May 28, 2020

Astro-Challenge: Spot a “Dark Asteroid.”

The path of 10 Hygiea during the month of May 2011.

(Created by the Author in Starry Night & Paint).

This week, the planetary conjunctions continue in the dawn skies, one of the better southern hemisphere meteor showers revs up, and we’ll turn you attention to an asteroid you’ve never heard of, but should have.

First off, the opening week of the month of May sees the rise of Jupiter, Mars, and Mercury out of the murk as the bright planet Venus dips to meet the trio. When is the first morning that you can spot all four? The angle of the morning ecliptic is set to a southerly inclination relative to the horizon; the farther south in longitude you are, the easier it will be to catch the grouping. Mercury reaches greatest elongation on May 7th, and Jupiter is headed towards a good pairing with Venus around mid-month.

Next up is the Eta Aquarids, one of the best meteor showers with a radiant south of the celestial equator. This shower peaks on the morning of May 6th. Expect a Zenithal Hourly Rate of 30-40 per hour emanating from just west of the circlet of Pisces. This year sees the Moon well placed (i.e. out of the morning sky) as it just passed New on May 3rd. Like the Fall Orionids, the Eta Aquarids originate from comet Halley, giving it the distinction of being one of the only meteor showers that shares a parent body with another named shower.

Finally, I want to turn your attention to a little known solar system body that might just be within binocular reach this month. 10 Hygiea is an asteroid 220×320 miles in size. Discovered in 1849 by Annibale de Gasparis, this places it 4th in size behind Ceres, Vesta, and Pallas. So, why the obscurity? Well, as a Class C-type carbonaceous chondrite type asteroid, 10 Hygiea has an albedo of 7%, about that of slightly worn asphalt. This generally assures that it shines at less than +10 magnitude… now for the good news. 10 Hygeia has an orbital eccentricity of 0.117 and will reach opposition May 13th very near a favorable perihelion of 2.77 Astronomical Units from the Sun. All this means that will be in the +9.2 magnitude range for most of the month, gliding from the border of the constellation Scorpius into the constellation Libra passing north of the star Sigma Librae. Sketch or photograph the region to reveal the asteroid’s slow motion against the background of stars night-by-night. The area transits around local midnite, and binoculars or a small telescope may enable you to check this “dark asteroid” off of your life list…hey; it’ll be a first for us, too!           

The astronomical term for this week is Kirkwood Gaps. These are gaps in the orbital distribution of minor planets in the asteroid belt and were first indentified and correctly explained by Daniel Kirkwood in 1857. These gaps are due to orbital resonances with the planet Jupiter; large gaps at the 3:1, 5:2, 7:3 and 2:1 resonances have been identified, with smaller gaps known. 10 Hygiea sits just before the 5:2 gap; if it orbited just a bit farther out it would soon get ejected into a different orbit.     

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