Learn Finnish: Satelliittia ei olisi jätetty kiertoradalle. - The satellite would not have been left in orbit.

It combines the negative word ei, the auxiliary verb olisi (would be), and the passive past participle jätetty (left). Notice that the auxiliary olisi is just the standard active third-person form—the main verb (jätetty) is what makes the whole structure passive.

Why is the object satelliittia in the partitive case?

Because the sentence is negative. In Finnish, the object of a negative sentence is almost always in the partitive case. If this were a positive sentence ('The satellite would have been left...'), the object in a passive sentence would take the nominative case (satelliitti).

The English translation says 'in orbit'. Why does the Finnish word kiertoradalle use the allative case ('onto the orbit')?

This is due to the verb jättää (to leave). In Finnish, when you leave something somewhere, the location is treated as a destination answering the question 'where to?'. Therefore, you leave something onto the orbit (kiertoradalle), not in the orbit.

The verb 'to leave' is jättää with a double 'tt'. Why does the stem change in jätetty?