NASA's New Horizons space probe passed by Ultima Thule, a tiny icy world four billion miles from the sun. It's the farthest target that humans have ever reached and will give scientists insight into the earliest days of our solar system. From NASA and the Johns Hopkins Applied Physics Laboratory:
"New Horizons performed as planned today, conducting the farthest exploration of any world in history — 4 billion miles from the Sun," said Principal Investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado. "The data we have look fantastic and we're already learning about Ultima from up close. From here out the data will just get better and better!"
Images taken during the spacecraft's approach — which brought New Horizons to within just 2,200 miles (3,500 kilometers) of Ultima at 12:33 a.m. EST — revealed that the Kuiper Belt object may have a shape similar to a bowling pin, spinning end over end, with dimensions of approximately 20 by 10 miles (32 by 16 kilometers). Another possibility is Ultima could be two objects orbiting each other. Flyby data have already solved one of Ultima's mysteries, showing that the Kuiper Belt object is spinning like a propeller with the axis pointing approximately toward New Horizons. This explains why, in earlier images taken before Ultima was resolved, its brightness didn't appear to vary as it rotated. The team has still not determined the rotation period.
As the science data began its initial return to Earth, mission team members and leadership reveled in the excitement of the first exploration of this distant region of space.
And from Kenneth Chang in the New York Times:
The New Horizons spacecraft itself was first able to spot Ultima Thule beginning in August, although as no more than a speck of light until this weekend. As the spacecraft approached, scientists had expected to see a rhythmic brightening and dimming of Ultima Thule, which would reveal how fast it is spinning. Instead, the brightness seems to have remained steady the whole time.
"It's really puzzling, because we know the shape is irregular," (New Horizons principal investigator S. Alan) Stern said. Over the next couple of days, preliminary looks at the data, including what the scientists hope will be striking images of Ultima Thule, will be beamed back to Earth. Twenty months will pass before scientists have the full set of measurements. And they will be eagerly awaiting every bit of that stream.
"We are ready to science the heck out of Ultima Thule," Dr. Stern said.