An asteroid for Asteroid Day (or nearly)

I’m not sure why the United Nations picked June 30 (yesterday) as Asteroid Day.

But tomorrow (July 2) one of the major asteroids, 3 Juno, will be at opposition.

Asteroid 3 Juno chart 2017

(The instant of opposition, as measured in longitude, is 6 hours Universal Time, so it falls in July 1 for some American time zones.  But that makes no discernible difference for observing.  Opposition is just the center of the best weeks.))

Juno is the third of the Big Four asteroids, or First Four as I prefer to call them – the first discovered, in the first few years of the 19th century, and for a while the only ones known.  Because the typical asteroids (those of the “Main Belt”) orbit at distances between those of Mars and Jupiter, their periods are around 3 or 4 or 5 years, and so we overtake them each year a few months later.  Juno was at opposition in January 2015, and April 2016, and now we overtake it at the beginning of July.

Though discovered so early, and though named for the queen of Roman goddesses, Juno is smaller than the others of the First Four (Ceres, Pallas, and Vesta), and is in most years surpassed in brightness not only by them but by at least one of those discovered later.  This time it reaches magnitude 9.9, somewhat brighter than last year’s 10.2; it can be as bright as 7.7 (as in 2009 and 2018) or rarely 7.6 (as in 2044), or as dim as 10.3 (as in 2012 and 2021).

In other words, Juno is always far fainter than the naked-eye limit of about 6, which is reached only by asteroid 4 Vesta.

An interesting feature of Juno’s orbit is that it always comes to opposition close to the celestial equator (within about 5° of it).  This time, it is at opposition in the northern end of the little constellation Scutum, which lies not far from the center of the Milky Way and contains the famous Scutum Star Cloud.  Thus Juno creeps against a dense background of stars, some as bright as it.

And let’s mention that the midpoint of this non-leap year is at July 2.5.


5 thoughts on “An asteroid for Asteroid Day (or nearly)”

  1. Asteroid Day is intended to promote awareness of the risk of asteroid impacts on Earth and to support efforts to defend the Earth from such events. On 30 June 1908 an asteroid exploded over the Tunguska River valley in Siberia, flattening a lot of trees.

    It seems prudent to keep track of potentially hazardous asteroids, and some of the ideas for diverting an asteroid from an Earth-impacting orbit are interesting. But I think we could easily come up with a list of 365.25 human-caused problems that are both more pressing and more amenable to solution than an asteroid (or comet) impact, and thus more deserving of a special day to raise awareness and concern.

    I’m curious why Juno’s oppositions always happen to appear near Earth’s celestial equator.

    1. It has to do with the fact that the descending node of Juno’s orbit is rather close (10 degrees) to the vernal equinox point (where Earth’s orbit ascends through the ecliptic plane), and the inclination is only about 13 degrees. This matter of the locus of planets’ and asteroids’ oppositions is discussed at length by Jean Meeus in his first Morsels book, p. 240-245, and in the asteroid section of Astronomical Calendar 2015 I drew a chart showing a curve through the loci of Juno’s oppositions. Sometimes Juno is crossing through the equatorial plane when it is at opposition; other times, as in 2017, it curls upward or downward so as almost to kiss the equator!

      1. Thanks. I looked at the 2015 Astronomical Calendar. I still can’t wrap my head around it, and that’s okay. I’ll file this tidbit under “Interesting Coincidences”.

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