Editors note: This story was updated December 31–January 1 with dispatches from astronomy writer Lisa Grossman, who was at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., with the New Horizons team.
Updated 1:20 p.m., January 1
The last best view of Ultima Thule that New Horizons sent back before last nights flyby gave a rough view of the objects shape. It revealed an elongated, bowling pin–like shape, team members announced at a news conference this morning.
“Its a pixelated blob, but its a better pixelated blob,” said mission project scientist Hal Weaver, of Johns Hopkins University Applied Physics Laboratory in Laurel, Md.
That shape, plus a new sense of the objects rotation (shown at right in a sequence of three images taken by New Horizons), helped solve one big pre-flyby mystery: What was going on with Ultima Thules brightness? The team was pretty certain that Ultima Thule was elongated and must be spinning, which should have made its brightness appear to vary over time from New Horizons point of view. But it didnt.
Before the flyby, team members speculated that New Horizons was seeing Ultima Thule along one edge, and the object was spinning in front of the spacecraft like a propeller. The best pre-flyby images seem to confirm that view, Weaver said. “That explains everything.”
Its still not entirely clear whether Ultima Thule is a single object with two lobes, or two objects orbiting each other extremely closely. But principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colo., is betting on it being a solitary world.
“If its two separate objects, this would be an unprecedented situation in terms of how close theyre orbiting to one another,” Stern said in the news conference. “That would be spectacular, and Id love to see it, but the higher probability is a single body.”
If Ultima Thule is a single, two-lobed body, it could have formed in a slow-motion collision between smaller objects in the solar systems distant past. More high-resolution images will be downloaded in the next few days to help clear things up.
“This will all be revealed tomorrow or the next day,” Weaver said. “Ultima Thule will turn into a real world.”
Updated 10:55 a.m., January 1
Its official: New Horizons just flew past Ultima Thule, the most distant object ever to get a robotic visitor. After a tense night of waiting, the New Horizons team received the spacecrafts “phone home” signal just after 10:30 a.m. Eastern time.
“We have a healthy spacecraft,” said mission operations manager Alice Bowman, who was tallying the status updates from mission control. “Weve just accomplished the most distant flyby. We are ready for Ultima Thule science transmission … science to help us understand the origins of our solar system.”
The team is still waiting for more details. But if all went as planned, the spacecraft skimmed past Ultima Thule at about the distance between Washington, D.C., and the west coast of the United States at 12:33 a.m. As of last night, the spacecraft was expected to arrive within a minute of its scheduled flyby time, said principal investigator Alan Stern. “I cant remember the last time an aircraft took me across the country and was on time to within a minute,” Stern quipped.
You can watch the 11:30 news briefing here:
Updated 9:49 a.m., January 1
Were about 40 minutes away from the phone-home signal from New Horizons. Meanwhile, whats next for New Horizons?
The spacecraft will keep looking around at Kuiper Belt objects (KBOs) from a perspective we cant get from Earth. It might even do a flyby of another KBO, although it will never get as close to another KBO as it will to Ultima Thule.
The probe may have already seen signs of a “hydrogen wall” at the edge of the solar system. New Horizons will keep looking for the wall about twice a year.
And the New Horizons team hopes to keep watching the spacecraft until it crosses the heliopause, the boundary between the suns influence and interstellar space. Only two other spacecraft have done that: Voyager 1 in 2012 and Voyager 2 in December.
Updated 12:33 a.m., January 1
Its closest approach time! New Horizons is at its nearest point to Ultima Thule, soaking up data, taking images and learning as much as it can about this space rock.
In a few hours, New Horizons will phone home to let mission control know its status and how much data it collected. And it will take six hours, eight minutes for that signal to reach Earth. If the calculations are correct (and they usually are), well know how the flyby went around 10:28 a.m. later today, January 1. Well be with the New Horizons team while they wait for the signal. Until then, Happy New Year!
Updated 9:40 p.m., December 31
Were getting closer! Right now, New Horizons is 149,000 kilometers from Ultima Thule, with just under three hours until closest approach.
The New Horizons hazard avoidance team — nicknamed the “hard hats” — spent weeks before the flyby looking for rings and other hazards around MU69. Hitting a dust particle the size of a grain of rice could cripple the spacecraft.
With the path clear, the team announced December 18 that New Horizons would target a closer approach to Ultima Thule rather than on one three times as far away.
At its closest approach, New Horizons will fly about 3,500 kilometers from the space rocks surface — about the distance from Washington, D.C., to Las Vegas. Thats 3.5 times closer than New Horizons was to Pluto in 2015.
Updated 8:00 p.m., December 31
NASAs Deep Space Network of radio telescopes spends most of its time listening to spacecraft. It will receive New Horizons all-clear signal tomorrow morning, after the flyby. But nearly two hours ago, two of the DSN's telescopes — Goldstone in California and Canberra in Australia — sent a signal to New Horizons. At about 6:15 p.m. Eastern time, the telescopes beamed a strong radio signal at Ultima Thule.
“Were shouting as loud as we can in hopes New Horizons will hear,” says New Horizons deputy project scientist John Spencer, of the Southwest Research Institute.
