# ISS barely misses space junk-but why the speed differential? **URL:** \ **Category:** Factual Questions\ **Created:** [July 1, 2011, 3:26am UTC](https://boards.straightdope.com/t/iss-barely-misses-space-junk-but-why-the-speed-differential/587472 "2011-07-01T03:26:47Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![rbroome](https://avatars.discourse-cdn.com/v4/letter/r/838e76/32.png) [@rbroome](https://boards.straightdope.com/u/rbroome)\ **Post date:** [July 1, 2011, 3:26am UTC](https://boards.straightdope.com/t/iss-barely-misses-space-junk-but-why-the-speed-differential/587472/1 "2011-07-01T03:26:47Z")
According to NASA a piece of space junk narrowly missed the space station. The relative velocity is given as 29000 mph. Well, they are in orbit… Orbital velocity is about 18000 mph-depending on the altitude. For the relative velocity to be 29K, the junk would have to be traveling in almost the opposite direction. How many satellites have been launched in a totally retrograde orbit-I think the answer is zero. Polar orbit I can understand. It doesn’t matter which direction the satellite travels. But the space station isn’t in a polar orbit. What is going on here? How can the relative velocities be so high? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 1, 2011, 3:34am UTC](https://boards.straightdope.com/t/iss-barely-misses-space-junk-but-why-the-speed-differential/587472/2 "2011-07-01T03:34:08Z")
The Space Station isn’t in a polar orbit, but it is in a highly inclined orbit (52 degrees). A piece of space junk in a similarly inclined orbit, but tilted the opposite way, could be in close to a head-on trajectory with the station. ---
**Author:** ![mlees](https://avatars.discourse-cdn.com/v4/letter/m/ba8739/32.png) [@mlees](https://boards.straightdope.com/u/mlees)\ **Post date:** [July 1, 2011, 3:36am UTC](https://boards.straightdope.com/t/iss-barely-misses-space-junk-but-why-the-speed-differential/587472/3 "2011-07-01T03:36:32Z")
How do you know the piece of junk was some old satellite? The only story I saw said the object was not completely identified. > **[Space Station's Brush with Space Junk Highlights Growing Threat](https://www.space.com/12107-space-junk-threat-growing-space-station.html)** > > A piece of space debris narrowly missed the International Space Station on June 28. More than 20,000 pieces of space junk are tracked every day and the problem keeps growing. ---
**Author:** ![rbroome](https://avatars.discourse-cdn.com/v4/letter/r/838e76/32.png) [@rbroome](https://boards.straightdope.com/u/rbroome)\ **Post date:** [July 1, 2011, 2:10pm UTC](https://boards.straightdope.com/t/iss-barely-misses-space-junk-but-why-the-speed-differential/587472/4 "2011-07-01T14:10:19Z")
> [@mlees](#): > > How do you know the piece of junk was some old satellite? The only story I saw said the object was not completely identified. > > [Space Station's Brush with Space Junk Highlights Growing Threat | Space](http://www.space.com/12107-space-junk-threat-growing-space-station.html) good point. I jumped to that conclusion based on all the discussion about the increasing chance of collision. But of course a meteoriod can approach at any direction/velocity. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 1, 2011, 5:03pm UTC](https://boards.straightdope.com/t/iss-barely-misses-space-junk-but-why-the-speed-differential/587472/5 "2011-07-01T17:03:28Z")
They said it was junk, not a meteroid. But junk can be traveling +/- several degrees of inclination from whatever orbit it was in just before it became junk. I’d also read (in a non-specialist & therefor suspect source) that the closing rate was 21K, not 29K. Which makes the range of possible approach geometries a lot more plausible. ---
**Author:** ![mlees](https://avatars.discourse-cdn.com/v4/letter/m/ba8739/32.png) [@mlees](https://boards.straightdope.com/u/mlees)\ **Post date:** [July 1, 2011, 5:57pm UTC](https://boards.straightdope.com/t/iss-barely-misses-space-junk-but-why-the-speed-differential/587472/6 "2011-07-01T17:57:34Z")
I hope my tone was not interpreted to be hostile in post #3. I was thinking that just from playing billiards, we can see that collisions between objects can result in some pretty fancy changes in vectors. So, any piece of manmade “junk” in orbit colliding with others may adopt a non-standard orbit, especially when compared with the ones we put our satellites into. The junk can be anything from a host of sources. The Wiki article states that only 1/30th of the objects estimated to be bigger than 1cm are tracked. > **[Space debris](https://en.wikipedia.org/wiki/Space_junk)** > > Lists > Categories > Space debris (also known as space junk, space pollution, space waste, space trash, space garbage, or cosmic debris) are defunct human-made objects in space – principally in Earth orbit – which no longer serve a useful function. These include derelict spacecraft – nonfunctional spacecraft and abandoned launch vehicle stages – mission-related debris, and particularly numerous in Earth orbit, fragmentation debris from the breakup of derelict rocket bodies and spacecraft. In addi...