The International Space Station (ISS) crew prepared for collision with a particle of Chinese debris in space.
On 5 April 2011, a particle was detected and found to be in collision course towards ISS.
The 1.5cm in diameter particle (approximately the size of a bullet), identified as of Chinese origin (debris from a satellite) was detected approaching at high speed, able to cause serious damage should it hit the station.
A red alert was issued because the particle was believed to get as close as 6 km to the Space Station, well within its 25km security zone.
Further tracking form the ballistics team sent good news to the crew when they changed the red alert to green as they calculated the trajectory and found it to be secure.
When objects are detected in time, the crew of the station can take measures to manoeuvre and avoid contact, a procedure known as Debris Avoidance Manoeuvre (DAM). But when objects are small they may avoid detection until it is too late to change course, therefore, when found late, a red alert is issued.
When the object is expected to enter the security space, all parties involved are alerted and the crew prepares for collision. The procedure is that they secure all areas of the station, close hatches, change into spacesuits and take shelter in the Soyuz module at the centre of the station. Upon evacuation the Soyuz is ready to detached and return to earth.
To close or not to close
There was a controversy about closing the hatch of the Soyuz in preparation for impact. The Russian control centre recommended to be “prepared” to close, while NASA preference was to actually close the hatch. The latter recommendation was finally adopted. The concern regarded a potential of hatch failure, which would prevent re-entry into the ISS, but no Russian data of hatch failure is available to date.
An impact to the station – So what?
Because ISS orbits the earth 16 times a day (an orbit every 90 minutes), it travels at a speed of 7 km per second (approximately 25,000 km/hr). Even a static object would impact the station at that speed. Objects travelling in opposite direction would add their speed to that of the station, therefore any object is potentially a high speed projectile that can cause major damage depending where it hits.
The walls of the station are designed with 3 layers, an inner shell that contains the pressurized interior (a pressure of 101.3 kPa (14.7 psi) similar to sea level), an insulation middle layer, and and outer armour plating strong enough to resist impact of small particles.
But what is out there? Isn’t space empty?
After more than 50 years of space exploration there is plenty of rubbish out there, from entire spent rocket stages and defunct satellites, to explosion fragments, paint flakes, slag from solid rocket motors, coolant released by RORSAT nuclear powered satellites, small needles, and many other objects in addition to natural micrometeoroids.
Danger is proportional to the size and speed of a particle, therefore tracking particles is a necessity. Space Command (STRATCOM = U.S. Strategic Command) tracks particles larger than 10cm in size. A 10cm piece of debris would create, at minimum, a 20cm hole in the ISS pressure shell, which may also result in 2 penetration points (one in, one out). A 10cm penetration causes damage that exceeds module critical crack limit and will result in catastrophic loss of that module.
The following damage/effect scenario was outline by the US Navy & Marine Corps Public Health Center, Aerospace Medicine Literature.
- Pressure (Shock) Wave: The explosive release of the kinetic energy of a tracked object impacting ISS may incapacitate crew.
- Fog: Rapid pressure drop results in H2O condensation that may affect visibility.
- Loud Bang: May rupture eardrums and incapacitate the crew even further.
- Light Flash: May produce temporary partial loss of vision.
- Changes to Internal Environment status during the impact: As the Atmospheric Pressure starts to drop, the crew begin to suffer from “some adverse physiological effects, but they are unnoticeable.” This is followed by “Impaired thinking and attention and reduced coordination.”
- At 90 percent O2 Blood Saturation, the crew would suffer “Abnormal fatigue upon exertion. Emotionally upset. Faulty coordination. Poor judgment,” followed by “Very poor judgment and coordination. Impaired respiration that may cause permanent heart damage. Nausea and vomiting.
- The final stages are “Inability to perform vigorous movement. Loss of consciousness. Convulsions. Death.
Clearly, particles have to be taken seriously. So far ISS has entered in Red Alert twice on 6 April 2003 and 13 March 2009. This is the third Red Alert in its history.
Luckily, mission control called off the "shelter in Soyouz" operation before the astronauts completed the emergency procedures. They were happy to hear the good news and said: "That means we don't get to camp out tonight? ... Well, we will bring pizza next time."
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¤ ‘ISS Module Soyuz TMA-12’ (No Date). Dark Government. [Online]. Available here. (Accessed: 05 April 2011).
¤ ‘Space Debris No Threat to Station.’ (2011) NASA. [Online]. Available here. (Accessed: 05 April 2011).
¤ ‘Soyuz TMA-16 launches for journey to ISS – Safe Haven evaluations’. (2009). NASA Spaceflight. [Online]. Available here. (Accessed: 05 April 2011).
¤ ‘STS-111 International Space Station’ (No Date). NASA. [Online]. Available here. (Accessed: 05 April 2011).