Astrophotography: SHOOTING Test Report: 3 MINI Lunar Mystery: AN ECLIPSE ANALEMMA p. 66 MEADE SCOPES p. 58 THE BESSEL RAY p. 52 THE ESSENTIAL GUIDE TO ASTRONOMY Triangulum Galaxy in Depth p. 54 Observing: 12 Steps to Infi nity p. 24 DECEMBER 2016 Revealing Pluto’s Moons p. 36 The Comets of Dawn’s Discoveries Edgar Allan Poe p. 30 at Ceres p. 16 Visit SkyandTelescope.com Download Our Free SkyWeek App S olar System Expedition Dawn of Discovery at Ceres NASA’s Dawn spacecraft has found a world of salt deposits, water ice, and potentially even signs of an ancient ocean. NASA / JPL-CALTECH FOR MOST ASTRONOMERS, the arrival of dawn planets than to asteroids (S&T: Sept. 2012, p. 32). It then means the end of observing. Intriguing sights become left to continue its mission to Ceres. more and more diffi cult to see, eventually disappearing Dawn began approach photography on January 13, entirely. Not so with dwarf planet Ceres. The arrival of 2015, two months before it entered orbit. It was as far Marc D. NASA’s Dawn spacecraft in 2015 meant the beginning of from the dwarf planet as the Moon is from Earth. Dawn Rayman more than a year of extraordinary discoveries. was designed to conduct a reconnaissance of unex- The spaceship left Earth in 2007 on an ambitious plored worlds from orbit, and it is not equipped with a journey to explore two giants of the main asteroid belt, powerful telescope. From a distance of 383,000 kilome- Vesta and Ceres. Propelled by its ion engines on a mis- ters (238,000 miles), Ceres was only about 27 pixels in sion to boldly go where — well, you know the rest — diameter. With a resolution of 36 km per pixel, these Dawn orbited Vesta in 2011–12, revealing a complex body initial images were not taken to learn about Ceres itself more closely related to Earth and the other terrestrial but rather to locate it against the background of stars. Distance: 383,000 km (238,000 mi) 46,000 km (29,000 mi) 7,200 km (4,500 mi) NASA / JPL-CALTECH / UCLA / MPS / DLR / IDA (3) 16 December 2016 sky & telescope Navigators needed the images in order to help pin down unravel Ceres’ secrets, scientists integrate results from all the position of Dawn’s destination accurately so that they of Dawn’s measurements and combine them with three could pilot the spacecraft into orbit. more ingredients: comparisons with other solar system But those very fi rst pictures showed what has become bodies, comparisons with laboratory measurements of the most famous and mysterious feature: a bright spot. candidate minerals, and the results of extensive math- A decade earlier, the Hubble Space Telescope, with reso- ematical modeling of the physics, geology, and chemistry. lution of about 30 km per pixel, had detected the spot as It will take years to put the pieces together to understand well. By the time of Dawn’s next pictures, on January 25, the big picture. For now, however, we have only begun to 2015, the resolution was one-third better than Hubble’s, digest the extraordinary bounty from Dawn. and the bright spot only grew more salient with those and subsequent pictures. The spot seemed to glow, A Salty Spotlight casting its mesmerizing light out like a cosmic beacon, Before Dawn brought Ceres into focus, one popular idea guiding the way for our interplanetary ship as it sailed was that the spots could be exposed ice. This idea’s appeal the solar system’s seas. might have been due to a report in 2014 that the Herschel Dawn continued to photograph Ceres occasionally Space Observatory had detected the faint but unmistak- prior to its graceful entry into orbit on March 6, 2015, able signal of water vapor around Ceres. From its orbit using the gentle thrust of an ion engine. And it has around the Sun near the second Lagrangian point, about achieved better and better resolution since then, operat- 1.5 million km from Earth, Herschel was much too far ing in a series of four circular, polar mapping orbits. from Ceres to see any detail. But scientists estimated that This has allowed us to progress from an overview of the the amount of water vapor could be explained by only entire world to an intimate portrait. The best pictures about 150 acres, or 0.6 square km, of ice on the surface. now, taken from an altitude of only 385 km, are one thou- As ice sublimated in the cold vacuum, transform- sand times sharper than the fi rst. Dawn orbits closer to ing directly from a solid to a gas, it could produce the Ceres than the International Space Station does to Earth. extremely tenuous