Be a Warrior — Tackle a Trojan Asteroid

You've seen Jupiter and its four brightest moons. Now meet the rest of the family — the Trojan asteroids.

Along for the ride

Jupiter's Trojans (green) follow the giant planet (orange) on its orbit around the Sun. This animation shows their movements along with the inner planets. Though not obvious here, many of the asteroids have large orbital inclinations, up to 40° above Jupiter's orbital plane. 
Petr Scheirich (Astronomical Institute of the Czech Academy of Sciences)

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.

Trojan locator

This map gives you an approximate idea where the L4 Trojan group is currently located. It begins about 60° to the east of Jupiter and extends well above and below the ecliptic into Pisces and Pegasus (and possibly further).
Stellarium, with additions by the author

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.

Take an imaginary flight to the contact binary 624 Hektor and its moon Skamandrios. From Franck Marchis

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.

Telescopic view of Hektor

624 Hektor, the largest Jupiter Trojan at 403 kilometers across, appears stellar in all amateur scopes. This asteroid, like the other Trojans, is located about 5 astronomical units from Earth.
Kevin Heider CC BY-SA 3.0

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.

Lucy's path

The convoluted path Lucy will take to reach six Jupiter Trojans and the carbon-rich main belt asteroid 52246 DonaldJohanson.
Southwest Research Institute

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.

Getting there

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).

Big picture

Use this wide-field map to locate the general vicinity of each of the featured asteroids. The stars labeled here are also circled and labeled on each individual chart.

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!

624 Hektor Mag. (14.5 and brightening)
659 Nestor (15.4 and slowly fading)
911 Agamemnon (14.8 and " ")
1143 Odysseus (14.8 and " ")
1437 Diomedes (15.4 and brightening)
16 Psyche (9.9 and fading)

9 thoughts on “Be a Warrior — Tackle a Trojan Asteroid

  1. RodRod

    Bob, I enjoyed this report. As you said “…with magnitudes that range from — take a deep breath…”. Yes these are indeed faint targets 🙂 The origin of Jupiter looks like the Grand Tack model “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.” Modeling the origin of our solar system is not easy. Today in exoplanet studies, we now see exoplanets larger than Jupiter that are extremely eccentric orbits around their host stars. HR 5183 b,, and HD 20782 b,, nothing like our Jupiter or solar system.

    1. Bob KingBob King Post author

      Thanks, Rod! I hope you try to see one. Go for Hektor, the brightest. Big number magnitudes don’t scare me away like they once did. I’ve seen 14th magnitude galaxies. Those are tough with their light spread over 30 arcseconds or more. All the light of a 14.5-magnitude asteroid is concentrated in a tiny point, making it relatively easy in contrast.

  2. Tom-Reiland

    All but Psyche are too faint for me to observe from my location. I have observed Psyche many years ago Though it’s not about this post, V3890 Sagitarii, a re-occurring Nova is 9th magnitude or brighter. It’s about a degree and a half West of M22. The AAVSO has an excellent chart on its webpage at I observed it at 1:20 AM EDT (8/29/19) and it was very easy to see at about 8.6 magnitude. There’s an 8.3 mag star very close to it to the ESE of the Nova. V3890 is as faint as 18.4 mag at minimum. It’s almost 10 magnitudes brighter now than it is at minimum or 10,000 times brighter. I love observing objects with wide ranges in brightness.

      1. Tom-Reiland

        I was wondering why you hadn’t mentioned it and I can understand the limits on posts. I know that your posts have helped me locate and learn more about objects I would have missed, such as CTA 102. I look forward to learning more, even though I’ve been observing for more that 47 years. There’s always something new that awaits us on the next clear night. Quick correction of my previous reply. I should have typed “recurring Nova” instead of “re-occurring.” I observed V3890 last night and it is still rather bright at 9 magnitude.

  3. Joe StieberJoe Stieber

    I was out to a new site in the New Jersey Pines last night (29/30-Aug-2019) with my 16-inch Dob. What a treat to have a clear, transparent sky at new moon (so prominent Milky Way), as well as cool temperatures and few mosquitoes!

    Anyway, I decided to try for the low-hanging fruit on the list, (16) Psyche and (624) Hektor. Psyche was, at magnitude 9.9, not much of a challenge, but still nice to see. I then tried for Hektor, working my way up from Neptune (at the time, Psyche was about 3.3° north of Neptune). Even though the nominal magnitude of 14.5 was a tad dimmer than Pluto at 14.2 (observed in mid-July 2019), Psyche wasn’t that difficult to see, certainly easier than Pluto. I suppose it’s because of the greater altitude, about 41.5° when I was looking last night vs. about 28° for Pluto last month. Of course, last night’s fine sky conditions probably helped too.

    I now regret that I wasn’t prepared for the other Trojans on the list, especially (911) Agamemnon and (1143) Odysseus, both magnitude at 14.8. But I now have increased optimism for spotting them in the hopefully-near future.

    1. AstroNerd7

      Bob, I ran 624 Hektor’s position and brightness for every 7 days for the next 10 years or so and it seems like a more opportune time to attempt an observation is around February 19, 2025. It should be in Leo (near Regulus), have a Mv=14.1 and have a declination of +16. Additionally, darkness occurs earlier so it doesn’t necessarily have to be such a late affair. I have a C-11 so I’ll try to observe it in the next week or so.

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