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The video Why is the International Space Station 400 km above the Earth? first mentions that there are fairly hard limits at 300 km (due to imminence of reentry) and 700 km (loss of protection from radiation associated with the lower Van Allen radiation belt), and so a value of 400 km, close to the lower limit was chosen based on cost; Space Shuttle launches being expensive.

I'm guessing that the cost argument was based on the need for additional Shuttle launches because they were often maxed out on weight, rather than the cost of extra LOX.

Question: Now that the ISS is built, and unmanned rockets bring the majority of resupply weight to it, are these generally or at least frequently still maxed out on payload weight so that a significant increase in ISS altitude would require significantly more launches and therefore higher cost? Or would a move from 400 km to say 500 or 600 km not significantly lower the number of launches per year for the same resupply and crew transport?

Any other reasons why 400 km would be better than 500 or 600 km?

For the purposes of this question assume the ISS was equipped with solar/electric ion propulsion so that the raising maneuver itself didn't incur substantial costs due to hauling of a lot of fuel, and that there was some reason why say 600 km was considered advantages to 400 km.

Also, while the cost of transport may be decreasing due to developments such as reusability, ignore that since one source of savings would not make another source of savings obsolete.

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  • $\begingroup$ having a frequent resupply schedule means they can lose two launches in a row and still be OK $\endgroup$
    – user20636
    Aug 26, 2018 at 11:21
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    $\begingroup$ At the beginning of ISS operations in 2000 Soyuz spacecraft lift capacity was at the limit to launch 3 persons at 400 km orbit. Current version Soyuz-MS probably can launch them at 600 km orbit. If not - it would mean reduction of crew to 2 persons an negate all benefits. Also SpaceX and Boeing future crafts are planned to carry 4 astronauts and 200 kg of cargo to 400 km orbit. For 600-km orbit it would mean 4-crew without cargo, I suppose. $\endgroup$
    – Heopps
    Aug 26, 2018 at 12:14
  • $\begingroup$ Lower orbit also mean shorter ones; for observations (one of the ISS missions) it is an advantage. It also have shorter night times (ie smaller batteries)... $\endgroup$
    – Antzi
    Aug 27, 2018 at 2:40
  • $\begingroup$ Very related (maybe dup) space.stackexchange.com/questions/10494/… $\endgroup$
    – Antzi
    Aug 27, 2018 at 2:41
  • $\begingroup$ @Antzi Space debris is a good argument, but I'd like to see the actual math worked out for the eclipse duration (and also of interest is frequency and fraction of time spent in eclipse). As far as duplicate, I'm asking specifically about the cost to resupply here and I don't see that there. I don't think you've thought through your "maybe dup" very carefully. I don't think "dupe" should be tossed around casually, excused by a "maybe". $\endgroup$
    – uhoh
    Aug 27, 2018 at 2:54

1 Answer 1

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ISS orbits during shuttle were often kept low to increase the shuttle payload. It had a lot of losses going much above 360km. After the shuttle was retired, the ISS orbit was boosted a bit to 415km+ to further reduce the drag losses.

The main problems you get going higher are:

  • Reduced payload on resupply vehicles
  • Increased sunlight time (needs more cooling)
  • Increased radiation
  • Increased debris collision risk (object dwell time is much higher above 400km, so debris density rises rapidly with altitude)

In 2014 the altitude was decreased from 415-420km to below 405km. In 2015 although the altitude was much lower than it had been for the past several years, it was decided that doing a normal phasing burn might take the station "too high" and they actually propusively lowered the orbit to allow it to stay closer to 405km. This maneuver makes zero sense if higher altitudes were preferable. The press release suggests that this was mainly due to debris risk.

So at least under times of reduced solar stress, the threat of MMOD appears to be such that they don't want to go much higher. It doesn't appear to have been raised above 410km since 2015.

ISS altitude from http://calsky.com

Altitude from calsky.com

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  • $\begingroup$ Excellent! I hadn't thought about reducing the sunlight at all, but now that you mention it, yes those ammonia-based coolers have been the source of a lot of drama over the years. I really appreciate the explanation of the altitude history as well. $\endgroup$
    – uhoh
    Aug 27, 2018 at 4:23
  • $\begingroup$ I am a little bit confused by your graphic showing orbit dimensions. How can the perigee be higher than the apogee (eg 2005)? $\endgroup$
    – le_daim
    Sep 7, 2018 at 6:50
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    $\begingroup$ They use different axes. Perigee on the left, Apogee on the right. More confusing than it has to be, but I didn't find a better source. $\endgroup$
    – BowlOfRed
    Sep 7, 2018 at 8:11
  • $\begingroup$ I've just now noticed that you can write a short answer "well, at least once" to the question How often has (or how common is it for) the ISS's orbit been propulsive lowered intentionally? $\endgroup$
    – uhoh
    Nov 21, 2019 at 6:27
  • $\begingroup$ Some work we did on the ISS orbital debris team indicated that increasing the ISS altitude by 10 km increased the orbital debris flux by 20%, a huge amount for such a small altitude change $\endgroup$
    – Tristan
    Nov 21, 2019 at 15:03

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