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A Digital Supplement to Astronomy Insights Astronomy Magazine © 2018 Kalmbach Media Comets & Asteroids More Alike than Different? October 2018 • Astronomy.com How are comets and asteroids related? The solar system is a complex place. Aside from the Sun and major planets, it is filled with hundreds of thousands of smaller bits of debris left from the solar system’s formation some 4.6 billion years ago. In the 19th cen- heating up and putting on a show with their tury, astronomers were concerned primarily spectacular tails. with discovery. Whether moons, comets, or Asteroids, on the other hand, are rocky asteroids, finding and cataloging objects bodies — micro-planets, if you will — most was the primary means of employment. And of which reside in the asteroid belt, which it is still an important part of the scientific lies between Mars and Jupiter. Asteroids process because many unusual and import- show no traits similar to comets, such as the ant objects certainly remain to be found. outgassing of volatiles that form pretty tails But in recent times, planetary scientists in the sky. Moreover, their orbits are dis- have turned increasingly to interpreting tinctly different from those of comets. what they have found. Analyzing objects But a view developed during the past with a growing number of techniques has 25 years suggests comets and asteroids, allowed astronomers to paint a sharper pic- although distinct types of bodies, are more ture of the solar system’s formation and the alike than astronomers originally thought. RUBBER DUCKY. Comet 67P/ relationships between objects in it. Trillions of comets probably exist in our solar Comets with Churyumov-Gerasimenko 1 For many years, astronomers kept com- system; the long-period ones exist in a vast hyperbolic or para- is seen close-up in this ets and minor planets, or reservoir called the Oort Cloud, named after bolic orbits appear image taken January 31, SPACE SPUD. This set of asteroids, in separate Dutch astronomer Jan Oort, who hypothe- only once, after which 2015, by the Rosetta nine color images of the categories. After all, com- sized its existence in the 1950s. they are flung outside spacecraft. Comets and asteroid 243 Ida reveals ets are frozen balls of ice Periodically, a gravitational kick from a the solar system. asteroids are leftover debris the potato-shaped structure and gas that live in the passing star or a disturbance in the Oort Lastly, main-belt com- from the solar system’s of this space rock, which initial formation 4.6 billion solar system’s distant Cloud sends long-period comets inward ets, a group consisting measures 36 by 14 miles in years ago. ESA/ROSETTA/NAVCAM extent. The images were reaches. Occasionally, toward the Sun. Short-period comets, on of only about a dozen captured by the Galileo through a gravitational the other hand, reside closer in and have members, reside in spacecraft August 28, 1993. kick, they travel inward, orbits with periods less than 200 years. the main asteroid belt. Similar objects, with their circular orbits, may have deposited water on Earth billions of years ago. This class, which includes 133P/Elst-Pizarro, 118401 (1999 RE70), and P/2005 U1 (Read), exists at the crossroads between comets and asteroids. By contrast, according to the Minor Planet Center, scientists know of nearly 780,000 minor planets. Of these, about 520,000 have been studied well enough to warrant permanent numerical designations, and more than 21,000 asteroids have names. Astronomers believe the total number of asteroids in the solar system larger than 1 mile (1.6 km) across is over 1 million. The largest asteroid in the main belt is Ceres, which has a diameter of about 600 miles (970 km). Altogether, the mass of NASA/JPL 2 ASTRONOMY INSIGHTS • OCTOBER 2018 NASA/NOAO/NSF/T. RECTOR, Z. LEVAY, AND L. FRATTARE L. AND LEVAY, Z. RECTOR, NASA/NOAO/NSF/T. DIRTY ICEBALL. HOLY ASTEROID. From main-belt asteroids adds up to only about found a faint cometary coma surrounding Comet NEAT (C/2001 Q4) July 2011 to September 2012, 4 percent of the Moon’s mass. this body. Finally, 4015 Wilson-Harrington, typifies the kind of NASA’s Dawn spacecraft During the 1980s and 1990s, scientists’ an object first observed as a comet in 1949 soft coma and glowing captured the images used to neat distinction between comets and aster- and later independently discovered as an tail that amateur create this beautiful mosaic of oids began to blur. With the discovery of the asteroid in 1979, shares both designations. astronomers hope to see the massive asteroid 4 Vesta. Kuiper Belt, it became clear that a vast pop- The 1949 observations showed cometary through their telescopes. Vesta has a towering ulation of icy bodies existed on the solar sys- characteristics, while the later ones did not. Astronomers