Some argue that the US government should release its analysis on the risk posed by the USA 193 satellite reentry to justify the cost of the intecept performed last month. (credit: US Navy)

Letter: responding to satellite shootdown

by Yousaf Butt
Monday, March 3, 2008

The most “amazing” and “brilliant” aspects of the recent interception of the USA 193 spy satellite were assuredly not technological: the US had tested a similar ASAT weapon in 1985 against a satellite in an even higher orbit. Yawn. Neither was it particularly brilliant or amazing to outfox the “bureaucracy” as Taylor Dinerman asserts (see “Satellite shootdown overcomes bureaucracy”, The Space Review, February 25, 2008).

No, what was truly amazing and brilliant was how the administration successfully pulled the wool over the public’s eyes. Again. Without so much as mentioning even the outlines of a technical risk assessment, let alone a firm number for the risk to individuals from the hydrazine tank (the ostensible target of the interception), they managed to spend about $50 million of the public’s funds for a thinly disguised ASAT test. (Dinerman admits as much by saying the purported motivation for the interception “was unlikely, but not impossible”.)

Over 100 metric tons of man-made objects re-enter the atmosphere each year, and since the dawn of the space age, more than 5,400 metric tons of such materials are believed to have survived reentry with no reported casualties.

The well-regarded Aerospace Corporation estimates that “the risk that an individual will be hit and injured is estimated to be less than one in one trillion. To put this into context, the risk that an individual in the U.S. will be struck by lightning is about one in 1.4 million.” Furthermore, if the government is seriously concerned about hydrazine toxicity they ought not be carting the material around the country by rail, sea, and roads.

It is almost certain that the hydrazine tank would have exploded upon re-entry anyway, saving the taxpayers $50 million with next-to-no downside risk. NASA administrator Michael Griffin is quoted as saying that “the tank will have been breached – not probably, but the tank will have been breached – because the fuel lines will have been ripped out of the main spacecraft, and so that hydrazine will vent.”

The temperatures deduced from the ablation signatures on previously re-entered (ruptured and empty) containment vessels indicate temperatures in excess of a thousand degrees Celsius. Even if the hydrazine was frozen, the heat conduction across the thin-shell containment vessel (not to mention the re-entry plasma discharges) would have been sufficient to vaporize and auto-ignite the hydrazine in the contact layer. Hydrazine has a high melting point (approximately 1°C) so the fact it may have been frozen is hardly relevant; since the satellite spent half of its time in daylight the hydrazine would hardly be expected to be at deep-space temperatures.

Quite separately, the vaporized hydrazine in the containment vessel contact layer will also exit from the shorn off fuel lines and there will be ignition at those locations also (due to both heat and plasma discharges), leading to a flash back to the main tank and its subsequent explosion as well. This is very similar to what is known to have taken place with the pyrovalves in Telstar 402 (September 8, 1994) and Landsat 6 (October 5, 1993).

Even if the hydrazine had made it to the ground by some miracle, its toxicity is unclear: a major hydrazine rail spill in July of 1991 between Los Angeles and Santa Barbara led to absolutely no casualties, and even its classification as a carcinogen has been challenged.

If there were real reasons to believe that the tank (and the hydrazine) could have made it to the ground and somehow killed people, the odds of this possibility ought to have been released by the government before they spent $50 million of the public’s money on this exercise. Certainly, if saving lives was a concern, this large sum could have been better used, for example, to improve the health care of wounded Iraq war veterans.

Releasing the probability number of death or injury from USA 193’s hydrazine tank arrived at by the government’s analysts (one assumes such an analysis was actually done!) would have had no national security repercussions and yet would have allayed fears that the administration is being less than forthright with the American citizens.

On the other hand, if there were other reasons to carry out this interception those ought to be made public immediately: the American government is accountable to the American people even if it means dealing with some “bureaucracy” from time to time.

Yousaf Butt is a staff scientist at the Center for Astrophysics at Harvard University, where he worked on NASA’s orbiting Chandra X-ray Observatory project from 1999–2004. He was a research fellow at the Union of Concerned Scientists’ Global Security Program from 2005–2007. He holds a BSc in physics from MIT and a PhD in experimental nuclear astrophysics from Yale University.

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