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Asteroid 2015 BZ509

May 8, 2017

Naming Solar System bodies was easier in prior centuries, when there were just six planets (seven if the Moon is counted as a planet). Galileo's telescope revealed features on the Moon that begged to be named; but, more importantly, he discovered four of the Moons of Jupiter (there are now 67). Galileo named these the Medicean stars to honor some important patrons, but they were quickly renamed as Io, Europa, Ganymede, and Callisto.

The major features of the Moon, Venus, Mars, and Mercury have been named according to established conventions, but our technology has advanced to the point at which there are so many things to be named that naming is becoming difficult. The New Horizons imaging of Pluto gave an opportunity for naming Pluto's geological features.

The difficulty of finding appropriate names for Pluto's features is illustrated by such obscure references as Vucub-Came Macula, named after one of the Maya death gods from the Popol Vuh, and Sun Wukong Fossa, named after Sun Wukong, the Chinese monkey god who was imprisoned by Buddha.

Sun Wukong (Tsukioka Yoshitoshi)

The Chinese monkey god, Sun Wukong, can transform into various objects and animals, but with an incomplete transformation of his tail. This trait of incomplete transformation is the same for the character, Odo, in Star Trek: Deep Space Nine.

(Portion of an illustration by Tsukioka Yoshitoshi, via Wikimedia Commons.)

The first discovered asteroids were Ceres, Pallas, and Vesta. These had rather conventional names, but there are now more than 20,000 asteroids with names recognized by the International Astronomical Union.[1] That's a lot of names, but there are actually 25 million asteroids with diameter greater than 100 meters (see graph).

Asteroid size distribution

Distribution of asteroid sizes.

(Graphed using Gnumeric.)

The International Astronomical Union has tackled the problem of keeping track of so many objects with a simple year, number, and letter naming convention for discovered asteroids.[2] A generic designation for a new asteroid is YYYY αβn, in which YYYY is the four digit year (e.g., 2017); α is the half month of discovery, coded as A-Y with 'I' omitted (e.g., 'M' is the last half of June); β is the first reference to the sequence of discovery, coded as A-Z, with 'I' omitted, for 1-25; and n is a number that extends the sequence in the following fashion: A-Z is 1-25, A1-Z1 is 26-50, A2-Z2 is 51 to 75, etc.. Computer scientists will recognize this as a modulus operation.

As an example of this naming convention, let's examine 1993 TQ1, also known as 46610 Bésixdouze. This somewhat large, two kilometer asteroid was discovered on October 15, 1993. This date, in the first half of October (dates 1-15), gives us the 'T' to supplement 1993. The remaining part of the designation indicates that this is the 25+16=41st asteroid discovered in that period.

A team of planetary astronomers associated with the University of Western Ontario (London, Ontario, Canada), the Athabasca University Observatories (Athabasca, Alberta, Canada), the University of Nagoya (Nagoya, Japan), the University of Calgary (Calgary, Alberta, Canada), and the Large Binocular Telescope Observatory (Tucson, Arizona) has just published a study on the orbit of asteroid 2015 BZ509.[3-4] They found that 2015 BZ509 orbits the Sun in a retrograde direction; that is, it orbits clockwise when looking down on the Solar System, while nearly every other object orbits counter-clockwise.[3]

At the time of its discovery by the Panoramic Survey Telescope And Rapid Response System (Pan-STARRS), the orbit of 2015 BZ509 was largely uncharacterized, but it appeared to be close to Jupiter's co-orbital zone. This made it interesting enough for further study with the Large Binocular Telescope Observatory.[6] The surprising discovery from that study was the retrograde co-orbital nature of this asteroid.[6] The measurements indicate that the orbit of 2015 BZ509 has been stable for a million years, it should continue for a million years hence, and that other retrograde asteroids co-orbital with Jupiter and other planets may be more common than previously thought.[3-4,6]

Orbits of Jupiter and 2015 BZ509

The strange orbit of 2015 BZ509. The Sun and the orbits of the inner planets, Mercury through Mars, are at the center, and the orbits of Jupiter and 2015 BZ509 are shown. The green line indicates how the orbit of 2015 BZ509 appears from Jupiter. (Drawn using Inkscape from data in this Western University, Canada, video.[6]

2015 BZ509 may have originated from the same region as Halley’s comet, another object with a retrograde orbit.[3-4] It might be an inactive icy comet nucleus, and not a rocky asteroid, that was pulled into its orbit through the gravitational attraction of Saturn.[3,6] Only 82, about 0.01 percent, of the known asteroids have been found to be retrograde.[4-5] Retrograde asteroids will be in relative proximity to a planet twice an orbit, and gravity will eventually destabilize their orbits, eliminating them from the asteroid pool.[5] The retrograde co-orbital resonance state that makes the orbit of 2015 BZ509 stable was first conjectured in 2012.[6]

Comet 103P/Hartley (NASA EPOXI Mission)

Comet 103P/Hartley.

This comet has a diameter of about 1.5 km, while 2015 BZ509 has a larger diameter of about three kilometers.[4]

This image was taken by the NASA EPOXI mission spacecraft on Nov. 4, 2010.

(NASA image.)

2015 BZ509 shares the Jupiter orbital space with about 6000 other asteroids called the Trojan asteroids.[4,6] 2015 BZ509 avoids a collision with Jupiter by an unusual gravitational interaction in which the influence of Jupiter in one orbit is cancelled in the next.[5-6] Jupiter causes 2015 BZ509 to alternate between two different orbits, and it never comes closer to Jupiter than 176 million kilometers, which is about the Earth-Sun distance.[5-6] This is the first asteroid found to have such an interaction with a planet, and its orbit appears to be stable for at least the next million years.[6]


  1. Minor Planet Names: Alphabetical List, Smithsonian Astrophysical Observatory, Harvard-Smithsonian Center for Astrophysics.
  2. New- And Old-Style Minor Planet Designations, Smithsonian Astrophysical Observatory, Harvard-Smithsonian Center for Astrophysics.
  3. Paul Wiegert, Martin Connors, and Christian Veillet, "A retrograde co-orbital asteroid of Jupiter," Nature, vol. 543, no. 7647 (March 30, 2017), pp. 687-689, doi:10.1038/nature22029.
  4. Wrong-way asteroid plays 'chicken' with Jupiter, Western University Press Release, March 29, 2017.
  5. John Timmer, "Encounters with Jupiter send asteroid on a bizarre backward spin," Arstechnica, March 29, 2017.
  6. Paul Wiegert, Martin Connors, and Christian Veillet, "The first retrograde co-orbital asteroid: 2015 BZ509 - A Trojan in retreat, Department of Astronomy, Western University, Web Site, March 30, 2017.

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