that would put so much junk in space that it would be impossible to orbit a satellite for thousands of years. That would be a humanitarian disaster considering the benefits humanity derives from space, such as warning of ruinous floods and aiding in the relief of tsunami victims.

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.