The Captured Wanderer

You orbit Nereid at apoapsis - the farthest point from Neptune. You check the distance: 9.6 million kilometers. Neptune is barely visible - a blue-green dot only 0.14 degrees across, hardly larger than Venus appears from Earth.

"Six months ago, we were at periapsis," your companion says, checking logs. "1.4 million km away. Neptune filled the sky - nearly a full degree across. Now look at it. A dot."

You watch Nereid rotate below - a lumpy, irregular moon completing a rotation every 10.8 hours. Unlike most moons, Nereid isn't tidally locked. It spins freely, like an asteroid. Because that's probably what it once was - a Kuiper Belt object that wandered too close to Neptune and got captured.

"How long until we start falling back?" you ask.

"We're already falling," the companion replies. "We passed apoapsis three days ago. Now begins the long fall back toward Neptune. In 180 days, we'll reach periapsis again. Neptune will grow from a dot to a disk filling the sky. And then we'll swing back out, spending another 180 days reaching this point again."

You look at the distant Neptune, its blue color barely visible. Somewhere on that planet, a year passes while Nereid completes one orbit. On Neptune's surface (if it had one), you'd see 536 sunrises. But out here, on captured Nereid, you experience time differently. Wild swings in distance. 800 ten-hour days per orbital year. Neptune growing and shrinking like breathing.

"Time is a circle for most moons," you say. "But for Nereid, time is an ellipse. A long, stretched-out ellipse."

"An ellipse that proves Nereid doesn't belong here," the companion adds. "It was captured. And someday - in a billion years, or ten billion - it will either escape or collide. Nereid's time at Neptune is temporary. Geological temporary. But temporary nonetheless."

You watch the small blue dot that is Neptune, and wonder what Nereid saw when it was still in the Kuiper Belt, before it was captured, when its time was its own.