Astronaut chief Steve Lindsey claims the risk from orbital debris to the International Space Station (ISS) will be negligible to nonexistent, after the US Navy shoots down the failed USA 193 (NROL-21) military satellite.
Lindsey, who briefed the astronaut corp in an e-mail on Friday, also used the opportunity to re-emphasize that the only reason the US Government ordered the satellite’s destruction via an intercept mission is to mitigate the risk of the spacecraft’s toxic hydrazine fuel surviving re-entry.
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The National Reconnaissance Office satellite was launched on a Delta II vehicle from Vandenberg Air Force Base back in December 2006, before the spacecraft failed shortly after reaching orbit.
The paralyzed satellite is out of contact and thus out of control, meaning it will re-enter Earth’s atmosphere sometime in mid March, along with a full tank of frozen hydrazine – which might survive the fiery descent.
‘All, I wanted to provide some additional information to you about the US Government’s decision to attempt to ‘shoot down’ a malfunctioning satellite,’ noted Lindsey in the e-mail acquired by L2. ‘The satellite was launched into orbit in late 2006 – and subsequently lost communications.
‘It is about to re-enter and break up, and the concern is that the spacecraft’s hydrazine tank (nominally used for orbital maneuvering and controlled deorbit at the end of useful life) will survive to the ground, potentially putting people at risk of exposure to the chemical.’
Lindsey referenced the commonality between spherical tanks that survived re-entry intact when Columbia broke apart over Texas, and the tank on USA 193. However, those tanks on the orbiter were protected by Columbia’s Thermal Protection System (TPS) through the opening part of re-entry – to around Mach 18 – before the orbiter finally succumbed.
‘We learned from Columbia that spherical tanks (like this one) do survive entry intact,’ added Lindsey. ‘The area of risk is about the size of two football fields if the tank lands intact. In order to mitigate this risk, we will attempt to intercept the satellite just prior to entry (at about 130 NM HP) with a Navy missile.’
The Navy will launch one of its Standard Tactical Missile from an Aegis cruiser, once the spacecraft is at an altitude of around 150-160 statute miles. The astronaut chief noted the possible results that can be expected from the intercept.
‘There are three possible outcomes: We miss (no change to the situation), We graze it (perturbing the orbit of whatever pieces remain, and almost certainly causing them to enter more quickly), or we hit the hydrazine tank (destroys the satellite and vents the tank prior to entry),’ he added, before confirming the evaluations have been ongoing long before the official announcement last week.
‘NASA has been fully engaged in this for the past several weeks and will continue to be so. To be clear the only reason for this ‘shoot down’ is to protect against the hydrazine risk; if the satellite didn’t have a full tank we would let its orbit decay and break-up normally during entry.’
Lindsey’s e-mail, which was sent to all NASA astronauts prior to the questions relating to the US government decision were asked by the media during a press conference with STS-122 commander Steve Frick and ISS Expedition 16 commander Peggy Whitson, also briefed the corp on the risks associated with the intercept.
‘The primary concern for our human spaceflight program is obviously the risk of increased orbital debris. First of all, shooting at the satellite when it’s just about to enter the atmosphere significantly reduces risk,’ Lindsey added.
‘With the current plan, after the target is hit more than 50 percent of the debris will enter the atmosphere within two orbits. The rest will be down in no more than a few weeks. This is significantly different than the ASAT test last year (by China), in which a satellite was hit at around 850 KM – that debris will be in orbit for decades.
‘Additionally, the plan is to wait until STS-122 has landed before taking the shot, so there will be no additional risk to the Shuttle. Risk to ISS is negligible to nonexistent (orders and orders of magnitude less than normal ISS spaceflight risks) – and trajectories will be analyzed to pick the ‘optimum’ windows which decouple ISS and the satellite’s orbits.’