In January 2006 the New Horizons spacecraft was launched to explore Pluto and its moons and, if all went well, proceed onward to another object in the Kuiper Belt of the outer solar system, Pluto being one of the largest, closest, and best known members. New Horizons was the first spacecraft launched from Earth directly on a solar system escape (interstellar) trajectory (the Pioneer and Voyager probes had earlier escaped the solar system, but only with the help of gravity assists from Jupiter and Saturn). It was launched from Earth with such velocity (16.26 km/sec) that it passed the Moon’s orbit in just nine hours, a distance that took the Apollo missions three days to traverse.
In February 2007, New Horizons flew by Jupiter at a distance of 2.3 million km, using the planet’s gravity to increase its speed to 23 km/sec, thereby knocking three years off its transit time to Pluto. While passing through the Jupiter system, it used its instruments to photograph the planet and its moons. There were no further encounters with solar system objects until arrival at Pluto in 2015, and the spacecraft spent most of its time in hibernation, with most systems powered down to extend their lives, reduce staffing requirements for the support team on Earth, and free up the NASA Deep Space Network to support other missions.
As New Horizons approached Pluto, selection of possible targets for a post-Pluto extended mission became a priority. In orbital mechanics, what matters isn’t so much distance and speed but rather “delta-v”: the change in velocity needed to divert the trajectory of a spacecraft from where it is currently headed to where you want it to go. For chemical rockets, like the thrusters on New Horizons, this depends entirely on how much propellant is on board, and this resource would be scarce after expending what was required for the Pluto mission. New Horizons was launched with propellant to provide 290 metres/sec delta-v, but most of this would be used in course corrections en route to Pluto and maneuvers during the Pluto encounter (the scientific instruments are fixed to the spacecraft structure, which must be turned by firing the thrusters to aim them at their targets.) Starting in 2011, an observing campaign using large Earth-based telescopes began searching for objects in the Kuiper belt which might be suitable targets for New Horizons after Pluto. These objects are extraordinarily difficult to observe: they are more than four billion kilometres from Earth, small, and mostly very dark, and thus visible only with the largest telescopes with long exposure times under perfectly clear and dark skies. To make things worse, as it happens, during this time Pluto’s orbit took it past some of the densest star fields of the Milky Way, near the centre of the galaxy in the constellation of Sagittarius, so the search was cluttered with myriad background stars. A total of 143 new Kuiper belt objects were discovered by this search, but none was reachable with the 33 kg of hydrazine monopropellant expected to remain after the Pluto encounter.
It was time to bring a bigger hammer to the job, and in June 2014, time on the Hubble Space Telescope was assigned to the search. By October of that year three potential targets, all too faint to spot with ground-based telescopes, had been identified and called, imaginatively, potential targets PT1, PT2, and PT3. The course change to get to PT1 would use only around 35% of New Horizons‘ remaining fuel, while the others were more difficult to reach (and thus less probable to result in a successful mission). PT1 was chosen, and subsequently re-named “2014 MU69”, along with its minor planet number of 486958. Subsequently, a “public outreach” effort by NASA chose the nickname “Ultima Thule”, which means a distant place beyond the known world. A recommendation for an official name will not be made until New Horizons reveals its properties.
The fly-by of Pluto in July 2015 was a tremendous success, fulfilling all of its scientific objectives, and in October 2015 New Horizons fired its thrusters for sixteen minutes to change its velocity by 10 metres per second (equivalent to accelerating your car to 22 miles per hour), setting it on course for Ultima Thule. Three subsequent burns would further refine the trajectory and adjust the circumstances of the fly-by. This was the first time in history that a spacecraft was targeted to explore an object which had not been discovered when launched from Earth. After transmitting all the data collected in the Pluto encounter to Earth, which took until October 2016, New Horizons went back into hibernation.
In June 2018, the spacecraft was awakened and in August 2018 it observed its target with its own instruments for the first time. Measurement of its position against the background star field allowed precise determination of the inbound trajectory, which was used in final course correction maneuvers. At the same time, the spacecraft joined Earth-based telescopes and the Hubble in a search for possible moons, rings, or dust around Ultima Thule which might damage the spacecraft on a close approach. Had such hazards been found, the fly-by would have been re-targeted to be at a safer distance, but none was found and the original plan for a fly-by at 3500 km was selected.
Although New Horizons is bearing down on its target at a velocity of 14.4 km/sec, it will remain just a faint dot until hours before closest approach at 05:33 UTC on New Year’s Day, January 1st, 2019. Other than its position, brightness, and colour (reddish), little or nothing is known about the properties of Ultima Thule. We don’t know its size, shape, composition, temperature, rate of rotation, albedo (reflectivity), whether it is one object or two or more in close orbit or in contact, or anything about its history. What is almost certain, however, is that it is nothing like anything in the solar system we’ve explored close-up so far.
