An out-of-control satellite is expected to re-enter the Earth's atmosphere in the next couple of weeks, but U.S. officials say they have determined that the risk to humans does not warrant a pre-emptive intercept

"This is the largest NASA satellite to come back uncontrolled for quite a while," NASA official Nicholas Johnson told reporters last week. However, he said, "I hope [people] don't get too concerned, because this is something which is ... a very, very low probability of anyone being hurt, or anybody's property being damaged."

Satellite debris enters the Earth's atmosphere on average more than once daily, but most fragments are small and fall harmlessly into water, according to NASA officials. By contrast, large pieces of the size expected before month's end tumble out of space typically just once a year.

The impending re-entry of NASA's Upper Atmosphere Research Satellite - or "UARS" for short - is reminiscent of another dysfunctional U.S. spacecraft that in early 2008 was on the brink of leaving orbit and hurtling toward the Earth's surface.

Then-President Bush decided the Defense Department would shoot down the "USA-193" spy satellite using a specially modified sea-based Standard Missile 3, which the military initially built for regional and tactical missile defense.

Pentagon officials at the time justified the intercept on the basis that if the spacecraft were allowed to enter the atmosphere on its own, any wreckage could prove dangerous to someone discovering it on the ground, because it might still contain toxic hydrazine rocket fuel (see GSN, Feb. 15, 2008).

Shooting down USA-193 at a precisely targeted location in space instead would shatter the fuel tank into small pieces that would be more likely to burn up and fall harmlessly into an ocean, officials said back then. The Pentagon carried out the intercept on Feb. 21, 2008.

Many skeptics cried foul, unable to believe the government's scenario - that hydrazine would make it through descent without burning up, land on ground rather than in the Earth's vast oceans or unpopulated areas, and be encountered close-up by humans - actually justified the high-technology intercept.

A number of critics alleged that the move to take down USA-193 was secretly based on a desire to signal to China - which had tested an antisatellite weapon a year earlier - that the Pentagon had a satellite-elimination capability of its own (see GSN, Jan. 19, 2007).

Another possible Bush administration motive was to protect secret technologies on board the spycraft from even a remote chance of discovery on the ground by blowing them up prior to re-entry, some experts surmised.

Today's UARS spacecraft - the acronym is widely pronounced YOO-arz - entered into orbit in 1991 and no longer contains rocket fuel. So there is zero danger in this case that a person might unwittingly encounter toxic wreckage, NASA officials said last week.

However, the orbiter is huge in size - at 13,000 pounds, more than twice the mass of USA-193. NASA officials project that during re-entry, the satellite will break up into roughly 26 pieces and the largest of those could weigh approximately 300 pounds.

The space agency has estimated a 1-in-3,200 risk of human fatality as UARS debris hits the Earth. Those might seem like long odds, but the hazard is roughly three times more than NASA's threshold for taking action to protect human life from falling debris.

"NASA, the U.S. government, and some foreign space agencies seek to limit the risk of human injury by re-entering satellites to less than 1 in 10,000," Beth Dickey, a NASA spokeswoman, told Global Security Newswire this week.

The only measures the space agency can take to reduce risk, though, are to design the spacecraft from the outset "to burn up more completely" upon re-entry, or at the end of satellite life "to execute a controlled re-entry," she said.

Neither of these options was available for the $750 million UARS craft, space agency officials said. The satellite was designed and built decades ago, before such systems incorporated materials optimized for burn-up upon returning to Earth, as they are today.

Controlled re-entry also was not a possibility for the satellite because NASA controllers expended all its fuel in lowering the spacecraft's orbit after it ceased mission operations in 2005. UARS would have been orbiting until 2025 had the space agency not succeeded in hastening its re-entry by 14 years to 2011, according to Johnson, chief scientist at NASA's Orbital Debris Program.

If additional fuel had been available on board, scientists could have lowered its orbit even more and carried out a controlled re-entry specifically aimed at avoiding damage on the Earth's surface, Johnson explained during a Sept. 9 teleconference with journalists.

