New Horizons is about to reach some very rarefied space, but don’t expect the NASA probe to rest on its considerable laurels.

On Saturday night (April 17), New Horizons will reach 50 astronomical units (AU) from the sun, a distance achieved by just four other operational probes in the history of spaceflight. (One AU is the average Earth-sun distance — about 93 million miles, or 150 million kilometers.)