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LAUREL, Maryland — A day before NASA’s New Horizons spacecraft closes in on a frozen outpost nicknamed Ultima Thule 4.1 billion miles (6.6 billion kilometers) from Earth, basic facts about the city-sized object continued to elude scientists Sunday as the ground team braced for a deluge of data and imagery that should unmask the unexplored world at the frontier of the solar system.

No more than 20 miles (30 kilometers) long, Ultima Thule — officially named 2014 MU69 — is a billion miles beyond Pluto, the last world New Horizons visited. It’s reddish in color, and scientists have pinpointed its location with remarkable precision for an object just discovered in 2014.

Other than that, Ultima Thule’s appearance is relegated to the imaginations of scientists and space enthusiasts. That will change in a hurry once pictures snapped by the New Horizons spacecraft’s black-and-white and color cameras begin coming back to Earth on Tuesday and Wednesday.

“We don’t know a thing about MU69,” said Alan Stern, New Horizons principal investigator from the Southwest Research Institute. “We’ve never, in the history of spaceflight, gone to a target that we knew less about, and it’s remarkable that we’re on the verge of knowing a great deal about this.

“Today, I can’t tell you more than five facts about it,” Stern said in a briefing with reporters Sunday. “We know its orbit, we know its color, we know a little bit about its shape, and its reflectivity. We can’t even get the rotational period. I thought we’d have that 10 weeks ago.”

While scientists knew Ultima Thule would only reveal its secrets in the final days — or hours — of the flyby, the questions still unanswered have prompted New Horizons team members to tap into their creative sides.

“Our team has been making little clay figures (guessing) here’s what we think it looks like today based on the current information we have,” said Hal Weaver, the New Horizons project scientist at the Johns Hopkins University Applied Physics Laboratory, where New Horizons was built and home of the mission control center.

Nevertheless, scientists think they are beginning to see some details.

Ultima Thule is just starting to be resolved by New Horizons’s LORRI imaging camera, which so far has viewed the object as just a point of light — a single pixel in the camera’s field-of-view. That will change rapidly as the probe speeds toward it at 32,000 mph (14 kilometers per second).

The target is now nearly 2 pixels across, but that’s still not enough to resolve its shape.

“How fast is it rotating? A few hours, tens of hours, or days?” Weaver said.

“There’s some indication, some hint, that maybe this is a fast rotator,” Weaver said. “The little bit that we’ve been able to tease out suggests that it may be rotating pretty quickly, but we’ve been up and down on the team as to whether or not we believe that.”

If Ultima Thule is spinning relatively quickly, that would be good news for the researchers eagerly awaiting their first look at such a primitive world. A fast rotator would show more of its surface to New Horizons during the flyby.

One of the main mysteries so far in the approach to Ultima Thule has been that New Horizons has not observed any light curve, or change in brightness, from the object.

Scientists expected to see Ultima Thule dimming and brightening as it rotated, New Horizons has not detected any change.

“We thought as we came in and started observing it systematically from mid-September up until now that we would get someting called a light curve, which allows us to see the variation in the brightness of Ultima Thule that would tell us something about the shape,” Weaver said.

“We were systematically making these observations hoping to be able to convert these observations into a shape model of Ultima Thule, but every time we went back and made observations it was just completely flat.

“So it’s possible that the rotation is maybe pointing towards us, which is a highly unusual… It could be anywhere in space — the rotational pole — but pointing toward us is an unusual circumstance,” Weaver says.

“So it may be that it’s highly elongated, which we think because of the stellar occultation measurements,” he said, referring to observations made when Ultima Thule briefly blocked the light of a background star as seen from Earth, allowing scientists to put constraints on its shape and size.

Cathy Olkin, deputy project scientist at the Southwest Research Institute, agreed.

“I think, based on the occultation result, we saw a clear signature that its’ either elongated or two lobes … I do believe that we’re not going to see something that’s round,” Olkin said.

“I think that what we’re going to see is that we’re looking pole-on to the object. That’s one way to reconcile the fact that we don’t see a light curve on this object. We don’t see a variation in light over time repeatably.”

Scientists believe Ultima Thule is a relic from the early solar system 4.5 billion years ago, a type of object known as a “cold classical” because it stayed in roughly the same orbit where it formed. The discoveries will open a new window into how all planetary systems are born and evolve, said Jason Kalirai, executive for the civil space mission area at APL.

