Asteroid Vesta at Opposition February 22nd 2010

I should first like to refer to an earlier Sky Notes feature on asteroids or minor planets (see site archives August 2006), in which we have for the definition of a planet:

The IAU recommendation is as follows: "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."

As a definition this leaves something to be desired in my opinion. (I have already objected to the use of the word “round”!) But then, if I am honest, I prefer the term minor planet or planetoid, to asteroid!

Of the four “major” asteroids—Ceres, Pallas, Juno and Vesta—only Ceres (diameter 952 kilometres) would be entitled to planetary status on the above definition. It should also be noted that this grouping of four has to do with the date of discovery rather than size, since there are seven asteroids larger than Juno, in addition to its three “companions”.

The orbits of all four major asteroids fall between the orbits of Mars and Jupiter. Vesta’s orbit in relation to the orbits of Jupiter, Mars and Earth is shown in Dia. 1.

Dia. 1: Diagrammatic representation of the orbits of Earth, Mars, Vesta and Jupiter—roughly to scale.

Although not the largest asteroid, Vesta (mean diameter 529 kilometres) is by far the brightest of the “big four” as seen from Earth.

This year Vesta makes its closest approach to Earth (opposition) less than a month after the more spectacular opposition of Mars. This means Vesta will be found in the constellation Leo and at a magnitude rendering it just bright enough (magnitude 6.1) to be seen with the unaided eye. Vesta will be an easy object in a small binocular.

By no means at its brightest this year (in 2011 it will reach magnitude 5.7 at opposition low down in Capricornus), the small asteroid is well placed for observation from the northern hemisphere. Moreover, there will be an interesting line up with the binary star Algieba (gamma Leonis, components mag. 2.4 and 3.6, separation 4.64" in 2010) and the 4.8 magnitude star 40 Leonis. (See Dia. 2)

Dia. 2: A large-scale diagram showing the apparent path of Vesta relative to Algieba etc. from 13th to 19th February 2010. Click on the image for an enlargement.

So, Vesta joins the small number of planets regularly orbiting the Sun that may be seen without optical aid. (Interestingly, Uranus is likewise on the threshold of naked-eye visibility at close to magnitude 6.)

Fig. 1 is a composite of three exposures to show the area of sky with Saturn and Mars at the extreme left and right of the frame respectively.

Fig. 1: A composite of three images (see text) taken 00h 05m UT January 19th, 2010. Nikkor 28 mm f/2.8 lens attached to a D100 SLR; a 2 minute exposure guided at sidereal rate, ISO 500. Click on the image for an enlargement.

Fig. 2 is a close-up in the region of Algieba showing Vesta imaged with a 28 mm f/2.8 lens, together with Mars and stars of Leo to magnitude 12. Vesta is just below and to the left of TYC 1421-860-1, magnitude 6.6. Both images from January 19 2010 00h 05m UT.

Fig. 2: Selected region from fig. 1 to show the image of Vesta in relation to Mars and neighbouring stars. Click on the image for an enlargement.

Dia. 3 Shows the movement of Vesta against the star background from February 1 to March 3 1010/

Dia. 3: The path of Vesta against the stars of Leo, 1st February to March 3rd 2010. Click on the image for an enlargement.

Those wishing to chart the course of Vesta could do so using a digital camera. A static camera (mounted on an ordinary tripod) would capture the image in 15+ seconds set with a wide aperture of f/2.8 at ISO 400 or thereabouts. Of course there will be some elongation of images due to diurnal motion which would show at high magnification but that hardly matters for the present purpose. A digital SLR with a standard 50 mm lens attached would be ideal.

JVC 28/01/10