Artist rendition of New Horizons visiting a proposed post-Pluto cold classical Kuiper belt object in January 2019. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Steve Gribben)

Aug 31, 2015 What's next for New Horizons

Sen—Currently at 33.3 AU away from the Sun and receding ever further away, New Horizons is safely beyond the Pluto system, zipping into the icy depths of the outer Solar System. As of today, the NASA spacecraft is 0.38 AU away from Pluto, moving at a heliocentric velocity of 14.49 kilometers/second. To give you some scale, New Horizons is now about as far away from the dwarf planet as Mercury is on average to our Sun. Starting in mid-September the mission will switch from Pluto observation mode to data download. There's also one more thing scheduled for the next few months; trajectory burns to put the spacecraft in a position to eventually encounter a small planetesimal in the Kuiper belt.

Last week, the New Horizons team announced the selection of their post-Pluto target, currently with the license plate moniker of 2014 MU69. One might think that an ideal choice would be one of the other large dwarf planets in the Kuiper belt that are similar in size to Pluto. Those do make intriguing targets especially in how their surface composition and properties are similar and different to Pluto, but unfortunately due to a quirk of nature, they are all literally on the other side of the Solar System. New Horizons is equipped with 16 hydrazine-fueled thrusters, but there's not enough fuel on board to make the huge velocity change needed to turn the spacecraft around and swing back to one the other icy dwarf planets. Instead, New Horizons basically needed a target in the region in front of the spacecraft.

The hunt for a place to visit after Pluto has been going on since 2011. New Horizons has had a team using ground-based and later the Hubble Space Telescope to search for such a Kuiper belt object (KBO). You might think that this is an easy task, but since its discovery in 1930, Pluto has moved in the plane of the sky closer to the center of our Milky Way. That meant that where Pluto would be in 2015 and beyond is in a region highly crowded by background stars, making it difficult to find moving Solar System objects. In the wide-field surveys I've been a part of looking for distant Kuiper belt objects, we ignored the regions around the galactic plane because there were just too many stars crowding the images. To look into the galactic plane you have to use a technique known as image subtraction where you can remove the light from stationary stars leaving moving Solar System bodies, and variable stars. To make matters worse, with what we know about the size distribution of the Kuiper belt to find something meant being able to go deep to have a chance at even finding a few KBOs, this means a least 4-10m class telescopes on the ground were needed.

The team had been searching for a viable candidate using large ground-based telescopes with no luck after ~3 years. They had found several targets that New Horizons will be able to observe from a distance but none that it can get close to. It took the help of the Hubble Space Telescope to find 2014 MU69. Last June to August 2014, Hubble spent some of its time pointed at the portion of sky that New Horizons could reach, taking several images spaced out over time to detect any motion of a Solar System body. The campaign was successful with 5 KBOs discovered. The list was culled to two potential candidates that could be reached. The first of these candidates PT1 (now known as 2014 MU69) has made the final cut, providing the best fuel budget wiggle room.

There are several dynamical subgroups that compose the Kuiper belt, but broadly speaking one can divide the Kuiper belt into two categories: the 'excited' population and the cold classicials. The cold classicals are objects with very low eccentricity and inclination orbits. 2014 MU69 should be an ordinary run of the mill cold classical Kuiper belt object, which makes it so cool. The cold classicals tend to be smaller and redder than the other populations in the Kuiper belt. They also tend to have a higher binary fraction, so it’s very possible that 2014 MU69 has a companion orbiting it. The observational properties of the cold classicals determined in the past decade have led to the hypothesis that these objects formed in place at their current distance from the Sun, unlike Pluto and the rest of the excited population which was emplaced into its orbit during Neptune's migration outward approximately 800 million years after the Solar System was born. The excited population formed closer to the Sun and represent a different part of the planet formation story. Getting a peek at something that formed out in the cold and icy depths beyond Neptune, will provide us with unprecidented view of our Solar System's history.

The detour to 2014 MU69 is proposed, as New Horizons' prime mission was the Pluto encounter. This next phase requires an extended mission and additional funding from NASA to cover the operation and science costs. Whether or not the mission is extended, the first course correction burn to send New Horizons hurtling towards 2014 MU69 will happen sometime in October of this year. Later a NASA review panel and NASA headquarters will review the extended mission pitch made by the New Horizons team. If this is successful, then the extended mission will officially begin in 2017 with New Horizons performing fly-by observations of 2014 MU69 in about 3 years time, in January of 2019.

2019 might see a long way off, but the New Horizons science team will be plenty busy in the meantime. Most of the data on the Pluto system captured during July is still locked away on the spacecraft. The New Horizons probe beams back data to Earth at a rate of approximately 2,000 bits per second, significantly worse than data download you're using to reading this blog. It will take a total of 16 months to get all of the Pluto fly-by data back to the Earth. By the end of this year, a compressed low-resolution copy of the data will be beamed back to Earth. This first look dataset should be complete by mid November. Then the spacecraft will start the high-resolution playback. The final downlinks of the losslessly compressed high resolution versions of the recorded data will complete by November of next year. This will also be the time to characterize and get to know as much as we can about 2014 MU69 with likely future observations with Hubble and 8-10-meter class optical telescopes to refine the body's orbit and characterize its rotation, shape, and binarity.