Satellites and things that go bump in the night
By Associate Professor Allan Blackman
This article was orignally published in the Otago Daily Times on Monday 10 March 2008.
On the afternoon of February 21st, the USS Lake Erie fired a missile at a non-functioning spy satellite (or as the US Department of Defense [sic] called it “a National Reconnaissance Satellite”) which was about to re-enter the Earth’s atmosphere. The official reason given for the destruction of the satellite was that it had about 450 kg of potentially toxic hydrazine in its fuel tank, which would be dispersed over an area of “two football fields” if it crashed back to Earth. Now I’m not here to neither confirm nor deny the plausibility of this explanation – I’m here to talk about the chemistry of hydrazine.
Hydrazine is a colourless liquid whose melting and boiling points are very similar to those of water. However, that’s where the similarity ends, as hydrazine is an altogether more nasty proposition. The hydrazine molecule is made up of 2 atoms of nitrogen and 4 atoms of hydrogen; the nitrogen atoms are bonded together and each is bonded to two hydrogen atoms. In chemical terms, we speak of hydrazine as being a reducing agent – this means that it desperately wants to give up some of its electrons to anything that will accept them, and therein lies the reason that 450 kg of it was sitting in the satellite.
When liquid hydrazine gives up four electrons, it rapidly turns into nitrogen gas, which just happens to occupy 800 times the volume of liquid hydrazine. This expansion takes place with explosive force, and hence hydrazine and its closely related derivatives have found extensive use as rocket propellants. The Lunar modules on the Apollo Moon missions were powered on landing and take-off from the Moon by hydrazine derivatives, and these are also used in the space shuttle for manoeuvring while in orbit. They are also obviously used for the same purpose in spy satellites.
Of course, the reactivity of hydrazine and its derivatives means that they need to be handled with great care. This was most famously demonstrated in the so-called Nedelin catastrophe in the former Soviet Union in 1960, when an unmanned rocket exploded on the launchpad while it was being fuelled with a mixture of a hydrazine derivative and nitric acid, with the loss of over 100 lives. As was the custom in those days, the accident was hushed up, with relatives of the dead apparently being told that their loved ones had died in a plane crash, and it was not until after the fall of the Soviet Union 30 years later that the full facts of the story emerged.
However, it was not the explosive nature of hydrazine, but its toxicity, that prompted the US to shoot down the satellite last month. Hydrazine is undoubtedly a dangerous substance, and acute exposure to it has been shown to cause damage to the liver, kidneys and central nervous system in humans. But spending an estimated US$60 million to shoot down something that had a miniscule chance of landing anywhere populated (assuming the hydrazine tank survived the enormous re-entry temperatures in the first place) does seem like a remarkable piece of largesse on the part of the US government. Or am I just being cynical?
Subscribe