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Our Solar System’s First Known Interstellar Object Gets Unexpected Speed Boost

Release date: Jun 27, 2018 1:00 PM (EDT)
Our Solar System’s First Known Interstellar Object Gets Unexpected Speed Boost

Using observations from NASA’s Hubble Space Telescope and ground-based observatories, an international team of scientists have confirmed `Oumuamua (oh-MOO-ah-MOO-ah), the first known interstellar object to travel through our solar system, got an unexpected boost in speed and shift in trajectory as it passed through the inner solar system last year.

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Release date: Jun 27, 2018
Our Solar System’s First Known Interstellar Object Gets Unexpected Speed Boost

Interstellar object 1I/2017 U1, named `Oumuamua (pronounced oh-MOO-ah-MOO-ah) received an unexpected boost as it passed through the inner solar system last year, behaving more like an icy comet than a rocky asteroid.

This finding is based on observations with the Hubble Space Telescope and several ground-based observatories, conducted by scientists at the European Space Agency’s Space Situational Awareness Near-Earth Object Coordination Centre (NEOCC), NASA’s Center for Near-Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory (JPL), and the University of Hawaii along with an international team of astronomers.

The calculation, based on the telescopes’ high-precision measurements of `Oumuamua’s position in the sky, found that its motion was perturbed by a force in addition to the known gravitational influences of the Sun and planets.

Although the team considered several possible causes for `Oumuamua’s slight deviation in trajectory, they concluded that the most likely explanation is that the object was jetting out gaseous material — like a comet. This emission of gas could explain the small but measurable perturbation of the object’s path as it headed out from the inner-solar system. This hypothesized outgassing (not directly visible in any observations) was likely produced by heating from the Sun, which caused ices to sublimate and vent away from the object.

"Our analysis suggests that `Oumuamua behaved like a tiny, weird comet," said Marco Micheli of the NEOCC, in Frascati, Italy, and lead author on the paper describing the team's result, published in the June 27, 2018, issue of Nature.

Hubble observations of the interstellar visitor were combined with other precise ground-based observations from the Canada-France-Hawaii Telescope, the European Southern Observatory's Very Large Telescope, and the Gemini South Telescope. Co-author Davide Farnocchia of CNEOS in Pasadena, California, assessed the direction and magnitude of `Oumuamua’s position over a two-month period in late 2017 and early 2018.

"We have evidence from the data that the motion of `Oumuamua was continuously affected by a non-gravitational perturbation, all the way from its discovery to the last observations in January," Farnocchia said. "This additional force we see acting on `Oumuamua is very similar to the kind of perturbation we see in comets from our solar system — which is a result of outgassing."

Comets in our solar system normally eject large amounts of dust and gas when warmed by the Sun. This ejected material forms a cloud called a "coma," and a tail. Surprisingly, even though `Oumuamua passed very close to the Sun — within the orbit of Mercury — no dust or gas was detected, even in the most detailed images. "We did not see any coma, tail, or small dust cloud, which is unusual if this is a comet," said team member Olivier Hainaut of the European Southern Observatory.

The team estimated that if `Oumuamua’s outgassing had contained small dust particles, it could only have amounted to a couple of coffee cans full.

Karen Meech of the Institute of Astronomy at the University of Hawaii in Honolulu, speculated that small dust grains typically present on the surface of most comets may have eroded away during `Oumuamua's long journey through interstellar space. The researchers' computer models, however, don’t rule out the possibility that the interstellar visitor vented larger, coarse dust grains in its journey through the solar system. A sparse cloud of these larger particles would have been too faint to be detected by Hubble or ground-based observatories.

“Despite the many unknowns, we were able to develop a model that is consistent with the observed acceleration — provided this is an unusual comet,” Meech explained. "The more we study `Omuamua, the more exciting it gets. I'm amazed at how much we have learned from a short intense observing campaign. I can hardly wait for the next interstellar object!"

The unexpected perturbation acting on `Oumuamua’s path makes it more challenging for astronomers to accurately trace its trajectory back to the parent star system where it originally formed long ago.

`Oumuamua, which is less than a half-mile in length, was first spotted in October 2017 by the University of Hawaii's Pan-STARRS1 telescope. The interstellar visitor is now farther away from the Sun than Jupiter and traveling away from the Sun at about 70,000 miles per hour as it heads toward the outskirts of the solar system. In only another four years, it will exceed the distance of Neptune’s orbit on its way back into interstellar space.

`Oumuamua is the first interstellar object ever observed, the researchers cautioned, and so it’s difficult to draw general conclusions about this new class of celestial bodies. The observations point to the idea that perhaps low-mass cometary bodies are regularly ejected from other star systems and wander the Milky Way galaxy for billions of years. Therefore, there should be more of them drifting among the stars. Future survey telescopes, such as the Large Synoptic Survey Telescope (LSST) under construction in Chile or NASA’s planned space-based NEO detection and tracking infrared telescope, could potentially detect more of these orphaned vagabonds, providing a larger sample for scientists to analyze to better understand their nature.

The international team of astronomers in this study consists of Marco Micheli (European Space Agency and the INAF, Italy), Davide Farnocchia (NASA Jet Propulsion Laboratory, USA), Karen Meech (University of Hawaii Institute for Astronomy, USA), Marc Buie (Southwest Research Institute, USA), Olivier Hainaut (European Southern Observatory, Germany), Dina Prialnik (Tel Aviv University School of Geosciences, Israel), Harold Weaver (Johns Hopkins University Applied Physics Laboratory, USA), Paul Chodas (Jet Propulsion Laboratory, USA), Jan Kleyna (University of Hawaii Institute for Astronomy, USA), Robert Weryk (University of Hawaii Institute for Astronomy, USA), Richard Wainscoat (University of Hawaii Institute for Astronomy, USA), Harald Ebeling (University of Hawaii Institute for Astronomy, USA), Jacqueline Keane (University of Hawaii Institute for Astronomy, USA), Kenneth Chambers (University of Hawaii Institute for Astronomy, USA), Detlef Koschny (European Space Agency, European Space Research and Technology Centre, and Technical University of Munich, Germany), and Anastassios Petropoulos (NASA Jet Propulsion Laboratory, USA).

JPL hosts CNEOS for NASA's Near-Earth Object Observations Program, an element of the Planetary Defense Coordination Office within the agency's Science Mission Directorate. The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.