veil of water vapor Herschel detected. From this distance, Ceres appears as large as a soccer But the bright spots Dawn saw covered too much area ball would when seen from a mere 9 cm (3.5 inches). to be the source. Even just the central bright region is As the resolution improved, what initially had appeared 500 times larger than the area of ice scientists calculated as one bright spot became two and soon a multitude clus- would yield the detected water vapor. Exposed ice was not tered together. Scientists and the public alike were capti- a good explanation for the spots. vated by them. NASA’s Jet Propulsion Laboratory polled With similar reasoning, the bright region couldn’t the public for their votes on what the spot was, and more have been a cryovolcano or geyser, however exciting that than 190,000 responded. One question I often received might be. For such a phenomenon to be bright enough was whether it could be the illumination from an alien for Dawn to detect even with the early, low-resolution city. I fi nd such questions disappointing, and I despair of pictures, it would have been enormous. Once again, it critical thinking. After all, given how little we knew about would not have been consistent with Herschel’s fi ndings. Ceres, how could we possibly know whether Cereans even As the “spots” became better resolved, they no longer live in cities? Perhaps they only live in rural areas, or only resembled discrete points but rather a complex distribu- in large states. Maybe they only live underground. tion of refl ective material inside a 92-km-wide crater now Seriously, without taking advantage of the power of the named Occator. scientifi c method and thorough consideration of all avail- Based on all the data acquired so far, scientists con- able data, it is all too easy to reach mistaken conclusions. sider the most likely explanation for the highly refl ective If understanding an exotic world required only stun- ground cover in Occator to be some kind of salt. It may ning pictures, the work at Ceres would be complete. To be that the heating from the impact that created the MYSTERIOUS SPOT Ceres’ bright beacon looks like a smudge in Dawn’s fi rst approach image (far left). But as the spacecraft neared, the feature revealed itself to be a complex of refl ective patches in the crater Occator. Although Occator’s “spot” is the bright- est, it is only one of more than 130 of these features revealed 1,470 km (915 mi) 385 km (240 mi) NASA / JPL-CALTECH / UCLA / MPS / DLR / IDA (2) by Dawn on the dwarf planet. SkyandTelescope.com December 2016 17 Solar System Expedition 90° Jarovit ° 60 Datann Geshtin Messor Ezinu Kaikara 30° Fejokoo S Nawish Occator am ha in Ca Heneb HANAMI te na Kirnis e Liberalia Rongo 0° Mons Azacca Lociyo PLANUM Kumitoga Ahuna Mons Tibong Belun Consus a n e −30° t a C G l Lono erb a er C g Yalode atena n o P Meanderi Besua B altay Catena Urvara −60° Sekhet 180°E 210°E 240°E 270°E 300°E 330°E CERES’ SURFACE AREA molecules escaping into space or settling elsewhere on Ceres. When they departed, they would have left behind Ceres spans 2.8 million km2 (1.1 million mi2). That’s the dissolved salts. about the area of Argentina, or 35% of the area of the contiguous United States. We base Occator’s (tentative) age on the number and size of smaller, more recent craters both within it and on the blanket of ejected material surrounding the crater. crater 80 million years ago caused underground briny That makes Occator young in geologic terms, given that water to fl ow upward to the surface, perhaps through Ceres itself is more than 4.5 billion years old. the many cracks visible to Dawn’s camera. Once on the Further evidence for the crater’s youthfulness comes cold surface, the brine would freeze. And exposed to the from it being slightly bluer than most of Ceres. Mate- vacuum of space, solid ice would sublimate, the water rial tends to redden gradually with exposure to the 18 December 2016 sky & telescope Ysolo Mons Omonga Oxo Coniraya Vinotonus VENDIMIA Gaue Dantu Inamahari Homshuk Haulani Rao PLANITIA Anura Piuku Kerwan Kondos Wangala Tholus Jarimba Oltagon Niman Rupes Darzamat Sintana Toharu Chaminuka Hamori Zadeni NASA / JPL-CALTECH / UCLA / MPS / DLR / IDA 30°E 60°E 90°E 120°E 150°E BRIGHT SPOTS GALORE Occator’s spots are the most famous, but there are more than 130 other locations on Ceres that are com- paratively refl ective. Shown here in red are the bright spots’ approximate locations, along with a selection of feature names.