using the mountain at its south pole WIYN Telescope on Kitt (bottom) that’s twice the tem’s fringe, much closer than the Oort Cloud. All this points to a complex solar system in Peak, Arizona, caught height of Mount Everest, as Discoveries of main-belt comets further which the orbits and behaviors of asteroids this view of NEAT on well as a set of three craters blurred the distinction. Certainly, some and comets are related more than astrono- May 7, 2004. (top left) commonly referred to main-belt asteroids could be ancient mers originally envisioned. as the “snowman.” ex-comets whose volatiles evaporated long ago, leaving only a rocky core. The discoveries of other bodies with peculiar properties led to the cross-listing of several objects as both asteroids and com- ets. These include 2060 Chiron, an object discovered in 1977 by American astron- omer Charles T. Kowal, then at Palomar Observatory. It’s the first member of an asteroid class known as centaurs, with orbits between Saturn and Uranus. This unusual asteroid came under intense study, and in 1988, astronomers watched it surprisingly undergo an outburst in brightness, an act characteristic of comets, not asteroids. Additionally, 60558 Echeclus, a centaur discovered in 2000, appeared to be an inno- cent asteroid. But in late 2005, astronomers NASA/JPL-CALTECH/UCAL/MPS/DLR/IDA WWW.ASTRONOMY.COM 3 THE BEST IMAGES ARE IN HD Celestron’s EdgeHD optical system sets a higher standard for catadioptric telescopes, revolutionizing both visual observing and astroimaging. Its aplanatic Schmidt optical design produces a completely flat field—all the way to the edge of today’s ultra-wide eyepieces and large imaging sensors. The result is crisp, high resolution images devoid of visual defects like off-axis coma, field curvature, and astigmatism. So, are you ready to experience the EdgeHD difference for yourself? For the month of October, all German equatorial EdgeHD telescopes and optical tubes are on sale. Enjoy big discounts on all our customer favorites, from the portable and beginner-friendly Advanced VX 8” EdgeHD to our flagship CGX-L 1400HD, and everything in between. SAVE NOW on all German equatorial EDGEHD telescopes. Sales ends 10/31 CELESTRON PREMIER SELECT DEALERS B&H Photo – 800.947.9970 – bhphotovideo.com OPT Telescopes – 800.483.6287 – optcorp.com High Point Scientific – 800.266.9590 – highpointscientific.com Woodland Hills – 888.427.8766 – telescopes.net Optics Planet – 800.504.5897 – opticsplanet.com Adorama – 800.223.2500 – adorama.com SHOP THE SALE AT Astronomics – 800.422.7876 – astronomics.com Focus Camera – 800.221.0828 – focuscamera.com Exploring Jupiter’s TROJAN ASTEROIDS Astronomers have studied the giant planet’s captured asteroids only from afar. That’s about to change. by Joel Davis In 2021, NASA will launch the Lucy mission, which will investigate two primitive asteroid populations that congregate at stable points along Jupiter’s orbital path. By getting a closer look at these asteroids, called Trojans, Lucy may revolutionize our understanding of how the solar system formed. ASTRONOMY: ROEN KELLY 6 ASTRONOMY INSIGHTS • OCTOBER 2018 WWW.ASTRONOMY.COM 7 upiter is by far the largest and ancient bodies. What the spacecraft uncovers could most massive planet in the solar confirm some current theories of the solar system’s system. And befitting a world early evolution — or turn it all upside down. named for the Roman king of the gods, Jupiter has an impres- The sweet spots sive entourage. It includes a Every planet has several gravitational “sweet spots” set of faint and dusty rings, 67 where a relatively tiny body, like an asteroid, can known or suspected moons, and maintain a fairly stable position in relation to two two swarms of asteroids that larger bodies, such as the Sun and the planet, or precede and follow the planet in the planet and its moon. The gravitational pull its orbit. These last are the Trojan asteroids. between the two large bodies provides enough For all we’ve discovered about Jupiter, its centrifugal force to keep the smaller object orbit- Jmoons, and even its gossamer rings, we know pre- ing with them. These sweet spots are called cious little about the Trojans. Pioneers 10 and 11, Lagrangian points, named for Joseph-Louis the two Voyagers, Galileo, and Juno have all Lagrange, who identified two of them in 1772. returned a wealth of data about the jovian system. Five Lagrangian points exist for each such sys- Until now, though, the only way to study the tem. L1, L2, and L3 (discovered by mathematician Trojans has been from afar, with ground-based Leonhard Euler a few years before Lagrange iden- and Earth-orbiting telescopes. tified the other two) fall on a straight line drawn That’s about to change. In 2017, NASA gave the through the two large masses.
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