Pluto’s atmosphere studied during stellar occultation; New Horizons images distant KBO
This illustration shows how New Horizons’ Alice ultraviolet spectrometer instrument “watched” as two bright ultraviolet stars passed behind Pluto and its atmosphere. The light from each star dimmed as it moved through deeper layers of the atmosphere, absorbed by various gases and hazes. Image Credit: NASA/JHUAPL/SwRI
NASA’s New Horizons mission continues to send back valuable information about the Pluto system even as the spacecraft begins taking its first images of a more distant Kuiper Belt Object (KBO).
The most recent data returned from New Horizons’ digital recorders are images taken by the Alice ultraviolet spectrometer as two stars passed behind Pluto. As the points of light passed through the many layers of Pluto’s atmosphere, they dimmed due to absorption by the atmosphere’s gases and hazes.
New Horizons was 200,000 miles (320,000 kilometers) past Pluto when Alice observed the occultation on July 14, only four hours after closest approach. Just before the flyby, the instrument had used sunlight to collect data about the structure and composition of Pluto’s atmosphere.
Both the solar and stellar occultations showed the ultraviolet spectra of nitrogen, methane, acetylene, and haze in the atmosphere and yielded consistent reports about the upper atmosphere’s temperature and pressure.
The first two of the 20 observations that New Horizons made of 1994 JR1 in April 2016. The Kuiper Belt object is the bright moving dot indicated by the arrow while the dots that do not move are background stars. The moving feature in the top left is an internal camera reflection (a kind of selfie) caused by the illumination by a very bright star just outside of LORRI’s field of view; it shows the three arms that hold up LORRI’s secondary mirror. Image & Caption Credit: NASA/JHUAPL/SwRI
These findings indicate that concentrations of nitrogen, methane, and other hydrocarbons, are consistent in the upper atmosphere over many regions on the planet.
Pluto’s upper atmosphere is 25 percent colder and more densely packed than scientists had thought prior to the flyby. Its nitrogen escape rate is 1,000 times lower than initially predicted.
While the mission team awaits approval for an extended mission to fly by KBO 2014 MU69 on Jan. 1, 2019, the spacecraft has already conducted two distant observations of 1994 JR1, its first post-Pluto KBO.
The 90-mile (145-kilometer) wide object, orbiting more than three billion miles (five billion kilometers) from the Sun, was photographed by the Long Range Reconnaissance Imager (LORRI) from a distance of 170 million miles (280 million kilometers) in November 2015 and then from a distance of 69 million miles (111 million kilometers) on April 7–8 of this year.
November’s observation set a record for the closest imaging of a KBO, which the April observation then broke.
“Combining the November 2015 and April 2016 observations allows us to pinpoint the location of JR1 to within 1,000 kilometers, far better than any small KBO,” said New Horizons science team member Simon Porter of the Southwest Research Institute (SwRI) in Boulder, Colorado.
By constraining its orbit, the mission team also successfully ruled out a theory that JR1 was a loose satellite of Pluto.
JR1’s rotation period, found to be once every 5.4 hours, was determined by observations of the changes in the amount of light the KBO’s surface reflected.
“That’s relatively fast for a KBO,” said John Spencer, a New Horizons science team member, also of SwRI.
Spencer said these observations serve as practices for the extended mission, which, if approved, may include close-up observations of up to 20 more KBOs.
Jorge Nunez, deputy systems engineer for LORRI as well as a planetary scientist, published the latest in a series of blog posts by New Horizons team members May 18 describing his role during last summer’s Pluto encounter.
The post, titled Imaging the Encounter of a Lifetime, notes Nunez’s responsibility to make sure LORRI was healthy, functioning, and successfully capturing the desired images. He specifically focuses on the camera’s use for navigational purposes and scientists’ excitement as Pluto grew from a small dot to a complex planetary system during the seven months preceding the flyby.
During closest approach, the instrument captured more than 1,800 images of Pluto and its moons, which are still in the process of being returned from the spacecraft.
New Horizons scientists used light curve data – the variations in the brightness of light reflected from the object’s surface – to determine JR1’s rotation period of 5.4 hours. Image Credit: NASA/JHUAPL/SwRI
Laurel Kornfeld
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.
