Emily Lakdawalla & Casey DreierSep 01, 2015

New Horizons extended mission target selected

The New Horizons mission has formally selected its next target after Pluto: a tiny, dim, frozen world currently named 2014 MU69. The spacecraft will perform a series of four rocket firings in October and November to angle its trajectory to pass close by 2014 MU69 in early January 2019. In so doing, New Horizons will become the first flyby craft to pass by a target that was not discovered before the spacecraft launched. However, NASA has not yet committed to keeping New Horizons operational long enough to perform science at 2014 MU69; that decision will be made next year, when numerous other solar system extended missions are all up for review.

PT1, the potential target for New Horizons after Pluto
PT1, the potential target for New Horizons after Pluto PT1 is a small, likely lumpy object, only 30-45 kilometers across. The view takes in nearly the entire solar system, with the Sun and the four giant planets appearing as bright stars.Image: NASA, ESA, and G. Bacon (STScI)

The news was greeted by a flurry of excited conversation among astronomers on Twitter. The idea that humans discovered a distant would and would be able to fly past it less than five years later boggled all our minds. Of course, the New Horizons mission would have preferred to find this object earlier. The discovery of 2014 MU69 happened after a long and fruitless search for potential Kuiper belt targets beyond Pluto using ground-based telescopes. Hubble came to the rescue in the summer of 2014, and 2014 MU69 -- also called "PT1" for Potential Target 1 -- has been seen in Hubble images several more times since then, most recently on May 4 and July 4, 2015, allowing astrogators to determine its trajectory well enough to steer New Horizons closer.

After the October trajectory correction, a New Horizons close encounter with 2014 MU69 is guaranteed. Funding to actually do science is less certain. When NASA committed to build New Horizons, it committed to operate the spacecraft and support its science team through its primary mission goal: the Pluto encounter. NASA’s budget projections reflect this (page PS-32 in their 2016 Request, for example). You can see NASA requesting about $20 million per year for New Horizons until 2018, at which point the requests promptly drop to $0.

This isn’t as scary as it looks. It merely reflects that, on paper, the New Horizons mission is only approved through the Pluto encounter, the time it takes to downlink all of the data, and the time it would take to close out the mission. Continued funding is approved every two years based on scientific proposals submitted to NASA and evaluated by an independent review panel. These are called extended missions, and nearly every science mission gets one, owing to the fact that not scuttling a spacecraft actively returning great science to save 0.01% of your budget is a stupidly easy case to make (for the most part).

So while the New Horizons team must make a scientifically sound argument for extended mission funding, it won’t be too hard to do. New Horizons was submitted to a NASA proposal for a “Pluto Kuiper Belt Mission.” The first planetary science Decadal Survey (which prioritizes the scientific goals in the solar system for the decade New Horizons was launched) recommended a mission to Pluto and the Kuiper Belt. New Horizons itself is over-engineered for Pluto (just like Voyager was over-engineered for Jupiter and Saturn). The capability to reach and study a Kuiper belt object beyond Pluto is built in to this mission, and it would be a highly embarrassing and unlikely misstep for NASA to deny a mission extension, particularly a few months after its greatest public outreach moment since the landing of the Curiosity rover.

In the years that remain between now and the encounter, astronomers back on Earth will perform further observations to refine the accuracy of our predictions of 2014 MU69's path, and learn what they can about its physical properties. It won't be easy work. At a visual magnitude of 26.8, 2014 MU69 is too dim a target for most Earth-based telescopes. Astronomers might possibly get more information on it using Subaru and Gemini, but the New Horizons mission can only rely on Hubble for pre-encounter observations, until New Horizons can spot it.

When will New Horizons get us a view that's better than Hubble's? New Horizons mission planner Mark Holdridge answered this and a few of my questions about navigating to a newly discovered object. Conservatively, he said, they expect the first detection of 2014 MU69 to happen only 80 days before encounter, which will be in October 2018. And their first images will be taken in a search campaign, where they will capture a grid of images on the sky overlapping the potential positions of the object, watching over time for the characteristic motion of the nearby target.

As happened at Pluto, they'll have much better knowledge of 2014 MU69's X-Y position on the plane perpendicular to New Horizons' trajectory (called the B-plane) than they will of its position in the third dimension, along the line between it and New Horizons. As they approach it, they'll be able to update that X-Y position to refine their flyby distance and keep 2014 MU69 in the crosshairs of their approach images, but the uncertainty in that third dimension translates into an uncertain flyby time. Their best information on its range to New Horizons will come in the final optical navigation images, which will be taken three days before the flyby.

If you check the JPL small-body browser for information on 2014 MU69's orbit, you might be surprised to see just how uncertain that site thinks 2014 MU69's orbit is; the 1-sigma uncertainty on its perihelion distance is 16 astronomical units, more than a third of its distance from the Sun. Actually the orbit is known much more certainly than that, and therein lies a funny story, as astronomer Simon Porter explained to me:

The standard IAU MPC submission format for spacecraft astrometry is 80 characters wide; in other words, it's designed to fit on IBM punch cards. It leaves room for 15 milliarcseconds of precision in Right Ascension and 10 in Declination. However, the accuracy of our astrometry is better than that, because of both the stability of Hubble and accuracy of our special-purpose star catalog (or rather catalogue) made by Stephen Gwyn and JJ Kavelaars at the Canadian Herzberg Institute. So, Marc Buie invented a modified version of the standard format in order to submit it to the MPC. This is a really unusual thing to do, but we wanted to make sure that we didn't propagate inaccurately rounded data into all the services that pull from the MPC servers.

The automatic service at JPL assumed that the precision was the same as standard ground-based astrometry, and doesn't yet include the data from this year, leading it to produce those enormous (and not real) uncertainties. We're working with JPL to make sure that the correct orbit gets in there soon.

What about a name? Ordinarily, objects can't be named until they've been numbered by the Minor Planet Center, which usually doesn't happen until the object has been observed on several oppositions. There's no hope of precovery images for 2014 MU69 because of its faintness, so it ordinarily wouldn't receive a number for several years after its discovery -- which is to say, after the flyby. Simon told me that "the MPC has in the past made exceptions [to this timeline] for objects that are definitely real and of public interest. The most famous example is 99942 Apophis, which was officially named just a year after discovery, since at the time it was thought to be on a possible impact trajectory with Earth (better orbits since then have ruled out the possibility)." So we can, at least, hope to have something more mellifluous to call the object before New Horizons flies past it.

Here is what we know so far about PT1 or 2014 MU69. It's not much:

  • First imaged by Hubble on June 24, 2014 (then called 1110113Y, the significance of which is explained in a blog post by Amanda Zangari), and discovered in those images on June 27, 2014
  • Magnitude 26.8, meaning it is probably 30 to 45 kilometers in diameter (larger if its surface is dark, smaller if it's bright)
  • Orbit is circular (eccentricity = 0.04) and near the plane of the ecliptic (inclination = 2.5°), making it a Cold Classical Kuiper belt object
  • It takes about 300 years to orbit the Sun
  • New Horizons will fly past it in early January 2019, at a distance of 43.4 AU from the Sun
  • The flyby altitude has not yet been chosen, but it will be "a lot closer" than the Pluto flyby altitude of 12,500 kilometers, "as close as the nav team will let us go."

Things we don't know yet about 2014 MU69, but which New Horizons should be able to tell us:

  • Color
  • Size
  • Mass
  • Density
  • Surface composition
  • Does it have satellites?

What happens after the flyby? Unfortunately, even if Hubble surveys continued, they would be very unlikely to yield another reachable object after 2014 MU69. The little world is at the edge of the classical Kuiper belt; beyond that, the space around the Sun is far more empty.

But in between returning all the Pluto flyby data and encountering MU69, New Horizons will have opportunities to capture distant images of 10 to 20 other Kuiper belt worlds too distant for close flybys. They will never be more than points of light to New Horizons, but just as astronomers have been doing on Earth since the beginning of human thought, New Horizons team members will be learning about the solar system from the motions and variations in those points of light. We can learn their color, which hints at their surface composition; we might get more detailed spectral information on their color. We can see if their brightness varies as they rotate, to learn about the patchiness of their surfaces. We can look for companion points of light, to see if they have satellites -- and if they do, we can learn their masses. If we're very lucky, we'll see them occult background stars, or see binary pairs eclipsing each other.

So New Horizons has a lot of new science to do -- but it has to finish returning its priceless Pluto data to Earth first. Last week, principal investigator Alan Stern announced that the transmission of the browse data set -- including lossy-compressed versions of all the images taken by the LORRI and MVIC cameras -- will begin on September 5. So get ready for exciting new photos from the third zone of the solar system!

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