  We have performed a detailed dynamical study of the recently identified
Neptunian Trojan 2004 KV18, only the second object to be discovered librating
around Neptune's trailing Lagrange point, L5. We find that 2004 KV18 is moving
on a highly unstable orbit, and was most likely captured from the Centaur
population at some point in the last ~1 Myr, having originated in the Scattered
Disk, beyond the orbit of Neptune. The instability of 2004 KV18 is so great
that many of the test particles studied leave the Neptunian Trojan cloud within
just ~0.1 - 0.3 Myr, and it takes just 37 million years for half of the 91125
test particles created to study its dynamical behaviour to be removed from the
Solar system entirely. Unlike the other Neptunian Trojans previously found to
display dynamical instability on hundred million year timescales (2001 QR322
and 2008 LC18), 2004 KV18 displays such extreme instability that it must be a
temporarily captured Trojan, rather than a primordial member of the Neptunian
Trojan population. As such, it offers a fascinating insight into the processes
through which small bodies are transferred around the outer Solar system, and
represents an exciting addition to the menagerie of the Solar system's small
bodies.
