Introduction2014 MU69 – a Kuiper Belt object nicknamed “Ultima Thule” – will soon become the farthest, most primitive object that humanity has ever seen up close.

    NASA’s New Horizons mission will map the geology and composition of this mysterious far-flung object. It will also look to see if MU69 has a coma, a fuzzy haze around it made of vaporizing water and other ices, and if it has any satellites or rings. The spacecraft’s close approach will be Jan. 1, 2019 UTC.

    Fuzzy dot against a field of stars.
    MU69 as seen by New Horizons on Dec. 2, 2018.

    New Horizons, which made its historic flyby of Pluto in 2015, was the impetus for the discovery of MU69. Scientists were searching for an object in the Kuiper Belt that the spacecraft could study after Pluto, and their next target had to be reachable on New Horizons’ remaining fuel. Using large ground-based telescopes on Earth, researchers began looking in 2011 for candidate objects, and searched multiple times per year for several years. But objects that would work for New Horizons were just too distant and faint to be seen through Earth’s atmosphere.

    NASA’s Hubble Space Telescope came through, however. MU69 was first spotted by Hubble on June 26, 2014, discovered by members of the New Horizons team. Based on its position and the New Horizons’ trajectory, this object was ideal for the spacecraft’s post-Pluto adventure.

    MU69 is so small and far away that it is impossible to see directly from Earth, but scientists have been able to take advantage of a special type of astronomical event called an occultation. This is when the object passes in front of a star from the vantage point of Earth. This event is only visible from certain parts of Earth, however. The New Horizons team combined data from Hubble and the European Space Agency’s Gaia satellite to figure out exactly when and where on Earth’s surface MU69 would cast a shadow. They determined that occultations would occur on June 3, July 10 and July 17, 2017, and set off for places around the world where they could see MU69 cover up a different star on each of these dates. Based on this string of three occultations, scientists were able to trace out the object’s shape – which may actually be two objects stuck together, or one long object.

    In the summer of 2018, nearly 50 New Horizons team members headed to Senegal and Colombia for another occultation event. They reported success in obtaining valuable information about MU69 that will be useful for the spacecraft’s flyby.

    In August 2018, New Horizons took its first images of MU69 for navigation purposes. Images will get better into autumn, with best close-ups taken on Jan. 1, 2019.

    Key Dates

    Key Dates

    June 26, 2014 - NASA’s Hubble Space Telescope locates MU69.

    June 3, 2017 - MU69 passes in front of a star from the vantage point of Earth – an occultation event that allows for important new observations. New Horizons team members observe from Argentina and South Africa.

    July 10, 2017 – Another stellar occultation for MU69. NASA's airborne Stratospheric Observatory for Infrared Astronomy (SOFIA) used its powerful 100-inch (2.5-meter) telescope to see if MU69 had any debris around it that would pose a hazard to New Horizons

    July 17, 2017 – MU69 passes in front of a third star. Hubble checks for debris around MU69, while New Horizons team members observe from small mobile telescopes in Argentina in the path of the occultation.

    Aug. 4, 2018 – New Horizons team members braced themselves for another MU69 occultation, this time stationed in Colombia and Senegal. They faced challenging weather conditions, but prevailed in collecting data that will help prepare for the spacecraft’s 2019 flyby.

    Summer/Fall 2018 – New Horizons begins taking images of MU69 to help mission planners guide the spacecraft toward this target. Each batch of images will be better than the best, representing the first time MU69 has been seen so close.

    Jan. 1, 2019 (projected) – New Horizons flies by MU69. The farthest planetary encounter in history will include a close look at the object’s geology and composition as well as search for satellites and rings.


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