New Horizons scientists observe stellar occultation by Ultima Thule
As they did three times last summer, NASA’s New Horizons team will observe a stellar occultation by the spacecraft’s second target, Ultima Thule, from the narrow path where the Kuiper Belt Object (KBO) will cast its shadow.
Stellar occultations occur when objects, such as Ultima Thule, pass in front of a star, briefly blocking its light. Data collected during these transits inform scientists about the transiting objects’ shapes, sizes, environments, and various other conditions.
When Ultima Thule, also known as 2014 MU69, passes in front of a star Aug. 4, 2018, the event will be observed by mission members from two 18.5-mile (30-kilometer) strips in Senegal and Colombia identified by both the Hubble Space Telescope and the European Space Agency’s Gaia satellite as the locations where the shadow will fall.
Last year, when the KBO occulted three separate stars in June and July, New Horizons scientists traveled to Patagonia and Argentina and braved extreme winter conditions to observe the events with lines of telescopes set up along the shadows’ paths. Successful observations at five separate sites provided the mission team with crucial information about the shape and size of Ultima Thule and the ideal distance for closest approach.
Based on that data, mission scientists determined Ultima Thule might be a binary system or even a contact binary, in which two objects orbiting one another are so close that they touch each other. Another possibility is that it is a double-lobed objected. It may also have a moon.
Its size is estimated to be about 20 miles (30 kilometers) long if it is a single object. If it is two objects, each is estimated to have a length of nine to 12 miles (15-20 kilomters).
Additionally, using data from last year’s occultations, mission scientists decided the New Horizons spacecraft will fly within 2,175 miles (3,500 kilomters) of Ultima Thule during its closest approach on New Year’s Day 2019.
“Gathering occultation data is an extremely difficult task,” said Marc Buie of the Southwest Research Institute (SwRI), who first discovered Ultima Thule in 2014 and is leading the occultation observation team, as he did last year. “We are literally at the limit of what we can detect with Hubble, and the amount of computer processing needed to resolve the data is staggering.”
Buie said the team of almost 50 researchers using telescopes in Senegal and Colombia are hoping “lightning will strike twice” and they’ll see more blips in the stars.
“This occultation will give us hints about what to expect at Ultima Thule and help us refine our flyby plans,” Buie said.
Ultima Thule is located four billion miles (6.4 billion kilometers) away from Earth, one billion miles (1.6 billion kilometers) farther than Pluto, the spacecraft’s first target, which it flew by in July 2015.
Observing an occultation requires intense preparation, including travel to distant, out-of-the-way locations and bringing along sensitive equipment. Scientists often arrive at these locations several days ahead of the event so they can acclimate to local weather and environmental conditions.
The governments of both Senegal and Colombia have been highly supportive of the research team, according to a New Horizons public statement, which also thanked the U.S. embassies in both countries, the U.S. embassy in Mexico, and the astronomy communities of Senegal, Colombia, France, and Mexico for their assistance in the effort.
“If the team is successful, the results will help guide our planning for the flyby,” said New Horizons principal investigator Alan Stern, also of SwRI.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.