July 11, 2017 – NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) was in the right place at the right time to study the environment around distant Kuiper Belt Object (KBO) 2014 MU69, the next flyby target for NASA’s New Horizons spacecraft.
On July 10, researchers using data from NASA’s Hubble Space Telescope and the European Space Agency’s Gaia satellite positioned the flying telescope precisely where MU69 would cast a shadow on Earth’s surface as the KBO passed in front of a star, an event known as a stellar occultation.
SOFIA provided a unique vantage point from which to observe the occultation. Based on calculations from Hubble and Gaia, the New Horizons team knew that the shadow would fall in the middle of the Pacific Ocean, making observations inaccessible to smaller, ground-based telescopes. The flight observatory operates at an altitude above 40,000 feet, allowing the mission team to position a powerful 100-inch (2.5 meter) telescope in the path of the occultation’s shadow.
“Yesterday’s MU69 occultation flight on SOFIA went well. We got great data, but it’ll be weeks before analysis is done,” New Horizons Principal Investigator Alan Stern of the Southwest Research Institute (SwRI) in Boulder, Colorado, said on Twitter.
SOFIA observed the occultation using its guide camera to stare at the star that MU69 passed in front of. With this perspective, it can see ‘blips,’ or decreases in brightness, as objects pass in front of the star. Data from the blips can help determine if MU69 has rings or clouds of debris around it that might present a hazardous environment for the New Horizons spacecraft.
“Working with the New Horizons researchers onboard, our instrument team, and our flight crew, we optimized our observing strategy and collected data throughout the entire occultation event,” said Kimberly Ennico Smith, SOFIA project scientist. “We’re eagerly anticipating the results of the data analysis.”
SOFIA previously supported the New Horizons mission before it made its historic flyby of Pluto.
“Back then, SOFIA was able to be in the center of Pluto’s shadow during the occultation event on June 29, 2015, providing a valuable atmospheric dataset in support of New Horizons,” said Ennico Smith.
But there are differences that make SOFIA’s observations for the MU69 flyby more difficult than those done in support of the Pluto flyby. Because of its small size and large distance from Earth, the shadow cast by MU69 is about 100 times smaller than that of Pluto. This small size, relative to the positional accuracy of the aircraft, made the observation very challenging for SOFIA. The team is elated to have been in the right place at the right time.
In addition to SOFIA’s observation of the occultation, scientists have been sifting through data gathered from observing another occultation on June 3. More than 50 mission team members and collaborators set up telescopes across South Africa and Argentina, along a predicted track of the narrow shadow of MU69 on the Earth’s surface, and collected a two-second glimpse from varying perspectives.
Combined, the pre-positioned mobile telescopes captured more than 100,000 images of the occultation star that can be used to assess the environment around MU69. Although the KBO itself eluded detection, the June 3 data provided valuable and unexpected insights that have already helped the New Horizons mission team. In particular, MU69 may not be as large or as dark as previously thought.
Initial estimates of MU69’s diameter, based primarily on data taken by the Hubble Space Telescope since the KBO’s discovery in 2014, fall in the 12-25 mile (20-40 kilometer) range, but data from the June 3 occultation implies it may be smaller, or may even be a swarm of small objects. There’s a possibility that MU69 is also highly reflective, since it avoided detection from every observing site on June 3.
A third and final occultation will occur on July 17. The Hubble Space Telescope will check for debris around MU69, while team members form another ground-based “fence line” of small mobile telescopes in southern Argentina. This event will offer a much brighter star, allowing researchers to probe more deeply for debris around MU69.
The New Horizons team will use data analysis from all three occultations to plan the best possible journey for the New Horizons spacecraft. In addition to hazard avoidance when it races by MU69 at 35,000 mph (56,000 kph), knowing about the size and reflectivity of the surface will help the team set exposure times on the spacecraft’s cameras and spectrometers.
“Spacecraft flybys are unforgiving,” Stern said. “There are no second chances.”
When New Horizons flies by MU69 on January 1, 2019, it will become the most distant object ever explored by a spacecraft, over a billion miles farther from our sun than Pluto.
MU69 is one of the tens of thousands of KBOs orbiting beyond Neptune. Pluto is also considered a KBO, but it’s giant by KBO standards, and MU69 will reveal what a more typical KBO is like. Because KBOs are so far from the Sun, they are thought to represent a well preserved, frozen sample of what the outer solar system was like following its birth 4.6 billion years ago.
The New Horizons spacecraft carries extra hydrazine fuel for the KBO flyby; its communications system is designed to work far beyond Pluto; its power system is designed to operate for several more years; and its scientific instruments were designed to operate in light levels much lower than it will experience during the MU69 flyby.
New Horizons is the first mission in NASA’s New Frontiers program, managed by the agency’s Marshall Space Flight Center in Huntsville, Alabama. The Johns Hopkins University Applied Physics Laboratory designed, built, and operates the New Horizons spacecraft. The Principal Investigator is Dr. Alan Stern of Southwest Research Institute (SwRI) in Boulder, Colorado. SwRI leads the science mission, payload operations, and encounter science planning. Ball Aerospace built the camera (Ralph) that has delivered stunning images of Pluto and Charon. Students from the Laboratory for Atmospheric and Space Physics (LASP) in Boulder built the Student Dust Counter, the first student-built instrument to fly on a planetary mission.