# Angle of meteoroid and debris strikes on ISS

Is it more likely for a space station like the ISS to suffer micro meteor and space debris strikes on the forward side as oppose to others? (it certainly seems less likely to get them on the underside.) The implication of this is in the layout of a space station design. Would it make sense to locate less important components (like storage) on the forward side to protect more critical components behind.

• "Forward" in the sense of its orbital motion around the earth, or in the sense of its motion around the sun? – DJohnM Aug 23 '16 at 7:05
• For clarification, I am referring to the orientations designated on the ISS. Forward and Aft on the X axis, Port and Starboard on the Y axis and Overhead and Deck on the Z axis. (Deck is toward earth) This is all in orientation to earth orbit. You make a good point. The ISS low earth orbit speed is so much greater than the speed of the earth around the sun I would think it would have a bigger effect. But there may be a sorting there between space debris and natural meteors. – Johnny Robinson Aug 23 '16 at 12:15
• "Greater"? ISS orbital speed=7.66 km/s; Earth's solar orbital speed = 30 km/s – DJohnM Aug 23 '16 at 16:29
• Well then I guess I stand corrected. – Johnny Robinson Aug 24 '16 at 4:30
• The terms of art used in the orbital debris world are "ram" for forward, "wake" for aft, "zenith" for overhead, and "nadir" for deck. These terms go with the velocity vector and the earth vector, as opposed to the other terms, which go with the vehicle body axes. – Tristan Sep 19 '16 at 16:36

The ISS orbital debris threat is worst about thirty degrees to the left and right of the velocity vector in the local horizontal plane (see page 9 of the pdf linked here). At the ISS orbit, pretty much all collisional trajectories are due to differences in orbital plane.

The ISS meteoroid environment is almost completely shadowed in the nadir direction, with a slight bias toward the velocity vector (see page 4 of this pdf). It's important to note that the near-homogeneity of the meteoroid environment is due mostly to the environment being expressed in an earth-centered LVLH coordinate system. In actuality, the meteoroid environment is highly directional in a sun-centric coordinate system, but the orbit around earth, coupled with the steady precession of the orbital plane, tends to "peanut butter" out the variations.

Meteroides are on hyperbolic trajectories relative to earth. It is unlikely to encounter such an object coming from the deck-side, because earth masks most of the possible inbound trajectories and even if if misses the ground, the athmosphere would slow it and probably break it into pieces.

Space debris is on elliptical orbits. Since most missions start in eastward direction in order to make use of the earth rotation, the debris is also more likely to travel eastward at low to medium inclinations (read: angle relative to equator).

Debris hitting the Forward side would be slower than the ISS. That means, the perigee needs to be below the ISS orbit. Since ISS's perigee is only 401km and athmospheric drag starts at 300km, there is not much room for such objects and they will tend to have less relative velocity to ISS.

Debris hitting the Aft side is faster than ISS. There is no limit for the apogee because there is nothing above to brake or catch such debris. Higher relative speeds to ISS are less likely though (the higher the eccentricity, the more time the debris spends farther away from earth, so less likely to encounter such an object).

The most dangerous and the most likely encounter is with an object on nearly circular orbit, but with a different (worst is opposite) ascending node. That means, ISS and debris move in different planes, such debris is approaching from star or portside and will likely have a high relative velocity.