  During the formation and evolution of the Solar System, significant numbers
of cometary and asteroidal bodies were ejected into interstellar space$^{1,2}$.
It can be reasonably expected that the same happened for planetary systems
other than our own. Detection of such Inter- stellar Objects (ISOs) would allow
us to probe the planetesimal formation processes around other stars, possibly
together with the effects of long-term exposure to the interstellar medium.
1I/2017 U1 'Oumuamua is the first known ISO, discovered by the Pan-STARRS1
telescope in October 2017$^3$.The discovery epoch photometry implies a highly
elongated body with radii of $\sim 200 \times 20$ m when a comet-like geometric
albedo of 0.04 is assumed. Here we report spectroscopic characterisation of
'Oumuamua, finding it to be variable with time but similar to organically rich
surfaces found in the outer Solar System. The observable ISO population is
expected to be dominated by comet-like bodies in agreement with our spectra,
yet the reported inactivity implies a lack of surface ice. We show this is
consistent with predictions of an insulating mantle produced by long-term
cosmic ray exposure. An internal icy composition cannot therefore be ruled out
by the lack of activity, even though 'Oumuamua passed within 0.25 au of the
Sun.
