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This question may seem pointless for some people, but I think it actually may a real problem for space exploration.

Imagine an interstellar probe that is launched and accelerated up to 20% of the speed of light. This probe will aim at a planet that has been found in the Alpha Centauri system. If we don't want the probe to slow down, how can the probe launch a satellite towards the planet for it to orbit? If the probe zooms by at 20% light speed throughout space, it seems impossible!

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  • $\begingroup$ This is not possible because any probe that you launch has to slow down, all speed is relative and you will need to lose your 20% the speed of light to get into orbit. Also this isn't about world building. $\endgroup$ – A. C. A. C. Dec 12 '17 at 18:36
  • $\begingroup$ This is why we have guidance computers/nav systems and thrusters capable of making course corrections over the duration of unmanned missions $\endgroup$ – anon Dec 12 '17 at 18:36
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    $\begingroup$ This seems like more of a question of orbital mechanics than worldbuilding. $\endgroup$ – sphennings Dec 12 '17 at 19:35
  • $\begingroup$ Use a light sail to decelerate the satellite. Problem is you have to get really close to the star - see e.g. Niven & Pournelle's "The Mote in God's Eye". $\endgroup$ – jamesqf Dec 13 '17 at 4:32
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You specifically asked how to launch a satellite without the probe slowing down. The simple solution is that the soon-to-be-satellite is released from the probe at 0.2c, and through the satellite's own thrusters or gravitational assists (or both), it slows down to about 10 km/s in time to enter the orbit of the planet it will circle.

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  • $\begingroup$ Here is a question showing how Voyager used that gravitational slowdown maneuver. Shed your kinetic energy by moving a celestial body a very little bit! space.stackexchange.com/questions/10195/… $\endgroup$ – Willk Dec 12 '17 at 20:59
  • $\begingroup$ Gravity maneuvers are effectively useless at this sort of velocity. You're going to have to do it the hard way. (Unless you can build a heat shield good enough to aerobrake on the star itself. Yes--hitting the star gives you a longer braking path and thus lower heat than a planet would. We have no idea of how to build a shield like that and you're also talking acceleration exceeding what a gun puts out.) $\endgroup$ – Loren Pechtel Dec 13 '17 at 5:33
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You can't

In order to enter orbit, a satellite will have to slow down to an appropriate orbital velocity. Low Earth Orbit, for example, would be in the range of 10 km/s. If you are going much faster than that, you simply can't enter orbit. The force of gravity will not pull you around the planet; you'll be deflected slightly and then continue off into space.

So if your probe is at 0.2c, which is about 60,000 km/s, you are obviously going way to fast to enter orbit of anything.

So you have to slow down

The obvious solution is to slow down. Your assumption in the question, where you say "If we don't want the probe to slow down" is simply not tenable. Either the probe, or the satellite it launches, is going to have to slow down considerably in order to enter orbit of your desired planet.

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