Mission Support of the New Horizons 2014 MU69 Encounter via Stellar Occultations

Scientific Context


Stellar occultation observations will provide the first precise size and albedo results for this very small cold classical KBO, a full 18 months ahead of the 1-JAN-2019 New Horizons encounter. The ability to get this information for MU69, as well as an extremely sensitive search for potentially dangerous (and interesting) rings provides a rare opportunity to fine-tune the spacecraft encounter sequencing of observations. Given that there is no second chance at this flyby — and no other KBO spacecraft encounters likely for decades — the advanced information from the 2017 occultations will provide critical support to improve the scientific return from the upcoming New Horizons encounter. In the broader context of Kuiper belt studies, the cold classical population is thought to be one of the most pristine remnants from the period of planetesimal formation. The albedo of MU69 is a potential indicator of exposed surface frosts, which in turn would constrain heliocentric formation distances and subsequent impact, sublimation and UV-photolysis histories.


Obtain size, albedo and ephemeris information for 2014 MU69 — and perform a ring search in its vicinity — in support of the New Horizons mission to MU69.


Observe MU69 on three occasions as it occults or makes appulses with bright stars on June 3, July 10 and July 17, 2017. Use GAIA catalog star positions and HST astrometry of 2014 MU69 to predict the narrow occultation path. Observe at 2Hz or faster to resolve the object's size and search for narrow rings.

Note that there are currently two potentially useful MU69 occultations predicted for 2018, but neither is as good as the three 2017 events. More importantly, the 2018 events do not provide enough lead time (only about 6 months) to be useful for New Horizons encounter efforts.

Anticipated Results

We anticipate the following results from this observing program: size and albedo information for 2014 MU69 to support the New Horizons 1-JAN-2019 flyby; potential shape information (from multiple chords); an improved orbit solution for MU69 to aid targeting of the New Horizons flyby; a search for rings or other material around the object — potential hazards and new observing targets for the New Horizons mission.

Scientific Justification

The primary goal of this project is to provide mission support for the 1-JAN-2019 New Horizons flyby encounter of 2014 MU69. Occultation observations will be very sensitive to thin rings around MU69 and provide the first size and albedo constraints for this small KBO. With enough measurements we can even determine whether this KBO is non-spherical. The three 2017 events provide critical and fundamental information about the flyby target a full 18 months in advance of the encounter.
SOFIA and ground-based telescopes will both be used for one or more of these events. SOFIA will have four key advantages over the ground-based telescopes that we expect to deploy in South America and South Africa: a much larger aperture, immunity from cloudy weather, the ability to reposition with very short notice and the ability to produce high-SNR lightcurves at 20 Hz. SOFIA's aperture allows frame rates of 20 Hz with SNRs of 19.2 for the faintest of the proposed occultation stars. The event sky velocity is 24 km/s, which means that SOFIA will search for rings with 1.2-km sampling in the sky plane surrounding MU69. Furthermore, model-fitting of the light curves allows SOFIA to measure chords with precisions of 300 m to 50 m (depending on SNR) rather than 1.2 km.
Ground-based observatories, both portable and mobile, are better able to cover a wide swath around MU69 to greatly increase the chances of a successful solid-body occultation. The baseline time resolution we need is 0.5 sec (2 Hz). Given the sky-plane velocity, a D = 40 km body would give a 2 sec central chord with 4 data points. This baseline is set by the parameters of the June 3 event.
Table 1 lists the three occultations that we propose to observe. June 3 and July 17 are to be pursued with ground-based mobile telescopes. July 10 and perhaps July 17 will be attempted with SOFIA. The Feasibility section describes some of the technical parts of this effort.

Note: these paths show the prediction as of mid-2016. See the main page for current predictions.
table one

Science Background & Motivation

Over the past decade, larger KBOs than MU69 have been observed and characterized as follows:

Taken together, these various observations have led to various explanations for the diversity of colors and albedos in the Kuiper belt. For example, [11] consider heliocentric distance and the ability to retain various volatiles: they propose that objects formed at cold classical heliocentric distances (MU69 has a semi-major axis of 44.21 AU, for example) could "uniquely retain NH3, which has been shown to affect irradiation chemistry and could plausibly lead to the unique colors of these objects."
2014 MU69 is much smaller than any cold classical KBO for which size and albedo have already been determined. Because of its size, 2014 MU69 represents an important observational control: a cold classical object that occupies the same heliocentric distance as larger cold classical KBOs but has a much lower surface gravity. The comparative albedos of large KBOs vs. MU69 lets us isolate surface gravity as a factor in the retention or loss of volatiles on cold classical KBOs, or interpret the presence of volatile ices as evidence of a recent event on MU69.

Relevance to New Horizons

Spacecraft flybys are unforgiving — there are no second chances to re-observe the target in response to the flyby observations. The occultations are a rare opportunity to learn something about MU69 before the New Horizons encounter on January 1, 2019 and help this New Frontiers-class NASA science mission better plan its unique flyby of its main target. Here are ways that observations of the MU69 occultations or appulses can help plan the encounter observations.


The three bright stellar occultations by MU69 represent a significant observing challenge. The main difficulty is the small size of MU69 (expected to be between 20 to 50 km), which means that the shadow path on the Earth (also 20 to 50 km wide) is a thin target for SOFIA to hit at precisely the right time. On the ground, the size of the object sets the spacing between observing sites and the number of telescopes dictates the range of prediction error we can cover. The goal is to get a minimum of two chords on the body while covering ±3 sigma of the ground-track uncertainty. Furthermore, the sky velocity of the events are 20 to 24 km s-1, which means that the event durations will range from 1 to 2.5 s. Fortunately, the combination of the GAIA astrometric catalog and HST/WFC3 observations of MU69 will give us incredibly accurate astrometry of the occultation stars and MU69 itself, referenced to a common astrometric network of stars.
Note that getting good limits on rings or dust does not require getting a solid body occultation.


Ground-track predictions are based on HST observations of MU69 and GAIA measurements of the target stars. The HST data is still being collected. The final prediction update for the June 3 event is expected on May 27. The final prediction for the July events will be finalized just in time for the July 10 SOFIA observation. The error in the occultation predictions from HST imaging is close to the expected diameter of MU69 itself.


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