# Calculating drop time from a spaceship to a planet

In my story, we have a spaceship orbiting at "LEO" around a foreign planet. The planet has 0.6g gravity. same atmosphere density as Earth. The ship orbits at 600km with an orbit time of 100 minutes. I want to have a "Pod" drop from the spaceship to the planet, using as basic elements as possible (i.e parachutes for the last stage instead of "thrust brakes", minimal to no currection engines in-flight etc.)

I need a bit of help figuring out:

1. how much time would such a drop take?
2. what maximum speed and temperature will the pod reach?
3. how big of parachutes would a small trailer-size pod need to touch ground safely?

Many thanks to anyone who can help with those :) Noam

• What sort of flight profile are you looking for? A pod in orbit can't really be "dropped", it wouldn't go anywhere. You need to apply a significant amount of retrograde thrust in order to either lower the orbital perigee into the atmosphere or to within the surface of the planet Commented Jan 8 at 10:35
• Sorry but a planet with 0.6 (Earth gravity) all other variables being the same is NOT going to have a similar atmospheric density to the Earth itself. Same distance from its sun, identicle radiation output from that sun? Same basic ratios of elemental and chemical composition? The atmosphere of your planet is going to be much less dense than that of Earth
– Mon
Commented Jan 8 at 11:53
• Can you share more details of the pod, there are 1000+ ways to crash it based on your parameters... ok I did understated a little but ðŸ™„ Commented Jan 8 at 12:03
• @Mon What makes you so sure? Take a look at this answer and the comments to a Q of mine. The rate of loss might not be as severe as you think.
– BMF
Commented Jan 8 at 12:06

Estimating drop time is fairly straightforward. The deorbit burn occurs about 180Â° from the drop location on the other side of the planet. The burn faces the opposite direction of travel and slows the spacecraft, shrinking a circular orbit to an ellipse whose periapsis intersects the atmosphere.

The speed at that point will be comparable to circular orbital velocity at that altitude, though somewhat higher. The time to get there is about half the orbital period. If 100 minutes, then about 50 minutes.

Once there, re-entry can take 10-20 minutes. Check out this video of the Orion Spacecraft doing a skip off the atmosphere before re-entering w/parachute deployment:

https://youtu.be/U88DzZcsubs?si=7xGJhRbE06HtOTkV

• That's with a minimal deorbit burn. If they are willing to expend more fuel they can make the transition orbit intersect the atmosphere sooner, even much sooner. Commented Jan 8 at 12:31
• @AlexP OP said something about it being a bare bones type of drop pod, so I went with minimum fuel. But yeah, it wouldn't take too much more delta-v. You can imagine shrinking the periapsis a couple hundred km further, somewhere underground at that point, and where the ellipse would then intersect.
– BMF
Commented Jan 8 at 13:12
• You'd be limited by deceleration forces and heating on encountering the atmosphere. The lower gravity would help with this, both directly by decreasing the amount of velocity you gain from gravity during descent and by increasing the scale height of the atmosphere, giving you a longer path through thin atmosphere before you reach the denser regions. But if you're native, you might also be less tolerant to high accelerations. Commented Jan 9 at 2:14