Is Asteroid Expansion Pack free?

Yes. Asteroid Expansion Pack is released under Open Source / Community and costs nothing. Neither it nor Endless Sky has any paid content.

What do I need to run Asteroid Expansion Pack?

Endless Sky, which is itself free. Asteroid Expansion Pack is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from Endless Sky's mods list. You can also install it by hand — full steps are on this page.

Roughly 40 MB, on top of the base Endless Sky installation.

Yes. The source is published at github.com/chemtrailoperator/Asteroid-Expansion_EndlessSky under Open Source / Community.

Asteroid Expansion Pack — Free Endless Sky Mod · PlayBound