The first verified Interstellar Visitor to our Solar System is the object 1I/2017 U1 (aka ʻOumuamua), which is a spindle shaped body that flew in from the direction of the constellation Lyra, came closer to the Sun than Mercury, shot past Earth at a distance of 0.16 AU, then receded rapidly away. Ultimately 1I/2017 U1 will leave with a hyperbolic excess of over 26 km/s.
The Initiative For Interstellar Studies, of which I am a part, was naturally intrigued by the possibility of sending a probe to an Interstellar Object using near-term technologies on a merely decadal mission, rather than millennial. The resulting effort by a great team produced this preprint:
Project Lyra: Sending a Spacecraft to 1I/ʻOumuamua (former A/2017 U1), the Interstellar Asteroid
…which is a starting point for further research work on the options available. Technology Review even picked it as one of the best arXiv Preprints of the Week ending 17 November 2017.
My initial thoughts, for a very rapid mission preparation, would be a clone of the New Horizons vehicle, which successfully visited Pluto/Charon and is heading for a new encounter in 2019, but launched into a Jupiter gravity-assist that would throw it towards the Sun. Why? To maximise the boost, via the Oberth (or “Gravity Well”) Maneuver. I assumed a high-thrust chemical rocket, based on the JPL work for sending probes into the Local Interstellar Medium.
Other options, with lower technology readiness levels (TRLs), would rely on Solar Thermal Boosters, Solar Electric Propulsion, E-Sails, Mag-Sails, and so forth. They’re all good, but 1I/2017 U1 is *rapidly* leaving the Solar System behind. A decade delay in launching means the intercept is out in the “Great Big Dark” between the stars, thus complicating navigation, antenna pointing, and likely science return.
Even if we don’t launch after this particular interstellar vagabond, all our best theories of planet formation suggest immense numbers of such objects are passing by. We’ve only *just* become able to see them, thanks to powerful all-sky surveys that have gone online in recent years.