Views of Vesta: Unbroken Dawn

Asteroid Vesta - every other day from April 26 to May 2, 2025
as seen from my backyard.

A Bullet With Our Name on it.

They are dark, tiny and almost invisible - yet they can change life on earth in an instant. 

Asteroids.

Relatively small among space objects, these are especially hard to see in the darkness of space. Even with telescopes,  they are hard to find due to their distance and low reflectance of sunlight. But more than stealth, what makes them deadly is speed. With velocities of up to tens of thousands of miles per hour - they are like cosmic bullets zipping through space, silently. And from time to time, one of them has our name on it.  

Of all astronomical phenomena touching the earth, perhaps next to sunlight, these otherwise innocuous space rocks can have the greatest ability to impact life on earth. As some of them already have.

In 1908, the Tunguska event caused by airburst of a meteor estimated to be only 50 meters in size, destroyed some 80 million trees over 800 square miles of uninhabited space in Siberia. In 2013, the Chelyabinsk event in Russia caused by a 20 meter asteroid, resulted in a powerful shockwave that damaged buildings, shattered windows, and injured over 1,600 people. There are numerous reports of tiny meteorites that have crashed through roofs of houses or landed on driveways or on barren fields of snow. But perhaps the most infamous is the Chicxulub impactor, estimated to be a 10-15 km diameter asteroid that triggered the extinction of dinosaurs 66 million years ago. One might argue that humans owe their apex position as the most dominant life form due to that one asteroid. Fortunately some asteroids such as those in the asteroid belt between Mars and Jupiter are in stable orbits posing no immediate threat to us. 

Viewing Vesta.

Are asteroids visible by naked eye?

The question popped in my mind one spring day. It felt strange that in a decade of stargazing, I had never seen an asteroid, nor had the thought crossed my mind. 

In late spring of 2025, I tried looking for an asteroid for the first time. And the first question that made me pause was which one to look for. The answer turned out to be simple. The only asteroid I knew by name was Ceres. But Ceres was below the horizon at the time. Among named asteroids, Vesta, the second largest one happened to be above horizon. That night, I tried looking for Vesta in the eastern skies from my bedroom window. The location, as per the star chart app on my phone, was in the constellation of Libra. Poised just above a star with a rather beautiful name - Zubeneschamali. 

  

I found nothing by eye, but using my 15x75 astronomy binoculars, I was able to find a dot in that spot that looked no different than a star. Over the next few days, I had photographed this dot shifting against the starry background, using only a DSLR on a tripod and a 75mm telephoto lens. 

I downloaded the images to my computer, and a bit of alignment and compositing later, the dots lined up in a row. This marked the path of Vesta against the background of fixed stars. I had seen and photographed an asteroid and charted its path  - a personal first. It is amazing to thing that what was once pathbreaking discovery two hundred years ago, was within reach of an amateur stargazer on a random spring night.

Planet, Planetoid, Asteroid

Heinrich Wilhelm Olbers

The first discoverers of asteroids including Vesta did not have it easy. About 300 miles in diameter, Vesta is the second largest asteroid in the asteroid belt. It was discovered in 1807, by Heinrich Wilhelm Olbers, a German doctor by day and astronomer by night. 

The first decades of 1800s were an uncertain time for astronomy when the idea of asteroids itself was new. Even the term "asteroid" had not been formally coined. Astronomers of the day thought of these to be planetoids or small planets. Vesta was the fourth object of its kind to be discovered, after Ceres (1801 CE), Pallas and Juno. Many others soon followed. Olbers, like some of his peers, incorrectly thought that it, along the objects in the asteroid belt were fragments of a destroyed planet. He gave the honor of naming it to the young German mathematician Carl Friedrich Gauss who had computed its orbit. Gauss named it after the Roman goddess of home and hearth.  

A Rocket to a Rock

Vesta is a giant among asteroids. Yet, being 2 to 4 times as distant as we are from the sun, and only 300 miles in diameter (slightly more than the distance from New York to Boston), features on Vesta were outside the range of most earthbound telescopes including Hubble. Two centuries after its discovery, no one actually knew what Vesta looked like up close. I did not know at the time when I chose Vesta for viewing, that this very rock had been visited by the NASA Dawn mission 

in 2011. This ambitious mission involved sending a spacecraft to visit not one but two asteroids using a unique propulsion system known as an ion engine. 

It orbited Vesta for 14 months before shifting over to Ceres. At Vesta it took over 100,000 images mapping Vesta's surface while making gravity measurements to understand its internal structure.

 

Colorized view of craters of Vesta.
Source: Nasa / JPL

We now know that Vesta's surface is full of large craters, mountains and troughs that run its circumference. The craters were probably caused by billion year old planetary impacts in the early protoplanetary disc of debris around the sun. These gave rise to a family of fragments that move together in orbit with Vesta. 

One of the two massive impact craters near its south pole is called Rheasilvia (named after the Vestal mother of Romulus and Remus in Roman mythology) and at 315 miles diameter covers most of Vesta's diameter. The Rheasilvia Central peak is a mountain about 20-25 km. tall (compare with Mt. Everest at 8.8 km), among the tallest in the solar system. It is believed to be result of an impact as opposed to having volcanic origins. Troughs the size of the Grand Canyon run along the surface of Vesta that suggest fluid action on its surface in the past.

Dawn’s gravity measures indicate Vesta has a differentiated interior with a core, mantle and crust, like a rocky planet indicating at some point it had enough heat and fluidity to let the heavier materials sink to its center. Howver, it was not large enough or hot for long enough to reach a spherical shape like Ceres. For that reason, it will never be a minor planet, though it was on its way to being one. By our current estimates, at the dawn of our solar system billions of year’s ago, Vesta started out as a planet-in-the-making which never finally saw the light of dawn.

Twilight of Dawn

Dawn was a unique spacecraft in many ways. First, it tested a new propulsion system. Unlike chemical engines which provide short bursts of high thrust at great expense of fuel, ion engines are light, start slow but are highly efficient. They create thrust by ejecting ionized gas which builds up speed exponentially over long durations. Dawn's ion engine got it to only 100km/ hr after four days. But then it ran for 85% of the 4 year trip to Vesta burning only 72 kg of fuel. The mission itself was also a first in many ways. The engine enabled it to enter orbits of two extraterrestrial bodies and not merely multi-target flybys like Voyager. It was also the first spacecraft to visit either Ceres or Vesta, and the first to orbit a minor planet, giving us unique peek into how planets in the solar system may have started. 

In 2018, three years longer than its planned mission end, Nasa announced that Dawn had run out of its hydrazine fuel and the mission had come to a successful conclusion. