Our Predictions about Pluto: Getting it Right/Getting it Wrong

By Mark R. Showalter, Senior Research Scientist

Back in June, my research colleage Doug Hamilton and I put out a paper in Nature Magazine about the four small moons of Pluto. The timing was not accidental. Although the paper was the culmination of years of work with the Hubble Telescope, we knew that a lot of our predictions would be tested barely a month later, when the New Horizons spacecraft passed Pluto. Making predictions that might be proven wrong is part of the fun, and also part of the danger, of scientific research.

As we all know, the flyby was a great success and we are now waiting, patiently, for the slow trickle of images and other data to come back from the spacecraft. Today, NASA has released the first "family portrait" of Pluto's four small moons. As someone who has spent years studying these objects as nothing but faint dots, I find it is enormously gratifying to see them as resolved bodies, with shapes, colors and surface features.

Pluto's moonsFamily Portrait of Pluto’s Moons: This composite image shows a sliver of Pluto’s large moon, Charon, and all four of Pluto’s small moons, as resolved by the Long Range Reconnaissance Imager (LORRI) on the New Horizons spacecraft. All the moons are displayed with a common intensity stretch and spatial scale (see scale bar). Charon is by far the largest of Pluto’s moons, with a diameter of 751 miles (1,212 kilometers). Nix and Hydra have comparable sizes, approximately 25 miles (40 kilometers) across in their longest dimension above. Kerberos and Styx are much smaller and have comparable sizes, roughly 6-7 miles (10-12 kilometers) across in their longest dimension. All four small moons have highly elongated shapes, a characteristic thought to be typical of small bodies in the Kuiper Belt. Credits: NASA/JHUAPL/SwRI

So what did we get right? Well, based on years of studying how the brightness of each body varies, we were able to determine the rough shapes of the two larger moons, Nix and Hydra. On that point, we nailed it! Also, by making assumptions about how bright the surfaces are, we could make estimates of their sizes. We have learned that the moons are a bit brighter than we expected, and therefore a bit smaller, but overall our predictions have held up well.

...with one big exception--Kerberos. Our paper predicted that Kerberos would be big and dark, whereas the image clearly shows an object that is small and bright. What went wrong? Well, it all comes down to the moons' masses---how much they would weigh if you could put them on a scale. The only way to determine the masses of these moons is to study how each one subtly alters the courses of the others. Based on years of precise measurements, our colleagues believed that they had weighed Kerberos, and its mass was surprisingly high--about a third that of Nix and Hydra. On the other hand, from our measurements, it only reflected about 5% as much sunlight. The only way to make an object that faint and that massive is to make it very very dark. That darkness, comparable to that of a charcoal briquette, is not out of line with other bodies in the outer Solar System, but it certainly made it different from its Plutonian siblings.

KerberosKerberos Revealed. This image of Kerberos was created by combining four individual Long Range Reconnaissance Imager (LORRI) pictures taken on July 14, approximately seven hours before New Horizons’ closest approach to Pluto, at a range of 245,600 miles (396,100 km) from Kerberos. The image was deconvolved to recover the highest possible spatial resolution and oversampled by a factor of eight to reduce pixilation effects. Kerberos appears to have a double-lobed shape, approximately 7.4 miles (12 kilometers) across in its long dimension and 2.8 miles (4.5 kilometers) in its shortest dimension. Credits: NASA/JHUAPL/SwRI

We know now that Kerberos is small and bright. If the mass determination is right, then Kerberos is absurdly dense--many times denser than  lead. That seems unlikely. We therefore conclude that we probably had a broken scale. Because weighing the moons of Pluto is such a tough job, this is not out of the question. Now that we have new and more precise measurements of the orbits of the moons, I boldly predict that we will soon learn that the mass of Kerberos is much lower than we had previously thought.

One word of caution, however: my bold predictions don't always turn out to be right.

NASA Press release follows

Last of Pluto’s Moons – Mysterious Kerberos – Revealed by New Horizons

Images of Pluto’s tiny moon Kerberos taken by NASA’s New Horizons spacecraft –and just sent back to Earth this week – complete the family portrait of Pluto’s moons.

Kerberos appears to be smaller than scientists expected and has a highly-reflective surface, counter to predictions prior to the Pluto flyby in July. “Once again, the Pluto system has surprised us,” said New Horizons Project Scientist Hal Weaver, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

The new data, downlinked from the New Horizons spacecraft on Oct. 20, show that Kerberos appears to have a double-lobed shape, with the larger lobe approximately 5 miles (8 kilometers) across and the smaller lobe approximately 3 miles (5 kilometers) across. Science team members speculate from its unusual shape that Kerberos could have been formed by the merger of two smaller objects. The reflectivity of Kerberos’ surface is similar to that of Pluto’s other small moons (approximately 50 percent) and strongly suggests Kerberos, like the others, is coated with relatively clean water ice.

Before the New Horizons encounter with Pluto, researchers had used Hubble Space Telescope images to “weigh” Kerberos by measuring its gravitational influence on its neighboring moons.  That influence was surprisingly strong, considering how faint Kerberos was. They theorized that Kerberos was relatively large and massive, appearing faint only because its surface was covered in dark material. But the small, bright-surfaced Kerberos--now revealed in these new images--shows that the idea was incorrect, for reasons that are not yet understood. 

“Our predictions were nearly spot-on for the other small moons, but not for Kerberos,” said New Horizons co-investigator Mark Showalter, of the SETI Institute in Mountain View, California. The new results are expected to lead to a better understanding of Pluto’s fascinating satellite system.