Learn Italian: La loro astronave atterrerà su Marte domani. - Their spaceship will land on Mars tomorrow.

For regular -are verbs like atterrare (to land), the a in the infinitive ending changes to an e when forming the future tense stem. Then you add the future ending. Since l'astronave is an "it" (third-person singular), we add -à to make atterrerà.

Why does atterrerà have an accent mark on the last letter?

In the Italian future tense, the forms for "I" (ending in -ò) and "he/she/it" (ending in -à) always have a written accent. This reminds you to stress that very last syllable when speaking.

Why do we say la loro instead of just loro?

In Italian, possessive words usually need a definite article in front of them. Loro means "their," and because astronave (spaceship) is feminine singular, we use la. Also, unlike other possessives (like mio or tua), loro is invariable and never changes its ending to match the noun.

Why is it just su Marte and not sul Marte?