One Year Until New Horizons

New Horizons will reach Pluto a year from today, and the scientific community is abuzz with speculation about what the space probe might see when it gets there. Meanwhile, the New Horizons team scours the skies for a Kuiper Belt Object that New Horizons can visit after its Pluto flyby.

After hurtling through the solar system for nearly a decade, the New Horizons space probe will finally reach Pluto a year from today, promising exciting new insights about the object formerly known as the ninth planet.

Since its discovery in 1930, Pluto has largely remained shrouded in mystery. Even the Hubble Space Telescope can only discern features larger than a few hundred kilometers on Pluto’s surface (Pluto itself is less than 2,400 kilometers across). From sub-surface oceans to collapsing atmospheres, Pluto is a veritable wellspring of unresolved questions, but scientists anticipate that New Horizons will provide some long-awaited answers.

Speculation and Anticipation

Cathy Olkin (Southwest Research Institute) and colleagues have suggested that Pluto retains its atmosphere year-round. Astronomers previously predicted that as Pluto’s highly elliptical (oval-shaped) orbit carried it away from the sun, the gas in its thin nitrogen atmosphere would freeze and fall to the surface. When Pluto occulted (moved in front of) a background star on May 4, 2013, Olkin’s team studied the effects on starlight passing through Pluto’s atmosphere and fit this data to an atmospheric model developed in 1992. They concluded that their observations could only be explained if Pluto’s atmosphere never collapses.

Alan Stern (Southwest Research Institute), who was not involved in this study, points out that many factors of Pluto’s atmospheric activity were not accounted for in the model Olkin’s team used. “Whether Pluto’s atmosphere collapses or not is, in my view, an open question,” Stern says. However, New Horizons will soon provide astronomers with much more detailed information on Pluto’s atmosphere, including its air pressure, temperature, composition, and escape rate.

Alyssa Rhoden (NASA Goddard Space Flight Center) and colleagues have recently proposed that Pluto’s largest satellite, Charon, might have once hosted an ocean of liquid water beneath its surface. If Charon initially had a much tighter, more elliptical orbit around Pluto, then strong tidal forces could have heated Charon’s interior, possibly enough to allow a liquid ocean beneath the surface. Charon’s current circular orbit is not expected to produce substantial tides, though, so any internal ocean might have long frozen over.

Rhoden’s team predicts that a sub-surface ocean would have caused cracks in Charon’s surface. Other moons around gas giants have shown fractured surfaces with evidence for interior oceans, such as Europa and Enceladus. Observations of Charon’s surface from New Horizons could confirm Rhoden’s sub-surface ocean theory.

Similarly, Amy Barr (Brown University) and Geoffrey Collins (Wheaton College) speculate that such tidal heating might have created an ocean under Pluto’s surface, though remains unclear how long Pluto's interior ocean could remain liquid after Charon's orbit reached its current distance. Barr and Collins await New Horizons data to confirm the existence of tidally driven tectonic activity on Pluto, which would result from such an ocean.

Final Countdown: Hunt for a Kuiper Belt Object

Meanwhile, the New Horizons team is racing to ensure that their space probe will be able to complete the latter half of its mission: visit a Kuiper Belt Object (KBO). New Horizons’ visit to Pluto is only a flyby, because reducing the spacecraft’s speed to enter orbit around Pluto would require 1,000 times more fuel than New Horizons can carry. So the New Horizons proposal accepted by NASA  in 2001 was a twofold mission to explore Pluto and another member of the Kuiper Belt, a debris disk of icy bodies left over from solar system formation.

But finding a KBO for New Horizons’ post-Pluto mission has proved no easy feat. KBOs can be as small as tens of miles across and dark as charcoal. In 2011, the New Horizons team began a ground-based search for possible KBO targets, primarily using the 8.2 meter Subaru Telescope in Hawaii and the 6.5 meter Magellan Telescopes in Chile. The survey has turned up 50 hitherto undiscovered KBOs, but has not found a target that New Horizons can reach with its remaining fuel.

Fortunately, the Hubble Space Telescope recently identified two possible KBO targets. From June 16^th to 26^th, the New Horizons team conducted a set of pilot observations in 20 areas of the sky to test the feasibility of using Hubble to find KBOs. After scrutinizing about 200 images, the New Horizons team found two KBOs, with apparent magnitudes 26.8 and 27.3, much fainter than can be observed using ground-based telescopes.

We will not know if either of these KBOs, designated 1110113Y and 0720090F, is a suitable New Horizons target for another few weeks. However, the discovery of these objects is strong evidence that a wider KBO search with Hubble will prove fruitful. The New Horizons team has been given the green light to use Hubble for a more extensive survey for KBO targets that will examine a field of view with the angular size of the full moon. The new survey began on July 7^th and will continue into August.

References:

Barr and Collins. "Tectonic Activity on Pluto After the Charon-Forming Impact." 25 March 2014.

Olkin et al. "Evidence that Pluto's Atmosphere Does Not Collapse From Occultations Including the 2013 May 04 Event." Icarus online 27 March 2014.

Rhoden et al. "The Orbital and Interior Evolution of Charon as Preserved in its Geologic Record." American Astronomical Society, DDA meeting #45, May 2014.