Many moons in our Solar System have very special characteristics.
One of them is Nereid, Neptune’s third largest moon located behind Triton and Proteus.
It has a diameter of approximately 340 kilometers (211 miles) and its most interesting characteristic is that it has the most ﬂuctuating orbit of any moon in the Solar System!
Its orbital period is 360.16 Earth days and the age of this moon is estimated to be about 4.5 billion years.
Nereid is an irregular satellite orbiting Neptune in a very eccentric and inclined orbit, varying from 9.65 million kilometers (6 million miles) away from the planet to just 1.37 million kilometers (854,000 miles) at its closest position.
It was discovered by Dutch-American astronomer Gerard Kuiper, in 1949, while he was working at the McDonald Observatory in Texas.
Over the years, researchers have also reported Nereid’s unusual brightness according to observations conducted in different periods of time, also from night-to-night.
Nereid is the second of Neptune’s moons to be discovered (and last moon of Neptune to be discovered before the arrival of Voyager 2).
Astronomers have long tried to solve a mystery of Nereid’s eccentric trajectory.
According to one theory, the satellite might be an asteroid captured from the Kuiper asteroid belt in the outer Solar System, which could easier explain its unusual orbit.
It is also possible that Nereid was once an inner moon and was perturbed during the capture of Neptune’s largest moon Triton.
However, to learn much more about Nereid is not any easy task for astronomers.
It is difficult to make astronomical observations of a dark moon Nereid. This celestial body has a surface, composed mostly of ice and silicon, which means it reﬂects only 14 percent of sunlight it receives..
It is extremely faint and Voyager 2 was only able to take a low-resolution image of it when it passed in 1989.