Spot The Bright Asteroid Vesta

The next two weeks are an excellent opportunity to spot the brightest of the asteroids, Vesta.

In the first six years of the 19th century, astronomers discovered four new members of the solar system. All four were small objects moving in orbits between the orbits of Mars and Jupiter. Initially they were called planets, but by mid century, enough new objects had been found in this area that they were given a category of their own, much as Pluto was reclassified from a planet to a dwarf planet. They were called “asteroids” because all were so small that they looked just like stars in the telescopes of the day. Now there are tens of thousands of known asteroids.

Vesta is the brightest of all the asteroids, ranging between magnitudes 5 and 8, and one of the largest, measuring 318 miles (512 km) across. It reached 6th magnitude at opposition on September 29, meaning that it could just barely be seen by someone with perfect eyesight at a perfectly dark site.

The rest of us have to make do with binoculars. Here’s how to find it.

A wide angle view of the autumn constellations, showing the position of the asteroid Vesta in Cetus. Credit: Starry Night software.

The first chart shows its overall position among the constellations of autumn. The two left-hand stars of the Square of Pegasus, Alpheratz and Algenib, point southward across the circlet of Pisces to the constellation Cetus, the Whale. Look for a large triangle formed by Eta and Iota Ceti and Deneb Kaitos. The last is easy to spot because, although only second magnitude, it is by far the brightest star in this rather dim part of the sky. Eta and Iota are both magnitude 3.5, so quite a lot dimmer than Deneb Kaitos.

A close up of the westernmost stars of Cetus, showing the position of the asteroid Vesta over the next two weeks. Credit: Starry Night software.

The second chart shows these three stars in detail, and the path of Vesta over the next two weeks. The end of Vesta’s path with the label is its position on Wednesday, September 30, and the points on the trail to the right show its position each night after that.

Vesta should be quite easy to spot, since it is about two magnitudes brighter than any of the stars along its track. Just to be sure, make a simple plot of the stars in its vicinity, and then check again a night or two later. The “star” that has moved is certain to be Vesta.

Vesta is now one of the best known objects in the solar system because it had the NASA spacecraft Dawn orbiting it for over a year (July 2011–September 2012). This is a great chance for you to see it with your own eyes.

Spot The Asteroid Pallas In The Sky

Most of us have played video games shooting at asteroids, or watched a starship maneuvering through the Asteroid Belt on television. But have you ever seen a real asteroid in the sky? This week is an excellent opportunity to see one of the largest asteroids, Pallas, as it reaches opposition to the Sun.

The first asteroids were discovered in the early years of the 19th century. The first four asteroids, including Pallas, were discovered in a 6 year period from 1801 to 1807; a fifth asteroid was not discovered until 38 years later in 1845. This ushered in a rich period of asteroid discovery, with three dozen more discoveries in the next decade. Once photography came into play, thousands more asteroids were found.

Astronomers originally thought the first asteroids were very small planets, but once they realized how numerous they were, they created a special category called “asteroids” (because of their resemblance to stars) or “minor planets.” The vast majority of these asteroids have orbits between the orbits of Mars and Jupiter, a region of the Solar System that came to be known as the “Asteroid Belt.”

The Asteroid Belt in reality is quite different from what you see in science fiction programs. Rather than being crowded with space rocks, the Asteroid Belt is mostly empty space. Most of the time, if you were standing on one asteroid, you would need binoculars or a telescope to spot the nearest asteroid. The chances of two asteroids colliding is virtually zero.

The first asteroid discovered, Ceres, has now been reclassified as a dwarf planet, along with Pluto and Eris. At 592 miles (952 kilometres) in diameter, it is significantly larger than the remaining asteroids. Two asteroids, Pallas and Vesta, are almost identical in size, 326 miles (524 km) and 318 miles (512 km) respectively. The rest range from Hygiea (276 miles/444 km) on down through chunks of rock only 30 feet (10 meters) across. Anything smaller than that is called a “meteoroid.”

Although Pallas and Vesta are nearly identical in size, they are quite different in their composition and appearance. Pallas is a typical rocky asteroid, quite dark in surface colour, resembling a carbonaceous chondrite meteorite. Vesta, on the other hand, is highly reflective, the only asteroid sometimes visible with the naked eye.

For the rest, binoculars are needed to spot them. Asteroids, as their name implies, look exactly like faint stars. What gives them away is their rather rapid movement against the background stars.

This week Pallas reach opposition in the eastern part of Hercules, very close to the 4th magnitude star Lambda Herculis. It is motoring along from east to west at about one degree per week, so that in three weeks time it will be close to the 3rd magnitude star Sarin (Delta Herculis). Its movement from night to night, as seen in binoculars, is quite obvious.

The large asteroid Pallas will be in opposition to the Sun in Hercules on Thursday, June 11.  Credit: Starry Night software.

The path of Pallas over the next 14 days carries it parallel to the stars Lambda Hercules and Sarin. The labeled dot is Pallas’ position on June 11, and the dots to the right mark its daily travel westward. Credit: Starry Night software.

I particularly like watching asteroids when they are passing close to a bright star. In a telescope, you can see their movement over even a 15-minute period.

At opposition, Pallas will reach magnitude 9.4, making it easily visible in binoculars. It will be 2.405 astronomical units from Earth, or 224 million miles (360 million km). A tiny object, sure enough, but interesting to see with your own eyes.