The NASA spokeswoman did not include a Standard Missile 3 intercept or similar intervention as an option for the UARS system, noting that the lack of rocket fuel on the orbiter makes its debris less of a danger than the expiring military satellite posed three years ago.

"USA-193 was destroyed because it contained a large amount of hydrazine propellant that was expected to reach the ground," Dickey said in a written response to questions. "UARS has no hydrazine remaining on board."

Then-Air Force Lt. Gen. Henry Obering, who at the time directed the U.S. Missile Defense Agency, is reported to have said in July 2008 that the risk that USA-193 debris would harm anyone was between 1 in 25 and 1 in 45. The likelihood of harm in that case was chalked up largely to the presence of the toxic fuel that could lead to human injury or death, rather than the possibility of damage directly from impact.

At 1 in 3,200, the chances of human fatality from this month's UARS re-entry are significantly lower than those of USA-193, while still well surpassing NASA's 1-in-10,000 threshold for action to mitigate hazard.

"Throughout the entire 54 years of the Space Age, there's been no report of anybody in the world being injured or severely impacted by any re-entry debris," Johnson said.

Absent interception, was the USA-193 satellite likely to become the first exception to this trend, as it hurtled toward the Earth's atmosphere in early 2008?

Yousaf Butt of the Federation of American Scientists takes issue with the government assumption that rocket fuel aboard USA-193 - judged by the Bush team to be its primary threat - could have endured descent without burning up.

"Even internal NASA reports imply that the [fuel] tank would not have survived in the first place," he told GSN this week.

Based in part on government documents obtained through the Freedom of Information Act, Butt found that NASA and a contracting team had concluded that it was "highly unlikely that the tank would survive the high g-forces and dynamic pressure of re-entry" if left without intercept.

Butt, an FAS scientific consultant, additionally found that an incremental remaining risk of lingering hydrazine that NASA identified was based on false assumptions and "biases toward [fuel] tank survival." That led Butt to conclude in an August 2008 technical analysis that the chances were even slimmer that hydrazine could have posed a danger in any USA-193 satellite wreckage found on the ground.

"Although it should be presumed that the public-health reason for the interception was legitimate and made in good faith, the official study released so far certainly doesn't support the contention that the tank would have survived intact to the ground," he wrote. "In fact, despite its optimistic oversimplifications, the released study indicates that the tank would certainly have demised high up in the atmosphere."

If it was virtually impossible for hydrazine to be present in USA-193 debris, its uncontrolled re-entry ostensibly would have posed much less of a human hazard than the UARS orbiter does today, because USA-193 was considerably smaller in size and mass than the NASA science satellite.

Given that the U.S. government has no intention of intercepting that satellite in the coming weeks, "I have to ask were there were other reasons for carrying out the intercept of USA-193, a satellite less than half the mass of UARS," Butt said in a written response to a reporter's questions. "We will let UARS enter just like many other pieces of orbital debris."

Air Force Lt. Col. April Cunningham, a Defense Department spokeswoman, declined this week to say whether a military or intelligence requirement to protect classified technologies or information on board a descending satellite should be regarded as a legitimate factor in assessing whether to intercept the system.

The Pentagon "has been working closely with NASA to prepare for the UARS re-entry," said the spokeswoman, but she would not address whether defense officials had participated in a NASA assessment of UARS re-entry risks.

Cunningham did say the Defense Department had not considered the possibility of performing an intercept of the satellite prior to its entering the atmosphere.

"[The] USA-193 engagement was a unique circumstance," she said in an e-mail.

"The decision to make that [2008 intercept] was due to the fact that there [were] hazardous materials on board," said Air Force Maj. Michael Duncan, a deputy chief of space situational awareness for U.S. Strategic Command, who also spoke with reporters last Friday. "And since that doesn't exist in this situation, there doesn't appear to be a reason to do those same measures."

Meanwhile, Johnson minimized any impending danger from falling shards of the satellite, saying there were "obviously very low odds that anything or anybody is going to be impacted by this debris."

A NASA briefing released to reporters stated that "UARS does not meet the [international] definition of a risk object," but did not provide additional detail on that standard.

The space agency reports that UARS re-entry is anticipated for Sept. 24, plus or minus one day.

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