Despite clouds and rain, oppressive heat and humidity, and myriad logistical challenges, NASA's New Horizons science team members deployed to Sénégal saw what they came to see on Aug. 4. From there, the next flyby target for New Horizons, an ancient Kuiper Belt object nicknamed Ultima Thule, passed in front of a star and momentarily blocked its light. This brief obstruction of starlight (called an occultation) provided information on the size, shape and hazard environment around the distant object. In addition to the data gathered in Sénégal, NASA's Hubble Space Telescope also observed the event, looking for evidence of rings or other potential hazards around Ultima Thule.
"Personnel and logistical support came from around the globe for this event — Sénégal, Colombia, France, Mexico and others. We owe the success of these observations in large part to this amazing, multinational collaboration," said Marc Buie, New Horizons occultation event leader from Southwest Research Institute (SwRI) in Boulder, Colorado. "This work is in preparation for our next New Horizons flyby, coming up fast on Jan. 1, during which we expect to learn much more about Ultima Thule by examining it up close."
"We were there at the start of Sénégal's rainy season, so getting the data we came for was a fantastic achievement, and was accomplished against the odds," said New Horizons assistant project scientist Anne Verbiscer of the University of Virginia. "Our Sénégalese teammates were incredibly passionate about this event, taking full advantage of the opportunity to learn from their American and French counterparts while at the same time providing real mission support for New Horizons."
For this occultation, as well as a similar occultation observed in July 2017, the New Horizons team used NASA's Hubble Space Telescope to monitor the occultation star to probe the region around Ultima for potentially hazardous rings or debris. Hubble's position on the other side of Earth during the midpoint of the July 17, 2017, occultation meant that the orbiting telescope could only probe to a distance of about 12,400 miles (20,000 kilometers) from its target.
"This year the telescope was in a much better position and could probe for rings down to around 1,000 miles (1,600 kilometers) altitude," said New Horizons team member Josh Kammer, of SwRI. "There were no detectable ring signatures for either event, so using the Hubble data we were able to set important constraints on the presence of any rings or dust that could jeopardize a safe flyby."
Ancient Kuiper Belt objects like Ultima Thule hold clues to the formation of planets and the "third zone" of our solar system in which they reside, the wide expanse beyond the giant planets. Ultima Thule, officially known as 2014 MU69, orbits the Sun from more than a billion miles beyond Pluto. The New Year's Day flyby will be the first ever exploration of such a body and the farthest planetary encounter in history.
"We're thrilled with the occultation results," said New Horizons principal investigator Alan Stern, also of SwRI. "And we're excited to soon see Ultima Thule up close on Jan. 1. What a way to welcome in the new year!"
The New Horizons mission team will continue collecting navigation tracking data and sending commands to the New Horizons spacecraft's onboard computers to begin preparations for the flyby, including memory updates, Kuiper Belt science data retrieval, and a series of subsystem and science-instrument checkouts. In early October, New Horizons will make a course correction to refine the spacecraft's trajectory toward Ultima Thule.
Follow New Horizons on its voyage at http://pluto.jhuapl.edu.