9 ms·

How does it not happen more often?

Retric 8y ago
ISS travels at a very low orbit which the earth’s atmosphere does a great job of keeping clean. They occasionally manover around anything large enough to be a significant issue. On to of that space is huge and largely empty even at the speed they are traveling. The ISS is also designed to take a hit and can generally survive similar impacts without issue.
antonvs 8y ago
Micrometeoroid impacts happen fairly regularly, but they're usually not energetic enough to make a hole. For example, there was a instance in 2012 where one hit a window: http://www.spacesafetymagazine.com/space-debris/kessler-syndrome/micrometeroid-hit-iss-cupola/ http://www.spacesafetymagazine.com/space-debris/kessler-synd... But as that incident shows, such impacts are often relatively benign - in that case, they didn't even need to replace the outer layer window. The hull is reinforced with four or five layers of shielding - the above article has a nice diagram: https://i2.wp.com/www.spacesafetymagazine.com/wp-content/uploads/2012/06/MMOD-typical-shields.jpg https://i2.wp.com/www.spacesafetymagazine.com/wp-content/upl... This means the usual consequence of a micrometeroid impact is pitting of the outer layers that may not even be noticed. For larger objects that present a serious danger, the US Space Surveillance Network attempts to track them and predict "conjunctions" with a zone around the ISS which extends 2km above and below it, and 25 km in each direction on the orbital path. Anything that's expected to intersect with this zone is analyzed and may result in the station being moved to get out of the way. These are mostly due to space junk of human origin. In 2013, there were at least ~70 of these "conjunction notifications", although this results in only about one move of the station per year, on average. There's a description of how these cases are handled at https://arstechnica.com/science/2013/07/how-nasa-steers-the-international-space-station-around-space-junk/ https://arstechnica.com/science/2013/07/how-nasa-steers-the-... : > "Any object with between a 1-in-10,000 and 1-in-100,000 chance of colliding with the station meets the "yellow" threshold. Flight rules say that the station must be moved out of the way in response to a yellow threshold object unless such a move results in a mission impact—"Like, if we do the burn, we're going to miss an opportunity to launch a Soyuz, for instance," explains Parris. "Do we delay the Soyuz, or do we do the maneuver?" A "red" threshold is assigned to any collision with a likelihood of between 1 (in other words, absolutely certain) and 1-in-10,000. Flight rules are more strict for maneuvers in response to red threshold objects: the station is always moved for a red threshold object, regardless of mission impact, unless a maneuver represents more risk than not maneuvering (for example, if there's a piece of equipment that's damaged on the ISS and a maneuver would exacerbate that damage)." This doesn't help in the case of meteor showers, where the Earth is travelling through a zone full of small debris, typically caused by the trail of a comet. In those cases, the station has to rely on its shielding, but the debris sizes in those cases are almost all within the limits that the shielding can handle.
- hssys 8y ago
  Very informative post, thanks!

ISS travels at a very low orbit which the earth’s atmosphere does a great job of keeping clean. They occasionally manover around anything large enough to be a significant issue. On to of that space is huge and largely empty even at the speed they are traveling. The ISS is also designed to take a hit and can generally survive similar impacts without issue.

antonvs 8y ago

Micrometeoroid impacts happen fairly regularly, but they're usually not energetic enough to make a hole. For example, there was a instance in 2012 where one hit a window: http://www.spacesafetymagazine.com/space-debris/kessler-syndrome/micrometeroid-hit-iss-cupola/ http://www.spacesafetymagazine.com/space-debris/kessler-synd... But as that incident shows, such impacts are often relatively benign - in that case, they didn't even need to replace the outer layer window. The hull is reinforced with four or five layers of shielding - the above article has a nice diagram: https://i2.wp.com/www.spacesafetymagazine.com/wp-content/uploads/2012/06/MMOD-typical-shields.jpg https://i2.wp.com/www.spacesafetymagazine.com/wp-content/upl... This means the usual consequence of a micrometeroid impact is pitting of the outer layers that may not even be noticed. For larger objects that present a serious danger, the US Space Surveillance Network attempts to track them and predict "conjunctions" with a zone around the ISS which extends 2km above and below it, and 25 km in each direction on the orbital path. Anything that's expected to intersect with this zone is analyzed and may result in the station being moved to get out of the way. These are mostly due to space junk of human origin. In 2013, there were at least ~70 of these "conjunction notifications", although this results in only about one move of the station per year, on average. There's a description of how these cases are handled at https://arstechnica.com/science/2013/07/how-nasa-steers-the-international-space-station-around-space-junk/ https://arstechnica.com/science/2013/07/how-nasa-steers-the-... : > "Any object with between a 1-in-10,000 and 1-in-100,000 chance of colliding with the station meets the "yellow" threshold. Flight rules say that the station must be moved out of the way in response to a yellow threshold object unless such a move results in a mission impact—"Like, if we do the burn, we're going to miss an opportunity to launch a Soyuz, for instance," explains Parris. "Do we delay the Soyuz, or do we do the maneuver?" A "red" threshold is assigned to any collision with a likelihood of between 1 (in other words, absolutely certain) and 1-in-10,000. Flight rules are more strict for maneuvers in response to red threshold objects: the station is always moved for a red threshold object, regardless of mission impact, unless a maneuver represents more risk than not maneuvering (for example, if there's a piece of equipment that's damaged on the ISS and a maneuver would exacerbate that damage)." This doesn't help in the case of meteor showers, where the Earth is travelling through a zone full of small debris, typically caused by the trail of a comet. In those cases, the station has to rely on its shielding, but the debris sizes in those cases are almost all within the limits that the shielding can handle.

hssys 8y ago
Very informative post, thanks!