Imagine: an earth-like, inhabited planet slowly grows uninhabitable over hundreds of thousands of years and the atmpsphere no longer supports life. Would it be possible for that change to have made it easier for stuff to enter the atmosphere? To face less (but still some) resistance? I'm not really wanting to just straight-up weaken the atmosphere, but I need a space ship to crash on this planet and not burn up all the crew on entry while also being terraform-able. Thanks so much!
There are a couple of ways an atmosphere can change to make the planet less inhabitable for life-as-we-know-it.
1. The atmosphere can change it’s composition. For example, volcanic eruptions could kick enough bad stuff (sulfur dioxide and hydrogen sulfide can be very nasty) into the air. It would take a lot of volcanoes and quite a bit of time, but it’s plausible. Substitute regular asteroid or cometary impacts kicking up nasty stuff if you prefer that.
2. The atmosphere can get thinner. For many reasons the planet’s atmosphere is getting thinner. This could be from an increase in solar activity or a weakening of the magnetic field around the planet allowing the solar wind to blow the atmosphere away. Maybe the gravity of the planet isn’t strong enough to hold an atmosphere for long and the air has been escaping to space for a long time (it’s been calculated that if we gave our Moon a breathable atmosphere, it would eventually escape into space after a couple of million years). This is the only way to make it easier to get to the surface from space.
However, it’s entirely plausible to have a spaceship built to encounter a mostly-uncontrolled reentry into a planets atmosphere and protect it’s crew. A civilization that can build real spaceships probable has the materials technology to make stuff that can survive a few minutes of very very high temperature of reentry.Mind you, the ship might not be in any shape to re-fly, but that can lead to a great story about a scrappy bad of survivalists on a desolate planet.
Crashed spaceships surviving a crash onto a planet are a staple of science fiction stories, and are completely plausible.