Seeing into the Asteroid Belt
May 2014 :
For a few decades following the January, 1801 discovery of Ceres by G. Piazzi and the 1807 spotting of #4 Vesta by H. Olbers, such small bodies were thought of as minor planets in their own right. Their orbits showed them to be members of the Solar System at distances that coincided with a zone between the orbits of Mars and Jupiter, where earlier predictions of an undiscovered planet placed a hypothesized body at a certain radius of orbital progression outward from the Sun. Some thought that a planet in that zone had perhaps “exploded” in the distant past, leaving a gap between Mars and Jupiter. Some early 19th century writings, in fact, referred to Ceres, Pallas, Juno and Vesta as “planets”, along with Mercury, Venus, Mars, etc., out to Uranus. (The expansion of the Solar System’s frontier by the discovery of Neptune did not happen until 1846.) Although much too small in size to display visual discs in telescopes of that era, motion against background stars was obvious from one night to the next. Star-like objects that showed – through time and measurements – orbital motion but no visual features akin to comets had to be something else. Points of light having no apparent actual diameters that displayed such regular motion were interpreted, therefore, as minor planetary members within the Sun’s family. Many classes of asteroids are now identified, typified chiefly by the shapes and degrees of eccentricity of their orbits, their orbital periods and distances from the Sun, as well as their sizes, mass, presumed densities, compositions and spectral classes. Literally hundreds of thousands of asteroids – including many icy bodies and some with comet-like features – are now catalogued in the archives of the Minor Planet Center at Cambridge, Mass., which is affiliated with the Center for Astrophysics (C.F.A.) at Harvard. Astronomers did not make the 5th asteroid discovery, following Vesta in order, until 1845: a small body named Astraea. By 1852, the count had risen to 20.
The large size of #1 Ceres (590 miles in diameter) coupled with its semi-spherical shape are characteristics which influenced the I.A.U. (International Astronomical Union) to reclassify it as a “dwarf planet” in 2006, at the now-infamous conclave that chose to demote Pluto from its traditional status as a planet to, basically, the same kind of object they consider Ceres to be. Vesta is about 330 miles in diameter, but is by far the brightest of all the asteroids, due to its relatively high albedo (reflectivity value), which is thought to result from composition of surface materials unlike that of any other such bodies. Vesta actually seems to have a geologically “differentiated” interior and may have undergone partial melting – from impacts and internal radioactive decay heating – in the deep past. I wrote about viewing the 2nd asteroid to’ve been discovered, #2 Pallas, in the March issue of this newsletter. Despite its much higher apparent brightness, Vesta was 4th in line of discovery, following Ceres, Pallas (43 miles in diameter bigger than Vesta), and #3 Juno, which is 150 miles smaller in size than Vesta, as well as much darker. I’ve long been puzzled as to why the brightest asteroid was not noticed first! Here are averaged orbital distances from the Sun and periods of revolution for the first 4 asteroids, all of which reside in the “main belt” between Mars and Jupiter: #1 Ceres – 2.767 AUs, period 4.61 years; #2 Pallas – 2.773 AUs, period also about 4.61 years; #3 Juno – 2.669 AUs, period 4.36 years; #4 Vesta – 2.361 AUs, period 3.63 years. As a timeline recap, the years of discovery, in order from #1 Ceres to #4 Vesta, were 1801, 1802, 1804, and 1807.
The Moon was just 6 days old and skies to the southeast were clear when Tim and I began a star-hop to Vesta and Ceres at around 10:50 p.m. Both objects would come to their oppositions within a day apart in mid-April and both were located together in the same wide field of view in Virgo, a circumstance which was a first for me regarding observation of 2 asteroids simultaneously. The star Zeta Virginis, which shines at magnitude 3.4 just east of the main outline of the constellation Virgo, was my starting point and Mars was positioned just a few degrees to the northeast of Zeta Vir at that particular time on 4/5. The result was a star-hop beginning at Zeta, up and left to Mars (shining at about magnitude -1.5), and 7 degrees east-northeast from Zeta Vir to the magnitude 4.3 white star #93/Tau Virginis. Binoculars made this initial part of our star-hop easy. Tau Vir, at RA 14 hours, 01.6 mins. and Dec. +01 degrees, 33 mins., was key to spotting Vesta. The asteroid was 2.6 degrees west-northwest of Tau Vir and was seen not quite halfway along a line starting at magnitude 5.9 #92 Vir and extending northwest a short distance to the next semi-bright star in that directional line, magnitude 5.4 #84 Vir. Vesta was unmistakably identifiable at its magnitude of 5.8; no background stars anywhere close to its specific position were comparable in brightness.
I always study my Uranometria 2000 atlas thoroughly prior to doing a new star-hop so as to memorize the appearance of certain stars and helpful small asterisms in a given area, although I don’t actually refer to charts most of the times while at the eyepiece. That may sound odd, but I’d rather look at charts indoors first and then not have to bother with them at a telescope, provided the observing site is only a few steps away from the door. Having accurate, current positions of moving objects enables you to plot them with high precision on a detailed chart when planning a star-hop and such information is readily available in numerous sources, particularly on the web. I’ll admit that simply downloading a finder chart onto a single sheet is more easily done, but each to his own, as they say. Of course, handy finder charts with dated tick-marks showing changing positions of an object through time are printed in certain publications; this applies mainly to specific objects featured at a given time of year, considered as notable for viewing. Sure, a computerized go-to scope – if it’s working properly – can slew automatically to certain objects, but what if your selection looks just like a star, as asteroids do? If you haven’t familiarized yourself with a given starfield and a precise position for a moving object at a certain date and time in relation to background stars in the field, how will you know which point of light is which?
Ceres was spotted about 1.4 degrees north-northwest of #93/Tau Vir; it stood out as an extra point of light of comparable magnitude to at least 3 of 7 background stars which together comprised an asterism that could be recognized (after studying a chart) in the vicinity of both Tau Vir and the aforementioned #92 Vir. Although 1.2 magnitudes dimmer than Vesta, I could make a positive identification on Ceres because it constituted a point of light not where any stars of the same brightness level were printed on my charts, and it therefore appeared as an 8th “star” among my chosen grouping of 7. Asterisms are pretty much everywhere around the sky – pick any position at random on a chart showing stars to the 9th magnitude or fainter and you’re almost bound to visually construct shapes, triangles, little zig-zag lines, etc. out of stars at or near to your position of choice. Doesn’t matter if nobody else can see anything that makes any sense to them; if you imagine, say, a lopsided letter T somewhere close to what you want to plot a location on, that’s all that matters!
Little did I know that our own Jim Hendrickson was taking pictures of Vesta and Ceres from Seagrave Observatory at the same time Tim Dube and I were viewing them in my 65 mm. at East Douglas, Mass. By the way, I may have unintentionally implied that asteroids could only be seen at (or close to) their dates of opposition, if you read the very end of my March article. I neglected to take into account the Full Moon of April 15th when I wrote that piece, suggesting viewing Vesta and Ceres within a day or two of their oppositions at mid-month. You’ll be able to see them even weeks from now, if you know
how to track them down. Just pick a clear night without a bright Moon anywhere close to Virgo. Even binoculars can show Vesta, owing to its brightness. Here’s your chance to see a “minor planet” if you’ve never done so before. Check the web for information on space probe missions to asteroids. The “Dawn” probe, for example, orbited Vesta for a year, departing in July of 2012. The same probe is due to arrive at Ceres in February of 2015. Other probes have studied different asteroids and taken stunning pictures of some with small moonlets orbiting them.
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