Halimede (HAL-ih-MEE-dee) is a small irregular satellite (a satellite with a distant, often highly inclined, eccentric or retrograde orbit) of Neptune. Its average distance from Neptune is just over 10 million miles, but it has a somewhat eccentric orbit and is only 7 million miles from Neptune at perineptune, and 13 million miles from Neptune at aponeptune. The orbit is very highly inclined, making an angle of nearly 70 degrees with the rotational plane of the planet, and Halimede moves backwards (or retrograde) relative to the rotational direction of the planet. Given its large distance from Neptune its rotational period cannot have any relationship to its orbital motion, and is therefore unknown.
All the outer moons of Neptune are assumed to have albedos (or reflectivity) of about 4%. If that is correct, Halimede's brightness corresponds to an object about 35 miles in diameter. If it were assumed to have a density between 1 and 2 times that of water, its mass would be 100 to 200 trillion tons (1 to 2 times 10 to the 14th power). However, since the albedo could be substantially higher (or even a little lower) than the assumed value, its size and mass are essentially unknown.
Halimede's orbit is not much further from Neptune than Nereid
's, and possible perturbations of their orbits suggest a nearly 50% probability of a collision between the two over periods comparable to the age of the solar system. That and the fact that it has a dull gray color very similar to that of Nereid has led to the suggestion that the two may be fragments from some ancient collision.