Assessing
technological risk is a thorny enough problem here on Earth, even with our
experience and our intuition about familiar uncertainties, factors, and
processes. But transport the problem into the unearthly venue of outer space,
where human experience is limited, and sound assessment becomes astronomically
more challenging.

A
notable and illuminating case in point was the U.S. decision earlier this year
to use a missile to knock out a derelict spy satellite, to head off the
possibility of its splashing a half ton of toxic hydrazine fuel somewhere on
Earth. That official explanation of the shootdown—and, it turns out, an
entirely plausible and credible explanation—nonetheless met with a chorus of
public criticism and skepticism. Coming as it did barely a year after China shot down one of its satellites with a
missile, in what struck many observers as an obvious antisatellite weapon
demonstration, the U.S.
shootdown was widely, but I believe incorrectly, seen as a response to that
event.

To
be sure, the shootdown of the U.S.
satellite was an impressive feat of technology. The Navy missile launched on 21
February 2008 achieved a head-on collision with the 2.3-metric-ton USA 193,
dispersing the contents of the vehicle’s propellant tank [see photo, “Hydrazine
Bomb”] harmlessly in space. To accomplish the intercept, military teams had to
reprogram the guidance system of an antimissile missile designed to target much
slower and lower missiles. That meant designing an intercept orbit that
maximized the infrared brightness of the target and provided a ground track
subsequent to the intercept that passed mostly over water or lightly populated
regions for the first few hours. It also meant training the missile’s onboard
computer not only to home in on the rapidly approaching target outline but also
to shift position at the last possible millisecond to hit a “sweet spot,”
behind which the fuel tank was installed.

“We
were operating on the margins of a system well engineered for a different job,”
a high official at the U.S. Department of Defense’s Missile Defense Agency told
IEEE Spectrum, with a request that his name not be used. No wonder
reporters and the sources they tend to rely on were so quick to dismiss the
official story.

John Pike, who has been one of Washington’s most
frequently quoted space technology specialists for two or three decades, told
reporters that “the claim there was a danger from the fuel is not the most
preposterous thing the Pentagon has ever said—but it seemed to be a bit of a
stretch.” Science commentator Noah Shachtman, blogging for Wired magazine approvingly quoted an
unnamed space security expert‚ who told him: “The cynic in me says that the
idea that this was being done to protect the lives of humans is simply a
feel-good cover story tossed to the media. Having the U.S. government
spend millions of dollars to destroy a billion-dollar failure to save zero
lives is comedic gold.”

Despite
the superficial plausibility of such attitudes, a careful reconstruction of the
analyses and reviews that led to the shootdown decision shows that the official
explanation was in fact the real explanation and that compelling considerations
of health and safety required that the satellite be taken out.

NASA administrator Michael Griffin, whose specialists
performed an independent hazard analysis of US 193 that confirmed the Pentagon’s
conclusions, stated the reason for the shootdown. “The analysis that we’ve done
is as certain as any analysis of this type can be,” he told a press conference
on 14 February. “The hydrazine tank will survive intact [because] the hydrazine
in it is frozen solid. Not all of it will melt. So you will land on the ground
with a tank full of slush hydrazine that would then later evaporate.”

How
did NASA and the Pentagon arrive at that conclusion? To find out I talked with
the two top space officials involved in the deliberation: General Kevin
Chilton, commander of the U.S. Strategic Command, Offutt Air Force Base in Nebraska, and Nicholas Johnson, chief scientist for
orbital debris at NASA’s Johnson Space Center
in Houston.

Gen.
Chilton says he was first approached in a hallway at Cape
Canaveral while attending a December 2007 military summit meeting
on satellite launch costs. The head of the National Reconnaissance Office,
Scott Large, wanted to discuss a problem satellite. “I’m worried about the
reentry,” Chilton recalls Large telling him. “My experts tell me it’s going to
survive reentry.”

Large
told Chilton he had already approached the Missile Defense Agency commander to
alert him to the problem. Soon Chilton and his space staff would be sucked into
a Christmas holiday crash-study project. Within weeks, they were briefing the
National Security Council, and then the president.

When
they got to work, NASA’s Johnson points out, there was already a long-standing
risk-level metric for satellite operations. Hazard mitigation efforts are
deemed necessary beyond a certain threshold number—a 1:10 000 chance of human
fatality.

In
the past, some heavy satellites with faltering control systems, such as the
Compton Gamma Ray Observatory in early 2000, were deliberately deorbited over
open ocean before control was lost and a random fall became inevitable. In the
case of Compton,
NASA had estimated from the beginning that the chances of human casualties from
a random fall were 1:1000, 10 times as high as the safety threshold.