“It’s absolutely foundational breakthrough science,” said Kalirai, an astrophysicist.

Weaver said the New Year’s encounter with Ultima Thule is a once-in-a-lifetime event for most people on the New Horizons team — due to the time it takes to prepare a space mission and have it travel from Earth to the Kuiper Belt.

New Horizons launched from Cape Canaveral on Jan. 19, 2006, got a gravitational assist by Jupiter on Feb. 28, 2007, then reached Pluto on July 14, 2015. Weaver called Pluto the gatekeeper to the Kuiper Belt, a ring of icy primordial worlds extending beyond the orbit of Neptune.

Pluto is the largest known object in the Kuiper Belt, where scientists think short-period comets originated.

“There’s nothing else on the books to do anything like this,” Weaver said.

“I don’t think I’ll be alive when the next cold classical Kuiper Belt Object is encountered, so we’re all looking forward to this flyby. In that respect, this is the frontier of planetary science … As a civilization, we’re stepping out into this third zone of the solar system that was not even discovered until the early 1990s.”

Scientists have brought sleeping bags, pillows, and even a tent to camp out here at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, as New Horizons speeds toward Ultima Thule — its next target after Pluto.

Alice Bowman, the mission operations manager for New Horizons, said she came into work at 3 a.m. EST Sunday to get the latest navigation update and help prepare a “knowledge update” for uplink to the spacecraft.

The update changed the timing of the sequence of images and data to be gathered during the flyby by just 2 seconds, but that’s enough to require some adjustment to ensure the cameras and sensors get the best information possible during the one-shot encounter with Ultima Thule.

“This last day has probably been the most intense for us,” Bowman said.

“Whatever it takes, we’re here for the exploration and we’re happy to spend the night if that’s what it takes,” she said.

New Horizons is right on course for its encounter with Ultima Thule, and Bowman tweeted Sunday night that the “knowledge update” was successfully received by the spacecraft after taking 6 hours and 8 minutes to traverse the distance from Earth at 186,000 miles per second, or 300,000 kilometers per second.

In fact, the latest navigation update from pictures of Ultima Thule captured by the LORRI camera on-board the spacecraft indicate New Horizons is around 18 miles (30 kilometers) off its aim point 2,200 miles (3,500 kilometers) away from the object.

Not bad for a mission that’s nearly 13 years off the launch pad.

No further commands are expected to be sent to the spacecraft before the flyby.

Purely as a consequence of astrodynamics, New Horizons will reach its closest point to Ultima Thule at 12:33 a.m. EST (0533 GMT) on Tuesday, New Year’s Day. Around four hours later, the spacecraft will pause its observations to turn its 6.9-foot (2.1-meter) antenna toward Earth to phone home.

A giant 230-foot (70-meter) dish antenna part of NASA’s Deep Space Network near Madrid will receive the signals more than six hours later at 10:29 a.m. EST (1529 GMT). But the best images — with Ultima Thule spanning hundreds of pixels across — won’t arrive on Earth until late Tuesday, and are expected to be released to the public Wednesday afternoon.

The black-and-white LORRI camera is programmed to take around 1,500 pictures during the flyby. The other instruments aboard New Horizons will take color images, measure Ultima Thule’s composition, and take infrared data.

The flyby command sequence is already being executed by the spacecraft. Because of the vast distance between Earth and Ultima Thule, scientists and engineers are hands-off for the encounter.

New Horizons has instructions already loaded into its computer to handle any last-minute glitches and continue the data-gathering sequence.

“At this point, the navigation effort is effectively finished,” said Marc Buie, a member of the New Horizons team from Southwest Research Institute. “From here on out, it’s party time.”

The last thruster firing to actually tweak the trajectory of New Horizons was completed Dec. 18, and there are no more opportunities to do a course correction as the flyby fast approaches.

Buie led the team that observed Ultima Thule during a pair of stellar occultations when the object passed between two stars and Earth in July 2017 and August 2018.

Those observations gave scientists an idea of the shape of Ultima Thule, which Buie suggested was possibly the shape of a peanut, at least according to the occultation data. Some scientists believe Ultima Thule could be a binary pair of objects, but Buie says he has ruled out that possibility, based on the most recent occultation measurements in August.

“We just have to be patient and wait for the images to come in, and we’re going see more and more pixels,” Buie said.

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Follow Stephen Clark on Twitter: @StephenClark1.