About six hours later, the signal will bounce off Ultima Thule and hopefully be picked up by New Horizons Radio Science Experiment, or REX, just after the spacecraft has flown past. If its successful, that radio signal bounce could penetrate a few centimeters into Ultima Thule's surface, giving scientists on Earth a unique view of the object's shallow interior. “It's a long shot that this will work, but we're doing everything we possibly can,” Spencer says.
Updated 4:38 p.m., December 31
The latest image of Ultima Thule gives us the closest look yet at the space rock. But it comes with some big questions.
The left picture, taken on December 30, is just a few pixels across, but it's already intriguing. Image sharpening (right) reveals an elongated shape — consistent with the shape the team inferred from observing MU69 cross in front of a more-distant star last year.
That shape raises a question: Whats up with MU69's brightness? From afar, MU69's brightness appears nearly constant. That would be easy to explain if the rock were spherical — it would reflect the same amount of light no matter how it was spinning. But those few pixels show that it can't be spherical. What's going on?
It could be that MU69 is spinning in front of New Horizons, so that it shows the spacecraft the same face at all times, says New Horizons deputy project scientist John Spencer, of the Southwest Research Institute. “That's our current best theory,” he said at a news conference on December 31. “It could be wrong tomorrow.”
Updated 1:53 p.m., December 31
New Horizons is 553,000 kilometers from Ultima Thule! The second of two “fail-safe” data downloads is in progress right now. This is the last communication well have with New Horizons before the flyby. (You can see the status of the download on NASAs Deep Space Network website.)
And a news briefing with several members of the New Horizons team is scheduled for 2 p.m. You can watch here:
Updated 12:45 p.m., December 31
While we wait for this afternoons news briefings, here are some details on the New Horizons mission and its target:
- 2014 MU69 is nicknamed Ultima Thule (pronounced UL-tee-mah TOO-lee), which means “beyond the borders of the known world.” The New Horizons team picked the name from 34,000 submissions.
- New Horizons is the first spacecraft to fly through the Kuiper Belt on purpose. Previous missions like Pioneer and Voyager went through the belt before scientists knew it was there.
- Scientists think Ultima Thule is a “cold classical” Kuiper Belt object, which means its orbit hasnt changed much in 4-plus billion years. Its been in the back of the solar systems freezer at –240 degrees Celsius since the planets were born, or earlier.
- We didnt even know MU69 existed until 2014, when it was spotted with the Hubble Space Telescope. Even Hubble could barely see it — this was its view:
- It was one of just two appropriate choices for the flyby, and it edged out 2014 PN70 because it took less fuel to get to MU69.
- Scientists on the ground chased Ultima Thules shadow as it crossed in front of a distant star on July 17. The team had to know precisely when and where to set up their telescopes to catch the tiny eclipse. The results suggested MU69 might have a moon.
Updated 10:40 a.m., December 31
New Horizons is speeding toward Ultima Thule at a whopping 14 kilometers per second. On December 30, the New Horizons team made a two-second adjustment to the timing of imaging and data gathering the spacecraft will do during the flyby. And the first of two “fail-safe” downloads of data from New Horizons completed successfully, ensuring that some information is in hand even if something goes catastrophically wrong during the the flyby.
This was New Horizons view of Ultima Thule on December 24:
The image is a combination of three 0.5-second-exposure images taken using the highest resolution of LORRI, or the Long Range Reconnaissance Imager, on board the spacecraft.
The last images that New Horizons sends home before the flyby will show the space rock as just five to seven pixels across — but that may be enough to answer some questions about Ultima Thule. For example, its about 30 kilometers wide, as far as we can tell — but we dont yet know if its a single object or two objects orbiting close to each other. And it doesnt seem to have rings, but its not crazy to think it might, even though its tiny. Asteroids Chariklo and Chiron, and dwarf planet Haumea all have rings despite their diminutive sizes.
Original post, 6:00 a.m., December 30:
After a journey of almost 13 years and more than 6.5 billion kilometers, New Horizons is about to meet a tiny, mysterious space rock called 2014 MU69.
The spacecraft will zip past MU69, also called Ultima Thule, at 12:33 a.m. Eastern Standard Time on January 1, at a speed of 14 kilometers per second. (See our preview story for more on the mission.) The New Years Eve party at the Johns Hopkins University Applied Physics Laboratory will be broadcast on APL's YouTube channel and on the APL Facebook page. (The flyby activities are also scheduled to be broadcast on NASA TV, despite the federal government shutdown.)
The broadcast will begin with a news briefing at 2 p.m. on December 31, and will pick up again with scientific lectures at 8 p.m. The countdown to the flyby starts just into the new year at 12:15 a.m. on January 1.
While there will certainly be celebration at the time of the flyby, NASA wont get confirmation from New Horizons that all went well right away. Just as when it flew past Pluto in 2015, New Horizons will focus all its energy on collecting data as it passes by MU69, rather than transmitting anything to Earth. MU69 is so far away that it will take six hours for a signal to go from the spacecraft to Earth. And the data downlink is so slow at that great distance that it will take until about September 2020 for all the science data to return to Earth.
But hopefully, when we wake up in 2019, well know that New Horizons is safe and has lots to tell us about MU69. Stay tuned.
Editor's note: The 10:40 a.m. update was updated to correct the speed of New Horizons. It's traveling at 14 kilometers per second, not 14,000 kilometers per second.