Nereid (moon)

Since 1987 some photometric observations of Nereid have detected large (by ~1 of magnitude) variations of its brightness, which can happen over years and months, but sometimes even over a few days.

Thermal modeling based on infrared observations from the Spitzer and Herschel space telescopes suggest that Nereid is only moderately elongated with a maximum aspect ratio of 1.3:1, which disfavors forced precession of the rotation.

[13] Its spectrum appears to be intermediate between Uranus's moons Titania and Umbriel, which suggests that Nereid's surface is composed of a mixture of water ice and some spectrally neutral material.

[13] The spectrum is markedly different from minor planets of the outer solar system, centaurs Pholus, Chiron and Chariklo, suggesting that Nereid formed around Neptune rather than being a captured body.

[13] Halimede, which displays a similar gray neutral colour, may be a fragment of Nereid that was broken off during a collision.

[9] However, this determination was later disputed, and other researchers for a time failed to detect any periodic modulation in Nereid's light curve from ground-based observations.

[8] If selected, The Arcanum mission[19] would do a flyby of Nereid before its primary purposes of orbiting Neptune and observing Triton.

Gerard P. Kuiper, discoverer of Nereid
Nereid's highly eccentric orbit around Neptune.
True color NASA image of Neptune
True color NASA image of Neptune