One of the puzzles over Neptune’s moon Triton is its retrograde orbit: it moves in the opposite direction to the mother planet’s rotation.

Many of the small outer moons of Jupiter, Saturn and Uranus also have retrograde orbits, almost certainly because they are captured objects.

But Triton is different. It is the seventh largest moon in the Solar System, bigger even than Pluto. So explaining its origin is not as simple as saying it must be a rock that flew too close. And yet that’s what many planetary geologists believe.

The one good look we’ve had of Triton came in 1989 during the Voyager 2 flyby. It turns out that Triton is an icy body, probably with a metallic core. And it has a thin nitrogen atmosphere

It is also geologically active. One of Voyager 2’s most astounding discoveries were nitrogen geysers erupting from Triton’s surface. This surface is also relatively smooth, meaning the craters must be have been covered relatively recently.

The current thinking is that Triton was not a rock but probably a Kuiper Belt object that was somehow pushed Neptune’s way. But since we don’t really know what Kuiper Belt objects look like, it’s hard to tell.

So Bruno Christophe at ONERA - The French Aerospace Lab in Chatillon and a few amis want to go and have a look. Their plan is to send a spacecraft to fly past Neptune, take a good look at Triton, and then fly on to find a Kuiper Belt object to study. If they look remotely similar, then that’ll back up the idea that Triton really is from the Kuiper Belt.

That’s not a bad problem to solve in a single mission (although the choice of Kuiper Belt object will obviously be crucial)..

At the same time, Christophe and co want to use the mission to study gravity at great distances from the Sun. Their idea is to throw some light on the possibility that gravity is somehow different at large distances from the Sun, something that various theories predict and that the Pioneer space probes seem to have experienced, in an effect called the Pioneer anomaly.

That’s an ambitious medium sized mission (about 500 kg) .Christophe and co proposed their Outer Solar System Mission to ESA last year in the hope that it would also attract funding from NASA and launch in the 2020 timeframe.

Sadly it seems to have missed the cut this time. ESA announced its medium-sized mission candidates in February and OSS wasn’t in it. That’s understandable given the similarities to Pluto Express, which is currently en route for a fly past with the dwarf planet and then heading to an as-yet-undetermined Kuiper belt object.

But a journey to Neptune ought to be on the cards at some point in the future. Triton is spiralling into its mother planet. Some 3.6 billion years from now, the giant moon is destined to be torn apart by Neptune’s tidal forces, and so will eventually forming a ring.

Which means ESA better get a move on.

Ref: arxiv.org/abs/1106.0132: OSS (Outer Solar System): A fundamental and planetary physics mission to Neptune, Triton and the Kuiper Belt