- Conservation of momentum. Any vector you start with you keep -- including your motion relative to a nearby planet, the velocity of that planet around it's star (about 30km/s for earth around the sun), the proper motion of the start and it's motion around the galaxy (if we are headed for Barnard's Star that's 106 km/sec for the radial component of proper motion. Galactic orbital velocity here is about 250km/s but if we keep within a few dozen parsecs of earth that will not really rear its ugly head). These are relatively modest velocities -- if the total delta-v we needed was 500 km/s and our ship could do one G we would deal with the velocity change in about 14 hours. Part of the astrogator's job would doubtless be setting up our departure vector so it nicely complemented our arrival vector.
- Conservation of potential energy. If we try to translate directly from Earth orbit to Jupiter's orbit we would experience a gain of about 700 megajoules of gravitational potential energy per kilogram of our mass. That energy has to show up somewhere - probably as heat, and with no real way to redirect it. The ship probably melts, certainly all the water on this ship (including the crew) boils. On the other hand, if we precisely jump to the right place in Earth's orbit -- or a point in another solar system with the same gravitational potential -- everything is peachy.
- Sufficiently flat space. The acceleration due to solar gravity at 1 AU is about .006 m/s^2 or .0006g. Earth's gravitational acceleration is at a similar magnitude at about 1 light second and is an order of magnitude less at 2 ls -- lunar orbit is about 1ls. If we let folks jump when the sum of magnitudes of acceleration is below, say .001 g that would require a trip of about a light second from earth, or a bit less than a day at 1g - and closely compatible with out worst-case velocity correction time.

This does not address how far the jump gets you or how long it takes -- that can be another post.

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