The asteroid, dubbed 'Oumuamua, was first spotted on October 19 using the University of Hawaii's Pan-STARRS 1 telescope.
"For decades we've theorized that such interstellar objects are out there, and now - for the first time - we have direct evidence they exist", said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate in a statement about the new findings published Monday in the journal Nature.
"This history-making discovery is opening a new window to study formation of solar systems beyond our own", he added.
The VLT was intrinsic to the combined effort to characterize the fast-moving asteroid rapidly, as it needed to be observed before it passed back into interstellar space again.
Thanks to data obtained by the ESO's Very Large Telescope (VLT) at the Paranal Observatory in Chile, the brightness, color and orbit of this asteroid have been precisely determined. But based on its orbit, the astronomers realized that the object came from interstellar space.
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And fortunately for all fans of oddball interstellar interlopers, there will likely be news of more in the near future. Sometimes very elongated objects are contact binaries... but even so, the pieces would be longer than most things in the Solar System, and our analysis shows that it is rotating fast enough that they should not stay together. It is surrounded by the trails of faint stars that are smeared as the telescopes tracked the moving asteroid. Scientists will continue taking observations until 'Oumuamua is too faint to see. These first images confirmed that the object doesn't have a coma - the cloud of dust and gas that fizzes from a comet as it approaches the sun - and is therefore an irregularly shaped asteroid.
"This unusually large variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape". The different-colored dots represent measurements through different filters, covering the visible and near-infrared part of the spectrum.
"Looking at the asteroid light curve database, there are five objects (out of 20,000) that have light curves that would suggest a shape up to an axis ratio of about 7-8 to 1", Dr Meech told the BBC. Scientists estimate the rock is more than 1,300 feet in length.
The long and rocky cigar-shaped object has a burnt dark-reddish hue due to millions of years of radiation from cosmic rays.
Led by Dr. Meech, the team included members from the European Southern Observatory, the Osservatorio Astronomico di Roma, the European Space Agency's SSA-NEO Coordination Center, and the Institute for Astronomy at the University of Hawaii in Honolulu. "And now that we have found the first interstellar rock, we are getting ready for the next ones!" Any further studies will be key given that interstellar asteroids are faint and hard to spot despite the fact that they likely pass through the inner solar system about once a year.