Alan Stern, principal investigator of NASA's New Horizons mission, gives a detailed preview of what the spacecraft will be doing in the days up to, during, and after the impending flyby of the Kuiper Belt Object 2014 MU69.

Illustration of New Horizons flying past 2014 MU69
The New Horizons spacecraft flies by the Kuiper Belt Object 2014 MU69 in this artist's illustration.
NASA / JHU-APL / SwRI / S. Gribben

As I wrote in this blog last month, NASA’s New Horizons spacecraft is on approach to conduct the first ever, close exploration of a Kuiper Belt Object (KBO) on January 1, 2019. The time of closest approach will be 5:33 Universal Time (33 minutes past midnight on the U.S. east coast), with a planned closest approach distance of 3,500 kilometers (2,200 miles). Our KBO, 2014 MU69, was discovered in a dedicated Hubble Space Telescope search for flyby targets that we conducted in 2014. After a naming contest, the mission team gave “MU69” the nickname “Ultima Thule,” a Latin phrase appropriately meaning “a distant unknown region; the extreme limit of discovery.”

The primary objectives of the flyby of Ultima are to map its surface features and composition, determine if it has an atmosphere, and to search for any satellites or rings it might possess. New Horizons, which was launched in 2006 and which made the first exploration of the Pluto system in 2015, carries seven powerful scientific instruments to carry out this exploration. All will be used in the exploration of Ultima.

Meet the science instruments

New Horizons' scientific payload
The experiments on New Horizons are designed with overlapping capabilities, so that if one fails the mission's scientific objectives can still be met.

Three of the payload instruments are optical devices. LORRI, the Long Range Reconnaissance Imager, is comprised of a 20-cm diameter telescope that feeds a panchromatic CCD — a digital imaging device sensitive to all visible wavelengths — and is the highest resolution imager aboard New Horizons. A second imager named Ralph contains four CCDs with color filter channels and two CCDs for panchromatic imaging. Importantly, Ralph also contains an infrared spectrometer that will be used to map the surface composition of Ultima. The third optical instrument aboard New Horizons is Alice, an ultraviolet mapping spectrometer that will be used to search for gases emitted from Ultima and determine their density and composition.

The other instruments aboard New Horizons are: SWAP and PEPSSI, both charged-particle spectrometers; REX, a radio-science instrument that will be used to determine Ultima’s surface temperature and to measure its radar reflectivity; and SDC, a student-built interplanetary-dust counter.

Planning the encounter

This illustration shows the planned flyby trajectory of New Horizons as it passes the Kuiper Belt Object 2014 MU69 on January 1, 2019.

Because Ultima is small — probably just 25 km (16 miles) or so in diameter — it will remain just a point of light to New Horizons until about 2 days before the close flyby. However, in the final hours around closest approach, New Horizons will be able to map Ultima at higher resolutions than we achieved at Pluto, because we will fly by Ultima at a much closer range than we did at Pluto

We will obtain geologic mapping resolutions as high as 35 meters (110 feet) per pixel using LORRI. By comparison, our highest resolution Pluto mapping was about 80 meters (260 feet) per pixel.

With the Ralph imager, we also plan to acquire color images of Ultima with resolutions as high as 330 meters (0.2 miles) per pixel, and composition mapping at a resolution of 1.8 km (1.1 miles) per pixel. Stereo imaging made on approach will map the surface topography of Ultima at about 80 meters (260 feet) per pixel.The first detailed imagery of Ultima will be downlinked to Earth once the spacecraft has completed its main flyby objectives late on January 1st, and will be released to the public after processing and image analysis on January 2nd. More images, as well as spectra and other data sets, will be downlinked on January 2nd, 3rd, and 4th — so get ready to learn a lot about Ultima in the first week of the new year! Then the spacecraft will slip behind the Sun as seen from Earth and image transmissions will cease for 5 days until the spacecraft reappears and can resume data transmissions.

This plan illustrates what the spacecraft will be up to in the days around the flyby. The bottom panel zooms in on the hours right around closest approach and shows what instruments will be active and when.

The total data volume collected on the Ultima flyby will be close to 50 gigabits. Because New Horizons is so far from Earth, about 6 billion km (4 billion miles), its data transmission speed is now only about 1,000 bits per second. This limitation, and the fact that we share NASA’s Deep Space Network of tracking and communication antennas with over a dozen other NASA missions, means that it will take 20 months or more, until late in 2020, to send all of the data about Ultima and its environment back to Earth.

Looking ahead

This illustration provides a "big picture" view of New Horizon's trajectory from Earth to the Kuiper belt and beyond.

As you read these words, New Horizons is closing in on Ultima at a speed of nearly a million miles per day. The main tasks onboard the spacecraft now are navigation imaging that drives course corrections to home in on our aim point, and a three-week-long hazard search to determine if our close approach path is safe, or if we must divert to a greater distance of 10,000 km (6,000 miles) to avoid hazards such as dust or moons. We will make a final decision on the close approach distance on December 16th.

The flyby of Ultima will be orchestrated aboard New Horizons with a computer program called “command sequence” that will be uploaded by radio and will begin 7 days prior to the flyby, on Christmas day. This command sequence will direct all of the spacecraft and science instrument activities from then until 2 days after closest approach.

Our science team just completed its final meeting before the flyby. We’ll assemble for science operations beginning December 28th at our mission control, which is at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. We’ll be reporting results in press conferences and news releases every day from December 30th to January 4th. You can follow New Horizons on NASA TV, and on a variety of social media such as Facebook and Twitter by just searching for “New Horizons”. Our mission web site is at

I’ll blog here again the week before the flyby with an update on our hazard findings, our final selection of approach distance, and late-breaking other details. Stay tuned!

This blog is part of a four-part series covering the Ultima Thule flyby. Click on the links below for the other installments:

Part One: New Horizons on Approach to the First Exploration of a Kuiper Belt Object



Image of Peter Wilson

Peter Wilson

December 5, 2018 at 11:26 am

So as New Year's eve party-goers start "talking to ralph," the real Ralph will be taking pictures of Thule? How cool!

Alan: Just read your book, Chasing New Horizons, and if the Terms of Use allowed it, I would plug it. But one question. Nitrogen ice, unlike water ice, is heavier than liquid nitrogen, so when a lake of liquid nitrogen freezes, we would expect it to look "different" in some way. You state unequivocally in the plate-section that the curious geological pattern of cells, seen in Pluto's frozen nitrogen sea, but nowhere on Earth, is "caused by the churning motion of convection." That is hard to picture in solid ice. Is that a definite result, arrived at by computer simulations or some-such? Or is that the most plausible sounding guess?

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Image of Ike


December 7, 2018 at 4:26 pm

Are there any plans currently to look for another target for New Horizons, once it has left Ultima Thule?

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