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I suspect the question may be a trifle subjective; dependent upon the nature of the propulsion system, the energy levels, as also the nature of the object impinged

The exhaust from a space-craft is very high velocity (Wikipedia says as high as Mach 10. In atmosphere (perhaps as high as LEO) the exhaust probably loses energy relatively quickly, or at near distance.

When travelling in deep-space, how long/far would it take for the ejecta to lose energy? How closely could space-craft, as depicted in popular literature, 'fly' in 'formation'?

Say, a space-craft is under assembly in orbit, how closely could an astronaut equipped with an MMU follow his/her/it's buddy?

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It varies tremendously based on the engine involved and what's being protected. Back in the day when Shuttle deployed comsats with solid fuel boost stages, the satellite would be tens of miles away at ignition but the Shuttle would maneuver to protect the windows. For Shuttle EVA, the Space Shuttle Flight Rules show that the safe distance for a suited crewmember was 27 feet for the main jets, 3 feet for the vernier jets, and 3 feet for the APU exhaust. It's rule A-15-22 in the document. This rule is probably a simplified version of what would actually be managed to; I remember seeing "Keep Out Zones" (KOZ) defined for all kinds of things on the Orbiter.

The buzzword for this is "plume impingement" if you want to google for other specific cases.

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    $\begingroup$ One minor update from the Google Search. Plume impingement is the super-set for various types of impingement; this one is cross-impingement. $\endgroup$
    – Everyone
    Commented Feb 16, 2015 at 18:46
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    $\begingroup$ The type of fuel is important as well. The PAM-D motors used on so many satellites used solid fuel that created an exhaust full of aluminum oxide ejecta. Unlike liquid fuel exhaust, this stuff is super nasty from an orbital debris standpoint -- dense and hard. $\endgroup$
    – Tristan
    Commented Feb 16, 2015 at 20:20

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