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A lowest circular orbit to remain stable around the Earth is possible at an altitude of about 95 mi (155 km) while the lowest perigee for a stable elliptical orbit around the Earth would be around 55 mi (88.5 km). I'd like to know these values on the moons Titan and Triton.

Titan's atmosphere reaches very high, and I suppose a stable circular orbit around Titan isn't possible below 500 mi (805 km). Triton's atmosphere on the other hand is very weak but still a more essential one, and you can't orbit Triton just above the surface (like you could above Mercury and many moons) while in Orbiter2016 I approached Triton as close as 33 mi (53 km) above its surface and my spacecraft's instruments still didn't show any air pressure nor any dynamic pressure at all. On Triton's surface my spacecraft would show an air pressure of 1.45 pa.

What would be the lowest possible circular orbit and the lowest possible perigee around Titan and Triton?

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    $\begingroup$ I would say nothing is stable forever. Th elower you go the more atmosphere (if any). Thus I think a btter question would be 'how low could a perigee or orbit be when stable for $time period'. $\endgroup$ – Hennes Jun 10 '20 at 11:18
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    $\begingroup$ @Hennes The question is for one stable orbit (one full revolution around the body). $\endgroup$ – LoveForChrist Jun 10 '20 at 11:45
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    $\begingroup$ One orbit - doesn't count as stable... $\endgroup$ – Rory Alsop Jun 10 '20 at 11:53
  • $\begingroup$ @RoryAlsop One orbit that counts as stable, meaning the orbiter continues its flight and may or may not enter the atmosphere during the 2nd orbit / at its 2nd perigee. $\endgroup$ – LoveForChrist Jun 10 '20 at 11:56
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    $\begingroup$ Ok, quite a lot clearer. But a good answer would need to involve the mass of the object (heavier things are harder to slow down) and the surface area facing the direction of travel. So there will not be any one single answer unless it includes a formula. $\endgroup$ – Hennes Jun 10 '20 at 13:23

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