This is a setting that uses Mongoose Traveller 2nd edition for play.

The most straightforward way of engaging with Heavy Sky is for characters to become involved in the Martian Underground. This puts them at the outskirts of Inner System society. They will become embroiled in the internal struggles of the Underground at large. They will become equally embroiled in the struggle of the Underground to produce and distribute medicines and cures for space-born ailments. The first means they will need to navigate a landscape of rival dealers. If the players with to follow a selfless path, they will come into trouble with the more outwardly criminal parts of the Underground. If they wish to pursue a career in organized crime specifically, they will in turn come into conflict with those who would rather help people in less selfish ways. Both of these sides have to evade the law. And both of these sides have to make sure to earn the upkeep for their operations in some ways.

Avatars are encouraged as player options. An Avatar behaves in no way different from a regular person. Most Avatars have the same skillsets and characteristics of regular humans. The point of an Avatar’s existence is to lead the life of a person outside of the collectives, and after a couple of decades return home to add this life back into the consciousness of the Great House that birthed the Avatar. A player character Avatar is generated along the same lines. At the referee’s discretion, the Avatar character can be granted to use a few items of cybernetic implants that would not be available to regular humans.

The Tech Level of Heavy Sky does not map entirely onto Traveller. For one, Heavy Sky is strictly limited to the Solar System, and there is no faster than light travel technology. But there are other achievements that far surpass pre-stellar tech levels, especially with the technological advancement of the Great Houses. Most of the Inner System is hovering around TL12, with some aspects bumping things up to TL13 or TL14. The Great Houses are a distinct outlier, with their vastly advanced technology bumping things up to a baseline of TL15, with some aspects exceeding that.

Regardless, there is no gravity manipulation, no unified field theory, no manipulation of weak and strong forces. There is nothing that goes faster than light.

This means that players are generally restricted to items and ships between TL12 and TL13. Equipment can be selected from all Mongoose Traveller publications, but a focus should be given to the material found in the current edition of the 2300AD books.