This article was written by navigator Tali Talasea. All temperatures given in degrees Celsius.

It is often said that geometry drives cannot be used deep in a planetary or stellar gravity well, but the "why" is rarely touched upon. Allow me to cast some light on it.

In reality, it is not impossible to use a geometry drive in a gravity well. As long as both the disintegration and reintegration points are outside of the atmosphere, technically the drive should be able to work. However, the immense majority of modern drives will spit out errors, generally either a generic code 001 "translation failed due to wrong parameters" or a more specific 0011 "translation failed due to projected compensation outside of acceptable range."

So what does it all mean? See, while the geometry drive remains a paracausal device, conservation of energy still applies to it. When a ship translates "uphill" or "downhill" a gravity well, its potential energy is affected. Moving "uphill" means an increase in potential energy, while moving "downhill" means a decrease. Because conservation of energy applies, this difference has to go somewhere. The modification in potential energy is given by the following formula:

DU = -(m*g*DH) where DU is the change in energy in Joules, m the mass of the translated ship, g the acceleration due to gravity and DH the change in altitude related to the surface of the object considered.

Another formula allows to convert this difference into heat. If a ship was to drop from geostationary orbit to a low planetary orbit in a single translation, it would accumulate enough heat to melt on the spot -- and the crew would be killed instantly. If on the contrary a ship was to do the opposite journey, its temperature would drop to several minus thousand degrees, also killing the crew. However, the ship might survive...or would it? Remember, the absolute zero is at -273 degrees and counting, so the residual energy would have to go elsewhere. "Elsewhere" means the drive, which would shatter and become unusable.

This is why flight computers, by default, forbid "uphill" and "downhill" translations in deep gravity wells. The setting can be deactivated, but I strongly recommend to keep it on. Note that, unlike the built-in matter reintegration safety, it only relies on pre-established knowledge of the environment: thus, it is possible to drop deep inside a gravity well during a blind jump. This is part of the reason why explorers always translate right outside an unknown system for their first contact.

Of course, you'll notice that even in deep space, you're always under the gravitational influence of a multitude of objects, but their effect is negligible and thus will not create significant temperature changes. Dropping a bit too close to a planet (we call this "shaving") might heat the ship up slightly, but nothing dangerous.

Two additional notes for the keenest students:

-- Yes, it is technically possible to create an infinite energy machine by having a ship translate uphill with exactly the right parameters, but they are so constrained that said machine will be extremely, extremely pitiful.

-- And yes, it is also possible to cool down a cargo by translating "uphill" for just the right amount of energy difference (let's say -20°C). This is a relatively widespread if a bit unconventional method for flash-freezing sensitive cargo.

I thank Winchell Chung of the Atomic Rockets website for the formula.

Talasea was illustrated by ElenaFeArt as a commission for Starmoth.