Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Neptune: Despina
(work in progress)

Despina as photographed by Voyager 2 in 1989
(NASA, Calvin Hamilton)

Despina passing in front of Neptune (from 1989 Voyager 2 photos, but not noticed until 2009)
(NASA, JPL, Ted Stryk, apod090903)
     The white dots (considerably enhanced for visibility) show the position of Despina in four separate frames. The dark spots are the shadow of the moon on the planet. These images may considerably increase the accuracy of our knowledge of Despina's orbit.

Enhanced-brightness (left) and true-brightness (right) sections of the transit composite image.
     The left image shows a small portion of the four-image composite of the 1989 transit of Despina, above. The right image shows a truer view of the same area. In the left image Despina appears as a bright dot against the blue background of Neptune's atmosphere. In the other image Despina appears as a dark dot, similar to the shadows it cast. The reason for its appearing so dark is that its albedo or reflectivity is far less than that of Neptune. Since Neptune reflects more sunlight than Despina, the moon is darker than the planet behind it, though not quite as dark as its shadow. This is one reason it took twenty years for someone (amateur astronomer Ted Stryk) to notice the images of the moon and its shadow.

Data for Despina

Discovered on Voyager 2 photos, July 1989
Named after a nymph, the daughter of Poseidon and Demeter
Orbital size 52,500 km (about 32,500 miles), about 27,700 km (17,200 miles) from Neptune's surface
Orbital eccentricity essentially 0
Orbital inclination close to 0 degrees
Orbital period about 8 hours = 0.33 days
Rotational period unknown (but possibly synchronous)
Diameter about 150 km (about 90 miles)
Mass unknown
Albedo (reflectivity) 6%