Nearly 50 years after Neil Armstrong stepped onto the surface of the moon, humanity has begun the final phase of mapping the solar system, according to a top NASA scientist.
Paul Schenk, co-investigator on the agency's New Horizons mission, told RadioLIVE on Wednesday the probe's flyby of distant space rock Ultima Thule - deep in the mysterious Kuiper Belt - has been a success.
"We've begun the exploration of the outermost part of the solar system - the area that's populated by small, icy bodies out beyond the orbit of Neptune for the first time. We're basically finishing the exploration of the solar system."
Early on New Year's Day, New Horizons flew by the most distant object ever visited by a space probe - a 30km-wide rock dubbed Ultimate Thule.
When New Horizons was launched in 2006, Pluto was still deemed a planet and Ultima Thule hadn't even been discovered.
New Horizons passed Pluto in 2015, taking the first-ever close-up images of what had since been demoted to dwarf planet status. NASA scientists realised it had enough fuel in the tank to visit the newly discovered Ultima Thule, which it passed on Tuesday (NZ time).
The first images sent back, taken from about 2 million kilometres away, consisted of little more than a few pixels.
"I've never seen so many people so excited about two pixels," Alan Stern, lead scientist on the New Horizons mission, said on Tuesday.
High-resolution photographs are due on Thursday morning (NZ time). They'll have been taken from only 3500km.
At more than 6 billion kilometres away, Ultima Thule is more than 44 times the distance from the sun as Earth. Dr Schenk says they don't really know what to expect.
"We've never seen an object like this. There's a lot of mystery here."
They'll be looking at what it's made of and features on its surface such as craters and erosion, in an effort to find out how it was made.
"We're going to spend the next 20 months downloading all the data from the encounter which occurred over the past 20 hours," he told Newshub.
"It takes so long because it's such a small antenna. We didn't have the funds, or the thrust on the rocket, to get a larger antenna to that distance. It takes a long time to bring it all down."
And then, who knows.
"We're going to continue to look, at a very long distance, at all the other small... objects that we can detect in our cameras to determine their sizes, rotation state, any basic properties we can get from those bodies. We also have some instruments that are going to measure the solar wind and the dust particles... We're going to be doing similar things to what the Voyager spacecraft has been doing."
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Voyager, launched in 1977, isn't heading towards anything in particular. Its fuel will run out in about 2025, while New Horizons has enough to keep the lights on until about 2030.
"We don't have another target identified yet, because it's an area we don't have much data or information on yet," said Dr Schenk.
"If we see another object we can visit, we'll go to it."