Because
of such considerations, for most of the space age, almost all of the heaviest
satellites—the Soviet Salyut space stations, U.S. military reconnaissance
birds, Russian supply drones, and so on—have used their rocket engines to
terminate their flights safely. Yet a lot of supposedly expert commentary
failed to recognize the prevalence of that active safety measure. “In the
history of the space age, there has not been a single human being who has been
harmed by man-made objects falling from space, [so] there has to be another
reason behind this,” said Michael Krepon, a founding member of the Henry L.
Stimson Center in Washington, D.C.

Just how dangerous was USA 193? Lt. Gen. Henry Obering,
head of the U.S. missile defense effort, announced the quantitative reasons for
the likelihood of human casualty from the satellite’s reentry in a television
interview on 20 July: “It varied depending on which experts we talked to, but
[we got] anywhere between [a] 1 in 45 and 1 in 25 chance,” he said.

“Clearly
nothing prior to USA
193 rose to that level,” Johnson had told me earlier. “The risk posed was much
higher than any risk we’ve ever seen.”

What
made US 193 so singularly dangerous was its toxic fuel payload. “If it had just
been hardware, we would never have considered these extraordinary measures,”
says Chilton. The presence of the toxic chemical, in a tank that was completely
full because the payload had failed immediately after launch, was the unusual
driving factor, Johnson concurs.

A
graphic illustration of the nightmare scenario preoccupying Johnson and Chilton
occurred in October 2004, when an off-course Chinese spy satellite’s film
canister smashed through the roof of a four-story apartment building in the
village of Penglai in southwest Sichuan. But with USA 193, could the hydrazine have
actually reached the surface in a sufficient concentration to hurt anyone?
Wouldn’t the tank be consumed by the heat of atmospheric entry and disintegrate
high above Earth? Here, much press commentary was led astray by a popular
misconception.

There
is a widespread notion that meteorites falling to Earth arrive red hot,
sometimes releasing superheated fumes or setting brush fires, as a result of
the tremendous heating during their passage through the atmosphere. But this is
untrue. Small meteorites actually fall to the ground cold, and under humid
conditions they can even briefly form frost on their surfaces. Though a thin
outer layer is briefly exposed to very hot air, for most of the descent that
air is thinner than the purest vacuum inside thermal-shielding thermos bottles.

NASA’s
Johnson explains the factors used by his team to calculate the likely thermal
history of the hydrazine in the satellite’s tank and conclude that it would
still be frozen on reentry. Heat coming into the structure would be absorbed by
the thermal inertia of the ice, or if it reached sufficient levels, by the heat
of the fusing chemical itself as it partially melted. “Hydrazine requires a
tremendous amount of energy to go from solid to liquid,” he points out.

Those
results are described in a paper by a NASA contractor and have withstood the
scrutiny of independent specialists. Andrew Higgins, an associate professor of
mechanical engineering at McGill University, in Montreal,
researched the scenario and published his results online. Claims that the
tank would be destroyed were “written in apparent ignorance of well-established
heat transfer relations for spacecraft reentry,” he said. “Simple estimates of
the total heat transfer to the tank upon reentry, available in any number of
aerospace textbooks, show that the heating of the tank would probably not have
been sufficient to melt the hydrazine entirely, much less vaporize or ignite
it.”

When it came to making the final decision, Chilton and Johnson
refer to what they call the “regret factor”—the issue of what might follow from
a decision to simply do nothing. “At the end of the day,” asks Chilton, “how
could we look somebody in the eye who had relatives killed or injured?”

Chilton
attended the White House briefings where President George W. Bush was given the
options and the odds, and he remembers Bush’s specific directive that if
something could be done to mitigate the risk to human life, it had to be done.
But what about the risk of people saying it’s really an antisatellite test,
Chilton recalls asking the president. He says Bush responded, “I don’t care
what people will say. We’re doing it for the right reason, and it’s transparent.”

If
amateur experts were quick to express skepticism, real experts knew better.
Anatoly Perminov, a former Russian general once in charge of his country’s
military space program and now the head of the civilian Russian Federal Space
Agency‚ told Russian reporters on 16 February: “In the given situation—if the
satellite is indeed out of control—destroying it is the inevitable and right
thing to do, I think.”

Despite
such statements from people not always trusting of U.S.
motives, the U.S. press to a
great extent echoed those portraying the USA
193 shootdown as aggressive and militaristic, an excuse to threaten China, or a
backdoor gimmick to test space weapons. As a result, a well-defined and
thoroughly researched technological hazard assessment—of a kind that someday,
for better or worse, will be needed again—has wound up buried in obscurity and
obfuscation. This is not an encouraging starting point for the next time such
analyses might be invoked.

 

Guest columns do not necessarily reflect the views of Accuracy in Media or its staff.

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