Hubble detects New Horizons’ targets beyond Pluto

Artist's impression of a Kuiper Belt Object (KBO), like the ones New Horizons will flyby after Pluto. Image Credit: NASA, ESA, and G. Bacon (STScI)

From its earliest days, NASA’s New Horizons mission was viewed as having a dual purpose–the first being to fly by and image the Pluto system and the second to visit one or more of the tiny Kuiper Belt Objects (KBOs) beyond Pluto — relics left intact from the solar system’s earliest days. With the help of the Hubble Space Telescope, New Horizons now has a plan after Pluto. 

KBOs are remote and tiny cosmic bodies. As such, any efforts to find KBOs, especially those within New Horizons’ flight path, proved quite challenging.

The search was further complicated by the fact that the target region beyond Pluto is in the direction of the constellation Sagittarius, an area with an extremely high concentration of stars. Two citizen science projects, Ice Hunters and its successor Ice Investigators, enlisted volunteers to sift through many images searching for KBOs the spacecraft could visit after Pluto.

Members of the New Horizons team embarked on a serious search in 2011, using the world’s most powerful ground-based telescopes. Both projects found KBOs, but these were so far away that New Horizons would run out of fuel before reaching them. With only a year to go until the Pluto flyby, the Hubble Space Telescope (HST), in the words of mission team members, came to the rescue.

After applying for time on the HST, the New Horizons team was granted time for a pilot program in June of this year. That project located a few KBOs, all but one too far for a visit, but convinced the Space Telescope Science Institute to grant the team a longer stretch of time between July and September to continue the search.

The effort paid off. Five possible target KBOs, all approximately one billion miles beyond Pluto, were detected. The first, designated PT1 has been determined to be “definitely reachable” while the others, PT2 and PT3, are considered promising. PT4 and PT5 were ruled out. Between now and next summer, the mission team will track PT2 and PT3 to determine whether they are reachable by the spacecraft or not.

Illustration of the Kuiper Belt, and the orbits of both Pluto and Neptune. Image Credit: NASA

While they are about 10 times the size of the average comet, all three KBOs are tiny. Two are estimated to be 34 miles (55 km) in diameter; the third is estimated to have a diameter as small as 15 miles (25 km). All three are about one to two percent the size of Pluto.

“We started to get worried that we could not find anything suitable, even with Hubble, but in the end the space telescope came to the rescue,” said New Horizons science team member John Spencer of SwRI. “There was a huge sigh of relief when we found suitable KBOs; we are ‘over the moon’ about this detection.”

Unlike Pluto, these small KBOs are located in what is known as the Cold Classical region of the Kuiper Belt, well beyond the area containing KBOs in orbital resonances with Neptune.

PT1 was actually discovered during the June pilot project and has already been imaged several times, the first being June 26, the following on August 2, 3, 21, and 23. By studying these images, four separate groups of scientists determined New Horizons will reach PT1, located 43.4 AU from the Sun, in January 2019.

The trip will use up about 35 percent of the fuel remaining in the spacecraft. PT2 and PT3 are brighter than PT1, meaning they may be larger. Further study of all three objects will be done before a final flyby choice is made next summer. Whichever KBO is selected, New Horizons will be sent toward it in a burn late next year, once the Pluto flyby is completed.

The KBO flyby will require NASA approval for an extended mission and additional funding, which the New Horizons team will apply for in 2016.

Each pair of blue rectangles shows the size and position of a single Hubble field. The white stars show the approximate positions where three KBOs that are potentially reachable by New Horizons were discovered. The purple outline shows the Hubble field containing object PT1, and the red outline shows the tiny fraction of that field in which PT1 was found. Image Credit: NASA/JHUAPL

Studying the Kuiper Belt was one of the initial goals for the mission stated in NASA’s 2003 Planetary Science Decadal Survey.

“This has been a very challenging search and it’s great that in the end Hubble could accomplish a detection – one NASA mission helping another,” said Alan Stern of the Southwest Research Institute (SwRI) in Boulder, Colorado, principal investigator of the New Horizons mission.

Even today, little is known about the Kuiper Belt. Because KBOs have never been heated by the Sun the way planets and asteroids in closer orbits have been, they are viewed as frozen samples of primordial materials from the earliest days of the solar system. Understanding them could provide keys to the solar system’s formation.

The Kuiper Belt is home to several small planets such as Haumea, Makemake, and Eris, but all are far too distant for New Horizons to visit.

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Tagged: Hubble Space Telescope Kuiper Belt Object New Horizons Pluto

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.