The next few weeks offer a splendid opportunity to spot what is usually the brightest of the asteroids, Vesta. This minor planet, which is one of many thousands circling the Sun between Mars and Jupiter, will be at its best on January 18, 2017, when it reaches a point called Opposition.
It will then lie in the opposite part of the sky to the Sun and shine at a relatively bright magnitude 6.3, just inside the constellation of Cancer, crossing into Gemini the day after. This is theoretically at the limit of naked-eye visibility for anyone with perfect, dark skies, but most of us will need binoculars at least to see it.
Under normal circumstances, Vesta is the brightest of the asteroids, becoming brighter even than the largest of these bodies, Ceres, which is now classed as a dwarf planet. Like Ceres, it has been been studied in detail by NASA’s Dawn mission, but both bodies appear as no more than points of light in a telescope or binoculars. (I say “under normal circumstances” because a potentially hazardous asteroid called Apophis will make such a close approach to the Earth in April, 2029, that it will briefly shine at third magnitude and be visible without optical aid.)
Vesta will be easy to find around Opposition because the bright stars Castor and Pollux, also known as alpha (α) and beta (β) Geminorum, very conveniently point towards it. Our chart shows the track across the sky that Vesta will make between January 16 and February 25. It will fade from magnitude 6.3 to 7.0 during these few weeks, becoming twice as faint.
Don’t miss this opportunity therefore to catch Vesta while it is brightest and also easy to locate. It is one of the best opportunities to observe an asteroid that you will ever get!
How to photograph Vesta
If you have a camera that is capable of taking manual exposures lasting more than a few seconds, you can try to take a photo of Vesta yourself. You will need to keep the camera steady, preferably by mounting it on a tripod, and pointed towards the stars of Gemini.
Open the camera lens to its widest aperture if you can and set the camera’s “film speed”, or ISO rating to a high number of 400 or above so that it is more sensitive to faint light. If you in a location free from light pollution, and the Moon is not bright in the sky, you might be able to go up to 1600 or even 3200.
The camera will need to be set to manual focus rather than autofocus as starlight is generally too dim for most cameras to be able to lock onto it. Turn the lens to infinity (the little squiggle resembling a figure 8 lying on its side). You can achieve even sharper focus if your camera has “live view” and you can see a bright star on your screen while you are focusing.
Use a cable release, if you have one, to avoid shaking the camera. An alternative would be to set the camera’s shutter-delay feature (usually between 2 and 10 seconds) between pushing the button and the photo being taken. Usually this facility is used to allow you to get into your own photo, but it works equally well in allowing the camera to settle down once you’ve fired the shutter!
Using a standard lens, you should be able to open the shutter for 15 seconds or so before the motion of the stars across the sky becomes apparent. This is due to the rotation of the Earth. Longer exposures will record the stars as trails rather than points of light, which is still a pleasing effect if that is what you want! With a telephoto lens, the effect is more noticeable, and your exposures will need to be shorter than 15 seconds.
Take a range of shots, using different exposure times and ISO ratings, and see which gives you the best results. Nowadays, in this digital age, you can find out almost immediately by checking your camera’s screen. Later you can use Photoshop or an alternative piece of software on your computer to adjust brightness and contrast to enhance your photo if you need to. Good luck!