Having already made its closest approach to the Sun, comet C/2017 U1 is on its way back out of the solar system. Discovered only a few days ago on October 18, 2017, this comet is on an extremely hyperbolic trajectory, with the highest eccentricity (1.18) of any known comet.
This comet may be the first clear case of an Interstellar comet: a comet that is not gravitationally bound to our Sun! The IAU Minor Planet Center is highly interested in having astronomers do follow-up observations on this comet.
At the time of this writing, no size or mass estimates for this comet were available.
Interactive Orbit Diagram for comet C/2017 U1 (Requires Java): Link
UPDATE: Follow-up observations with the Very Large Telescope (VLT) show that the object is not showing any cometary activity; it looks completely star-like. The object has been reclassified as an asteroid, and renamed to A/2017 U1.
For more information, read this Sky and Telescope article: Link