  Asteroid 1I/'Oumuamua is the first observed interstellar object. Its
light-curve amplitude indicates that the object is highly elongated with an
axis ratio of at least 5:1. The absence of such elongated asteroids in the
Solar system, as well as the apparent lack of observed interstellar objects,
are intriguing problems. Here we show that 'Oumuamua may have originated as a
slightly-elongated asteroid about $500\times300$ meters in size. Surface
erosion, caused by interstellar dust bombardment, subsequently increased the
axis ratio. Simply traveling through the interstellar medium for 0.03 to 2 Gyrs
would have sufficed to give 1I its present shape. Passing through a 10 pc dust
cloud with a grain density of $10^{-23} \ \mathrm{g/cm^3}$ at 50 km/s would
have had a similar effect on 'Oumuamua's form. Smaller objects of around 100
meters in diameter can travel the Galactic disk for merely 30 Myrs before they
are disrupted. This could explain the small number of interstellar objects
observed to date.
