Only two planets in our solar system are moonless: Mercury and Venus, the planets closest to the Sun. But 50 years ago this month, it looked like Mercury might have to drop off the list — but only for a while.

The excitement began as Mariner 10 sped toward the planet, in 1974.

Mariner was the first spacecraft to visit Mercury. Before then, astronomers were limited to views through telescopes. But Mercury is so close to the Sun that it could be hard to see a moon in a tight orbit around the planet.

On March 27th, just two days before Mariner’s closest approach, one of its instruments showed a jump in the ultraviolet energy coming from the planet. Three days later, as Mariner flew away from Mercury, the source of energy seemed to pull away from the planet, as though it were a separate object — like a moon.

Mariner’s cameras didn’t see a moon, though. So scientists quickly realized that they were seeing not a moon, but a distant star. The star was so hot that it was producing a lot of ultraviolet energy, which Mariner picked up.

Mercury probably is moonless because it formed so close to the Sun. Solar heat and radiation made it impossible for Mercury to hold on to the material that might have formed a moon.

Mercury is in the evening sky the next few days. It’ll stand farthest from the Sun for its current evening appearance on Sunday. It looks like a bright star, quite low in the west not long after sunset.

Script by Damond Benningfield