Neptune trojan

Neptune trojans are bodies in orbit around the Sun that orbit near one of the stable Lagrangian points of Neptune. They therefore have approximately the same orbital period as Neptune and follow roughly the same orbital path. Seventeen Neptune trojans are currently known, of which thirteen orbit near the Sun–Neptune L₄ Lagrangian point 60° ahead of Neptune and four orbit near Neptune's L₅ region 60° behind Neptune. The Neptune trojans are termed 'trojans' by analogy with the Jupiter trojans.

The discovery of 2005 TN53 in a high-inclination (>25°) orbit was significant, because it suggested a "thick" cloud of trojans (Jupiter trojans have inclinations up to 40°), which is indicative of freeze-in capture instead of in situ or collisional formation. It is suspected that large (radius ≈ 100 km) Neptune trojans could outnumber Jupiter trojans by an order of magnitude.

In 2010, the discovery of the first known L₅ Neptune trojan, 2008 LC18, was announced. Neptune's trailing L₅ region is currently very difficult to observe because it is along the line-of-sight to the center of the Milky Way, an area of the sky crowded with stars.

It would have been possible for the New Horizons spacecraft to investigate 2011 HM102, the only L₅ Neptune trojans discovered by 2014 detectable by New Horizons, when it passed through this region of space en route to Pluto. However, New Horizons may not have had sufficient downlink bandwidth, so it was decided to give precedence to the preparations for the Pluto flyby.

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