The craft has gradually made its approach over the last two weeks as NASA scientists performed a series of checks and trajectory corrections to ensure New Horizons is on the right track to gather as much information as possible about the mysterious object,, without crashing into any debris that may be lurking in the outer reaches of our solar system.
"This last day has probably been the most intense for us," Alice Bowman, New Horizons mission operations manager at APL, told CNET sister site CBS News. "We had these optical navigation measurements coming down much closer together, that means a lot of the team was up all night."
She also said the spacecraft would pass within 19 miles of its aim point. That's about 2,200 miles from the object. To put that in perspective, remember? New Horizons cameras snapped those as it flew 12,500 kilometers (about 7,800 miles) from the surface of the far-off dwarf planet.
Thus, New Horizons will come three times closer to Ultima Thule than it did Pluto and provide NASA researchers with valuable images and science data of a world we know practically nothing about.
On Dec. 15, the 12 researchers who make up New Horizons hazard watch team confirmed that the approach path was safe using New Horizons' telescopic Long Range Reconnaissance Imager (LORRI). If they had discovered moons or rings near Ultima, NASA would have opted for a secondary flight path, with New Horizons course-correcting and flying past the object from a much greater distance.
"The team was in complete consensus that the spacecraft should remain on the closer trajectory, and mission leadership adopted our recommendation," said Mark Showalter, hazards team lead.
On Dec. 26, New Horizons entered Encounter Mode, a type of "safe mode" that ensures the science objectives of the mission will be carried out even if the spacecraft malfunctions. Under normal circumstances, a malfunction sees New Horizons phone home asking for help, but because this now takes 12 hours to perform, its risky to do so when the spacecraft is on its close approach.
Practically, entering Encounter Mode means the spacecraft is on its own now. With thousands of instructions loaded into its onboard computers, it has begun its delicate dance, 1 billion miles past Pluto.
Two days before we left New Horizons to its own devices, it snapped the highest-resolution image of the distant "worldlet" yet: the dark, pixelated blur below shows Ultima Thule at its center, 10 million kilometers (about 6.3 million miles) away.
Within a week, that tiny pixel of light in the distance will become a known world. We'll see what it looks like, what it's made of, how cold it is, its mass and whether it has any moons of its own.
New Horizons will be literally ringing in the New Year by flying past the most distant world we've ever explored, with the closest approach set to occur at 12:33 a.m. ET on Jan. 1. Although the US is currently in the middle of an ongoing federal government shutdown, you'll still be able to catch the reactions and live simulations of the flyby on the New Horizons mission website. Data and images from the flyby are expected later on New Year's Day, sometime after 11:30 a.m. ET.
, New Horizons will hopefully put 2019 on the right flyby track, too, so I suggest you whack on the Interstellar soundtrack on New Year's Eve, settle in and marvel at the new world we're about to uncover.
First published Dec. 26, 4:23 p.m. PT
Update, Dec. 27, 6:35 p.m. PT: Adds that New Horizons has gone into Encounter Mode and is on track for its historic flyby.
Update, Dec. 31, 8:55 a.m. PT: Adds comment from APL mission operations manager.
Update, Dec. 31, 5:22 p.m. PT: Adds NASA live stream.
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