Hi there! First I have to explain the title. Ultima Thule usually stands for any place beyond the known world… Actually, we will not leave the Solar System. Ultima Thule is an unofficial nickname for (486958) 2014 MU69, which is the next target of New Horizons spacecraft. Do you remember this spacecraft, which gave us outstanding images of Pluto and Charon in July 2015? That’s just the same one!
After having left Pluto, New Horizons changed its trajectory to 2014 MU69, which will be reached on January 1st, 2019. 6 months to wait then.
Interestingly, 2014 MU69 was unknown when New Horizons was launched in January 2006. Its primary mission was the binary Pluto-Charon and its satellites, and of course it was worth to extend the mission to another body. But choosing this second target was a difficult task, since the distant Solar System, here the Kuiper Belt, is very difficult to observe, and is pretty sparse. This is why observations programs of the Hubble Space Telescope (HST) were dedicated, and 2014 MU69 has been discovered in 2014.
Discovering an object is one thing, determining accurately its motion in view of a rendezvous with a spacecraft is another thing. This is the topic of the study I present today, High-precision orbit fitting and uncertainty analysis of (486958) 2014 MU69, by Simon Porter et al. This study has recently been published in The Astronomical Journal.
The New Horizons spacecraft
New Horizons is the first mission of NASA’s New Frontier program. It was launched in January 2006, and made its closest approach to Pluto in July 2015. Before that, it incidentally encountered the small asteroid (132524) APL at a distance of about 100,000 km in June 2006, and benefited from the gravitational assistance of Jupiter in February 2007.
It carries seven science instruments:
- the Long-Range Reconnaissance Imager (LORRI), which images the encountered bodies,
- the Solar Wind At Pluto (SWAP) instrument, which name is very explicit regarding its goal,
- the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), which supplements SWAP for the detection of high-energy particles,
- Alice, which is an ultraviolet imaging spectrometer,
- the Ralph telescope, which is a photographic instrument,
- the Venetia Burney Student Dust Counter (VBSDC) measures dust. This instrument has been built by students of the University of Colorado,
- and the Radioscience Experiment (REX), which measured the temperature and the atmospheric pressure of Pluto.
As you can see from some of the names of the instrument, Pluto-Charon was definitely the primary goal of New Horizons. Anyway, Pluto is now behind, and New Horizons is en route to 2014 MU69, also nicknamed Ultima Thule.
At this time, our knowledge of Ultima Thule is very limited. This body has been discovered in 2014, from a dedicated observation program on the Hubble Space Telescope, to identify potential targets for New Horizons. Finally, 2014 MU69 has been selected, partly for technical reasons, i.e. it is not so difficult to reach from Pluto.
It was discovered in June 2014, and has an apparent magnitude of nearly 27, and an absolute one of 11. We can guess its size from its magnitude, and its diameter should be smaller than 50 km. So, a very small body. A stellar occultation happening in 2017 has revealed that its diameter should be closer to 25-30 km, and its shape may be bilobal, or it could even be a contact binary.
Observations of its dynamics revealed that it is a cold, classical Kuiper Belt object. Its eccentricity and inclination are limited, since they are not excited by any resonance with the giant planets. So, it belongs to a region of the Solar System, which is quiet from a dynamical point of view.
As I previously said, discovering it is not enough if you want a spacecraft to reach it. You must know its motion accurately, and for that you need more data. And Ultima Thule can be observed only with the Hubble Space Telescope.
Hubble Space Telescope data
The authors disposed of 5 observations of 2014 MU69, by the Wide Field Camera 3 (WFC3) of the HST. Even with the HST, imaging 2014 MU69 requires 6 minutes of exposure, i.e. you need to accumulate photons reflected by 2014 MU69 during 6 minutes to have enough signal.
Another difficulty comes from the number of stars in that region of the sky. This is due to the galactic latitude of 2014 MU69, which is close to 0, i.e. close to the Galactic plane. The images have to be treated, to remove their incoming light. This is not just a pixel, but a diffraction spot, which needs to be modeled to be properly removed.
Once you have these data, you can start to determine the orbit of 2014 MU69, i.e. make ephemerides.
From astrometry to orbit
When you catch a body on a 2-dimensional image of the sky, you get two coordinates. Basically, these coordinates translate into a right ascension, and a declination. And to build ephemerides of a body, you need to integrate the equation ruling the orbital motion. This equations is a second-order 3-dimensional ordinary differential equation.
The motion is ruled by the gravitational perturbation of the Sun and the major planets, and for the problem to be solved, you need initial conditions. These are a position and a velocity of 2014 MU69 at a given date, which you derive from your astrometric observations, i.e. the 5 couples (right ascension, declination).
Easy, isn’t it? No, it’s not! Because of the uncertainties on the measurements, your 10 data, i.e. 5 right ascensions and 5 declinations, do not exactly correspond to an initial condition. So, you have to make a fit, i.e. determine the initial condition, which best fits the observations.
There are many potential sources of uncertainties: the accuracy of the positioning of the HST, the accuracy of the coordinates of 2014 MU69 (remember: this is not a pixel, but a diffraction spot), the duration of the exposure… and also the location of the stars surrounding 2014 MU69 in the field of view. To make absolute astrometry, you need to know precisely the location of these stars, and you get their locations from a star catalogue. Currently, the astrometric satellite Gaia is making such a survey, with a never reached accuracy and comprehensiveness. The Gaia Data Release 2 has been released in April 2018, and gives positions and proper motions (i.e. you can now consider that the stars move from date to date) of more than 1 billion stars! The authors had the chance to use that catalogue. This resulted in predictions, which were accurate enough, to predict a stellar occultation, which has been observed from the Earth.
Predicting stellar occultations
When a Solar System body occultates a star, you can indirectly observe it. You observe the star with your telescope, and during a few seconds, the star disappears, and then reappears, because of this object, which light is too faint for you. Multiple observations of a stellar occultation give information on the motion of the object, and on its dimensions. The rings around Chariklo and Haumea have been discovered that way.
For 2014 MU69 (or Ultima Thule), an occultation has been successfully predicted. It has been observed 5 times on 2017 July 17, in Argentina, giving 5 solid-body chords. This permitted us to infer that 2014 MU69 could be bilobal, or even a contact binary.
A stable orbit
And from these astrometric data, the authors propagated the orbit of 2014 MU69 over 100 million years, in considering the uncertainties on the initial positions. The outcomes of the simulations safely state that 2014 MU69 is on a very stable orbit, with a mean semimajor axis of 44.23 astronomical units (39.48 for Pluto), and an orbital eccentricity smaller than 0.04. This results in an orbital period of 294 years, during which the distance to the Sun barely varies.
We are looking forward for the encounter in 6 months!
The study and its authors
- You can find the study here. The authors made it freely available on arXiv, thanks to them for sharing! And now, the authors:
- The website of Simon B. Porter,
- the webpage of Marc W. Buie,
- the website of Alex H. Parker,
- the webpage of John R. Spencer,
- the one of Susan D. Benecchi,
- the one of Paolo Tanga,
- the one of Anne Verbiscer,
- the one of John J. Kavelaars,
- the one of Eliot F. Young,
- the one of Harold A. Weaver,
- the website of Catherine B. Olkin,
- the webpage of Joel W. Parker,
- and the Wikipedia page of S. Alan Stern.