NASA's New Horizons unveils unique shape of Ultima Thule

NASA's New Horizons spacecraft has beamed back new images of Ultima Thule, which show that the most distant world ever explored is much flatter than previously thought. The images of the KBO officially named 2014 MU69 were captured by the New Horizons as it raced away at over 50,000 kilometres per hour on January 1. The images were taken nearly 10 minutes after New Horizons crossed its closest approach point.

Mission principal investigator Alan Stern, of Southwest Research Institute in the US said, this really is an incredible image sequence, taken by a spacecraft exploring a small world four billion miles away from Earth. Nothing quite like this has ever been captured in imagery.

The newly released images also contain important scientific information about the shape of Ultima Thule, which is turning out to be one of the major discoveries from the flyby. The first close-up images of Ultima Thule with its two distinct and, apparently, spherical segments had observers calling it a snowman.

However, more analysis of approach images and these new departure images have changed that view, in part by revealing an outline of the portion of the KBO that was not illuminated by the Sun but could be traced out as it blocked the view to the background stars. Stringing 14 of these images into a short departure movie, New Horizons scientists can confirm that the two sections (or "lobes") of Ultima Thule are not spherical.

The larger lobe, nicknamed Ultima, more closely resembles a giant pancake and the smaller lobe, nicknamed Thule, is shaped like a dented walnut. 

Stern said, we had an impression of Ultima Thule based on the limited number of images returned in the days around the flyby, but seeing more data has significantly changed our view. It would be closer to reality to say Ultima Thule's shape is flatter, like a pancake. But more importantly, the new images are creating scientific puzzles about how such an object could even be formed. We've never seen something like this orbiting the Sun.

The departure images were taken from a different angle than the approach photos and reveal complementary information on Ultima Thule's shape. The object's illuminated crescent is blurred in the individual frames because a relatively long exposure time was used during this rapid scan to boost the camera's signal level, but the team combined and processed the images to remove the blurring and sharpen the thin crescent.

Many background stars are also seen in the individual images; watching which stars blinked out as the object passed in front of them allowed scientists to outline the shape of both lobes, which could then be compared to a model assembled from analysing pre-flyby images and ground-based telescope observations.

Simon Porter, a New Horizons co-investigator from the Southwest Research Institute said, the shape model we have derived from all of the existing Ultima Thule imagery is remarkably consistent with what we have learned from the new crescent images. 

Hal Weaver, New Horizons project scientist from the Johns Hopkins Applied Physics Laboratory said, while the very nature of a fast flyby in some ways limits how well we can determine the true shape of Ultima Thule, the new results clearly show that Ultima and Thule are much flatter than originally believed, and much flatter than expected.