You’ve seen Jupiter and its four brightest moons. Now meet the rest of the family — the Trojan asteroids.
You may have to fight an astronomical version of the Trojan War to capture the faint quarry we seek this week. When you do you can be proud of your observing accomplishment. I’m talking about the Trojan asteroids, two loose groups of faint, dark asteroids that share Jupiter’s orbit. They occupy one of two stable zones called Lagrangian Points.
The leading, or L4, group orbits ~60° ahead of Jupiter while the trailing L5 pack follows ~60° behind the giant planet. Together we know of slightly more than 7,000 Jupiter Trojans as of October 2018. Many more await discovery.
Most asteroids orbit between Mars and Jupiter in the main asteroid belt. The Trojans, named after the heroes of the Trojan War, were clawed away by Jupiter’s enormous gravitational pull as it migrated across the solar system 4 billion years ago, before it settled into its current location. Asteroids in the L4 group are named for the Greek heroes of the war; those at L5 for the Trojan heroes.
In late August, Jupiter stands near the meridian at sunset, close to eastern quadrature. The L5 group lies ~60 to 90° west of the planet in the Virgo-Leo region, too near the Sun to view. But the L4 group, located in Capricornus-Aquarius-Pisces, is well-placed in the evening sky for observation. Trojans at or near opposition in Pisces are currently shining at their brightest just like any planet would at opposition.
As skywatchers know well, bright is a relative term. I’ve selected five of the brightest Trojans in the “Greek camp” that are perfectly placed for viewing in the next week or two before bright moonlight threatens: 624 Hektor, 659 Nestor, 911 Agamemnon, 1143 Odysseus, and 1437 Diomedes. They’re all members of the leading group (located east of Jupiter, preceding it) with magnitudes that range from — take a deep breath —14.5 to 15.4. I chose the largest L4 objects assuming they were likely the brightest of the bunch.
Studies of the largest Trojan, 624 Hektor, with the 10-meter Keck II telescope reveal that it has two lobes, which may be a pair of co-orbiting asteroids known as a contact binary. It also has its own moon, a 12-kilometer-wide orbiting island of rock with the Star Wars-y name of Skamandrios. The others featured here are between 110 and 130 kilometers in diameter. Except for Nestor (a C-Type), all are dark, primitive D-type asteroids rich in organic compounds along with possible water ice and other volatiles. The Tagish Lake meteorite, which fell in British Columbia in January 2000, is composed of similar materials and may even be an ejected Trojan fragment.
Hektor is currently the brightest of the bunch at magnitude 14.5 and will continue to brighten in the next couple weeks to 14.3. I know that sounds faint, but it’s still within reach of even an 8-inch telescope from dark skies. I found all five from my home under a Bortle 4 (rural/suburban transition) sky with a 15-inch telescope using magnifications of 142x and 245x.
All you need is a good chart to track down these fascinating blips. Links to maps showing their nightly positions at 4h UT (midnight Eastern; 11 p.m. Central; 10 p.m. Mountain and 9 p.m. Pacific) through mid-September can be found below. Remember that Aug. 29 at 4h UT = Aug. 28 at 11 p.m. Central Time. Converting times in the Western Hemisphere means you have to back up into the previous night.
I’d hate for observers with smaller instruments to go asteroid-less, so I also made a chart for 16 Psyche as well. Compared to the Trojans, this ~225-kilometer-wide object practically glares at magnitude 9.8 in Capricornus and is well placed at nightfall. Psyche is a metallic object that may be the nickel-iron core of a larger, differentiated asteroid. NASA plans to launch an orbital mission (also called Psyche) to the metallic world in August 2022, to arrive on January 31, 2026.
Similarly, NASA’s Lucy Mission will launch in October 2021 on a 12-year survey of both the L4 and L5 clusters to study six Trojans and one main belt asteroid. Lucy is named for the 3.2 million-year-old early human ancestor, whose fossilized bones helped deepen our understanding of human evolution. In the same way, mission scientists hope to understand planetary evolution by studying primitive bodies, like the Trojans, that once helped build the planets.
Tote your scope out the next clear night and have a look at these Jovian captives while imagining the epic battle that happened more than 32 centuries ago.
Below are links to maps that locate each asteroid. Charts were created with MegaStar5 by Emil Bonanno, copyright 1992-2002 by E.L.B. Software and distributed by Willmann-Bell, Inc. North is up and stars are shown to magnitude 16 except for 16 Psyche (mag. 11).
Additional tidbits of information appear along the top of each map, such as the Uranometria 2000.0 chart that contains that particular field, field size and so on. Numbers to the left of the stars are magnitudes with the decimal points omitted. To make the charts as easy as possible to follow I have included a bright star in each field that you can reference using the wide-field map above. Center on that star and then star-hop to the asteroid. Easy!