Its orbit, unlike that of Pluto, is that of a conventional, well-behaved member of the Sun’s extended family. The orbit, which takes Ultima Thule around the Sun every 296 years, is almost perfectly circular (eccentricity 0.045) and close to the ecliptic (2.45°). (By contrast, Pluto’s orbit has an eccentricity of 0.25 and an inclination to the ecliptic of 17°.) This makes it probable that Ultima Thule has avoided the cosmic billiards game which has perturbed the orbits of so many distant objects in the solar system, making it a “cold classical Kuiper belt object” (the “cold” refers not to temperature but its analogue in dispersion of velocity). What this means is that it is highly probable that this body, unlike the planets and moons of the inner solar system, which have been extensively reprocessed from their original constituents, has been undisturbed since the formation of the solar system 4.5 billion years ago and is a time capsule preserving the raw materials from which the inner planets were assembled.
In 2017, predictions of Ultima Thule’s orbit indicated that it would pass in front of, or occult, a distant star, with the shadow passing through southern Argentina. Since the distance to the object and its speed in orbit are known reasonably well, simply by measuring the duration of the star’s occultation, it is possible to compute the length of the chord of the object’s path in front of the star. Multiple observing stations and precise timings allow estimating an object’s size and shape. A network of twenty-four small telescopes was set up along the expected path (there is substantial uncertainty in the orbit, so not all were expected to see the occultation, but five succeeded in observing it). Combining their results yielded this estimation of Ultima Thule’s size and shape.
The best fit was to a close binary or “contact binary”: two lobes, probably originally separate objects, in contact with one another. What does it actually look like? We’ll have to wait and see. The occultation observations found no evidence for rings, moons, or a dust halo, increasing confidence in the planned close fly-by.
Another mystery which will have to await close-up observation is the absence of a pronounced light curve. An irregularly-shaped object like Ultima Thule would be expected to vary dramatically in brightness as it rotates, but extended observations by Hubble failed to find any variation at all. The best guess is that we’re observing it close to the pole of rotation, but again it’s anybody’s guess until we get there and take a look.
Are we there yet? No, but it won’t be long now. As I noted, the closest fly-by will be at 05:33 UTC on 2019-01-01. Most of the scientific data will be collected in the day before and after the moment of closest approach. Coverage of this event will not be like what you’ve become accustomed to from other space missions. New Horizons will be 6.6 billion kilometres from the Earth at the time of the fly-by, more than 43 times the distance of the Earth from the Sun. It takes light (and radio waves) six hours to travel that distance, so anything transmitted to Earth will take that long to arrive. Further, since the high-gain antenna used to send data back to Earth is fixed to the same spacecraft structure as the scientific instruments, while they are collecting data during the fly-by, the antenna won’t be pointed in the correct direction to send it back to the distant home planet.
After the scientific observations are complete, the antenna will be pointed at the Earth to send “quick look” data, spacecraft health information, and the first images. These are expected later on the first of January and over the next few days. To those accustomed to broadband Internet, these data arrive excruciatingly slowly.
Even with a 70 metre Deep Space Network antenna, the downlink rate is 501 bits per second. If you have a 50 megabit per second broadband Internet connection, this is one hundred thousand times slower: comparable to the dial-up computer terminal (300 bits per second) I used in 1968. It takes around an hour to return a single image, even in the compressed formats used for quick-look data. Downloading all of the science data collected during the fly-by will begin on the 9th of January, when New Horizons returns to spin-stabilised mode (which requires no maneuvering fuel) with its antenna pointed at Earth, and is expected to take twenty months. When the data download is complete, the spacecraft will be placed back into hibernation mode. If another Kuiper belt target is identified which can be reached with the remaining maneuvering fuel before its nuclear power source decays or its distance to Earth becomes too great to return fly-by data (expected in the 2030s), it may be re-targeted for another fly-by.
Coverage of the New Horizons fly-by of Ultima Thule will be broadcast on the Johns Hopkins University Applied Physics Laboratory (who built the spacecraft and manages the mission) YouTube channel. Here is a schedule of mission-related programming. This is the mission Web site, with links to resources for the spacecraft and its destination. This article by Emily Lakdawalla of the Planetary Society gives more detail about the encounter, when data and images will be returned, and what we can expect to see when.
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Here is a Science Chat from September 2018 with New Horizons principal investigator Alan Stern looking ahead to the encounter with Ultima Thule.
This is a panel discussion at the American Geophysical Union meeting in December 2017 describing the preparations for the encounter with Ultima Thule and what may be learned from the fly-by.