Learn Spanish (Latin America): Para entonces, habrán instalado la gravedad artificial en nuestra cúpula. - By then, they will have installed artificial gravity in our dome.

We use the future perfect tense. This is made by taking the future tense of the helper verb haber (here, habrán for "they will have") and adding the past participle of the main verb, which is instalado.

Why use para entonces instead of por entonces?

We use para here because it sets a deadline or a specific target time in the future, meaning "by then." If you used por entonces, it would mean "around that time" and is usually used when talking about the past.

I don't see the word for "they" in this sentence. How do we know who is doing the installing?

The subject pronoun ellos or ellas (they) is dropped here. We know who is doing the action because the -án ending on habrán is specifically for the "they" or "you all" form.

Is there a trick to remembering that gravedad is feminine (la gravedad)?

Yes! As a general rule, Spanish nouns ending in -dad (like gravedad, ciudad, and verdad) are feminine. They often correspond to English words ending in "-ty" (gravity, city).