This column first appeared in The Tablet in April, 2006, and first published here in 2015.
It was April Fool’s Day at the Vatican’s telescope in southeastern Arizona, and we’d had three nights of cloudy weather. This night looked promising, however. Bill Romanishin, our colleague at the University of Oklahoma, had given us a list of Kuiper Belt Objects to observe, orbiting out beyond Neptune. Included were some Centaurs, objects that some day might become comets plunging close to the Sun.
At the telescope, I was with Steve Tegler from Northern Arizona University. Along with each object, we also observed bits of blank sky, so we could find and remove all the dust spots on the images; and fields of stars whose colours and brightnesses were already well known, so that we could calibrate our objects against these known standards. Around midnight, about halfway into a field of standard stars, Steve looked over his list again and remembered, “Oh, Bill wanted us to try 60558. It’s a Centaur that’s supposed to have a coma.”
A coma, a cloud of gas? Odd that this object, thirteen times the Earth’s distance from the Sun, would be warm enough to start vaporizing its ices into a nascent comet tail. So when we finished the standards, we pointed to 60558 and took a “test” image, a 30 second exposure to make sure we were pointed at the stars near the object. We didn’t expect to see the Centaur itself with such a short exposure; at 20th magnitude, it would be a million times fainter than the faintest star visible to the naked eye. But when the image was finally downloaded from the camera chip and displayed on our computer screen, a coma was visible even in the test image!
We took a three minute exposure; any longer, and the motion of the Centaur would have blurred its image. It showed an irregular cloud of light actually detached, off to the west of the little dot of light that marked the Centaur itself. Very strange. We ran through the colored filters, and then went on to other objects. But the longer we thought about it, the stranger it seemed. Was it real, or just an internal reflection of some bright star onto our camera chip? Would it be there the next night, after the Centaur had moved to a new field of stars?
The next night, our last at the telescope, started out cloudy again. Looking at the satellite weather maps, it looked like we’d have a hole in the clouds just as our object was rising. Fingers crossed… waiting impatiently for the object to rise… the clouds were especially thick in the southeast… the hole was beginning to close in the west… finally… success! The thin clouds cut a magnitude’s worth of light out, but the coma was bright enough that we could see it anyway. There it was again. It was real.
We sent Bill the images to analyze, and he found in our data an even larger, but fainter, cloud of light on the other side of the nucleus. Our first bright cloud, we estimated, must be half the size of Jupiter; the bigger cloud is a good fraction of the Sun’s diameter.
What causes these comae? We don’t know. We’ve never seen anything like them before. A splitting Centaur? The result of a collision, or a pocket of exotic frozen gases exposed to the sun? Is this a major breakthrough or a mere curiosity? We don’t know; only time and further observations will tell.
Usually discoveries are revealed to us gradually, understood only after a lot of “data reduction” has removed the false images of dust on our lenses and compared our faint signals to known standards of truth. But on wonderful rare occasions like this one the discovery blares at us, as obvious as the empty tomb.
In 2006, the object known as asteroid 60558 Echeclus was also given a cometary designation as 174P/Echeclus.
As of 2015, we still didn’t know for sure the origin of this coma. It was tracked for several months and seen to be on an orbit that is slightly different from that of 60558; our best guess is that the source of the coma broke off the main body about a thousand years ago and the two objects have been chasing each other around the solar system since then.
By 2019, Echeclus had its own Wikipedia page, where one can read that “In 2016, carbon monoxide was detected in Echeclus in very small amounts, and the derived CO production rate was calculated to be sufficient to account for the observed coma. The calculated CO production rate from Echeclus is substantially lower than what is typically observed for 29P/Schwassmann–Wachmann, another distantly active comet often classified as a centaur.”