Visually verify Vesta’s visibility

Preamble: This article was originally intended to be released in August near Vesta’s opposition. I wanted to get some more cell phone pictures and then the weather didn’t cooperate and then life happened. Vesta is now getting low in the west at dark and will soon disappear in to twilight. Think of this article as a precursor to the next opposition in Dec 2023.

During late summer and in to fall of 2022 there was a lot of press about 5 planets being visible, including a previous post here. But there’s more out there in the solar system than just planets!

The main asteroid belt, located between the orbits of Mars and Jupiter consists of chunks of rock from early in the solar system’s history. During solar system formation there may have been a total mass in that region that would equal the mass of the Earth. Gravitational interactions with Jupiter in the first few million years of the solar system kicked most of the material in towards the sun or out of the solar system.

The total mass left behind is now only a few percent of Earth’s moon mass. Of that paltry amount the four largest: Ceres, Vesta, Pallas, and Hygiea account for almost two-thirds of the total mass of the main belt asteroids.

The 17 largest asteroids. Despite Vesta being almost half the diameter of Ceres (529 km versus 952 km) it can appear a little over a whole magnitude brighter from Earth due to it’s much higher albedo. From “VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis.” A&A 654, A56. http://doi.org/10.1051/0004–6361/202141781

Vesta has a very reflective surface, its albedo is 4 times that of Ceres and twice that of Pallas. Vesta also orbits the sun slightly closer than the other large asteroids which further enhances its apparent brightness when viewed from Earth. At opposition (i.e. when the Sun, Earth, and Vesta are lined up and therefor Vesta also closest to Earth) it can get as bright as magnitude 5.1, which means from a very dark site you might see it without optical aid.

Vesta in natural color as imaged by the Dawn spacecraft in 2011. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA https://solarsystem.nasa.gov/missions/dawn/overview/

Since most of us live in places with light polluted skies we’ll need to at least use binoculars to see Vesta. As noted in the article about observing Neptune, it can be tough to tell if the the little point of light is what you think it is and not some star. Using the same technique as used there a cellphone (with added 60mm lens) was used to capture Vesta on 3 different dates and make a short video to show its movement.

Motion of Vesta n July and August 2022 as captured with a cellphone.

During that time period Vesta was undergoing apparent retrograde motion, something that was covered in this article about Saturn. It’s apparent magnitude, taking into account the dimming from Earth’s atmosphere, was around 6, well within the reach of the Pixel 3a used for imaging, and also easy to pick out even with small 8x35 binoculars.

Here’s a plot on a star chart of the motion of both Saturn and Vesta in August.

Motion of Vesta (Yellow trace) in August 2022. Since it’s considerably closer to the Earth than Saturn (blue trace) it appears to move across a much larger distance in the sky. Illustration created with Cartes du Ceil.

Fast forward to early January 2023 when this article was written; can we still image Vesta and/or find it with binoculars? Using Stellarium we can see that Vesta will be in the southwest around 6PM local time, which is dark in the Boston area. Stellarium calculates Vesta is shining at magnitude 8 now (remember that brightness magnitude is measured on a logarithmic scale), 2 magnitudes less than in August, or 6 times less bright. That shouldn’t be a problem since we had shown that the Pixel 3a with the lens should reliably capture 8.5 magnitude or dimmer.

That determination of limiting magnitude was from a location on Cape Cod with considerably less light pollution than the Boston suburbs where this latest attempt was made from.

Screen capture from Stellarium for Jan 8, 2023, showing Vesta near ψ 1,2,3 Aquarii, which makes a convenient asterism for locating Vesta. The field of view is about 2 degrees.

Cellphone capture made from Boston suburbs of same region in the Stellarium plot above. See text for explanation. Star magnitudes have the decimal point removed.

From the cellphone capture we can see that stars with magnitudes of 8.0–8.2 just barely show up. Yet the magnitude 7.7 star (in red) is really tough to pick out. This has been noticed before, the Pixel’s software isn’t consistent in the way it records dim stars.

The blue circle is where we would have expected to see Vesta.

Some of this variability may related to the stars color and that the camera’s image chip is not linear in its response with respect to RGB (red , green, blue) sensitivity; a lot of software finagling is needed. The brighter background from local light pollution as well as less than perfect seeing (atmospheric) conditions may also be a factor in the failure of Vesta being recorded.

Using 12x50 binoculars the same stars as seen in the cellphone capture are observed; this is with the binoculars braced but not tripod mounted so there will be a slight loss in magnitude from image shake in the binos. Under good conditions 12x50 should show stars down to 9th to 10th magnitude, so either light pollution, atmospheric seeing, or both are limiting what can be seen.

The forecast for the next week is for increased clouds so in the interest of getting this article posted we’ll just assume Vesta is where we think it should be and not worry that it couldn’t be imaged nor seen with binoculars now. Vesta was readily visible in September and asteroids just don’t disappear.