Pancakes Anyone? Ultima Thule Flatter Than We Thought


Departure movie of Ultima Thule made with 14 images taken after New Horizons made its closest approach on Jan. 1, 2019.

Farewell Ultima Thule! This evocative video was made just after closest approach to Ultima Thule on New Year’s Day when New Horizons was speeding past at over 31,000 miles an hour (50,000 km/hour). See all those stars? Not only do they provide a breathtaking backdrop, but it turns out that they were crucial to determining the object’s true shape. When the first photos were beamed back, it looked for all the world like two approximately spherical bodies joined at the neck. Some of us went so far as to call it a snowman. But further analysis of these latest photos reveal that Ultima, the larger of the two halves, is shaped more like a pancake, while Thule reminds me of a Milk Dud, a chocolate-coated caramel candy.

The top view is how we thought we understood Ultima Thule’s shape. The bottom view is the team’s current best shape model for Ultima Thule, but there’s still some uncertainty since the entire region was in shadow. The dashed blue lines show the uncertainty — the object could be either flatter than, or not as flat as, shown here. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Scientists “traced out” their shapes by examining how they blotted out the background stars as New Horizons raced by. They then compared the observations to a model made from pre-flyby photos and telescope observations from the ground. Amateur and professional astronomers use the same method when trying to figure out an asteroid’s shape from Earth.

In July 2011 the asteroid 90 Antiope occulted a star in Aquarius for observers in the western U.S. 46 stations watched the star blink out and 11 observed a miss. Each of these timelines are shown at right — one per observer. The solid lines show when the star was visible, the blank sections when it disappeared. The results clearly show the asteroid as two separate, different-shaped objects! Occult 4.0

If you know the path of an asteroid, you can predict when it will occult (hide) occasional bright stars along the way.  Using software, you can predict where on Earth the occultation will occur. Usually it’s visible in a zone, say 100 miles wide, like the path of totality during August 2017 total solar eclipse. Amateurs within the zone set up telescopes and time how long it takes the asteroid to cover and uncover the star. Someone near the edge of the zone of visibility will see it blink out for only a fraction of a second because only the asteroid’s edge covers the star, while an observer at the “centerline,” where the center of the asteroid crosses the star, will see the star disappear for several seconds. If a lot of people at different places within the path participate, you can convert all the different “blink out” times to create a profile of the asteroid. Pretty incredible.

New Horizons took this image of Ultima Thule Jan. 1, when the NASA spacecraft was 5,494 miles (8,862 km) beyond it. The photo at left is made from raw images and show motion blur from the longer time exposure. It’s been sharpened at right. NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute/National Optical Astronomy Observatory

If you’re ever interested in observing an occultation, be sure to check out the International Occultation Timing Association’s (IOTA) pages. Scientists watched Ultima Thule do the same thing from nearby to give us a much better idea of its shape:

“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,” said Alan Stern, the mission’s principal investigator. “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.”

I agree. Pancakes, Milk Duds — what next?

7 Responses

  1. Edward M Boll

    The weather has been so uncooperative like in December, still waiting to see Comet Y1. The second observation now with the naked eye has been reported to COBS, the second one put magnitude 6.1. But the average reports is about 6.6.

  2. Edward M Boll

    Yes, the brightest number I have seen for Iwamoto so far is 6.0, and that is not naked eye. But the average reports now are around 6.8.

  3. Doug Lowe

    UltimaThule has got to be one of the least interesting astronomical objects that Earth has visited to date. It has got a wow factor of minus 10. Sorry!.

    1. astrobob

      Sorry you feel that way about it, Doug. I imagine some find it uninteresting, but for me and other it’s a fascinating object for its says about the evolution of the planets. Plus it was a great way to squeeze more out of the New Horizons mission originally intended to only explore Pluto. Ultima Thule was gravy.

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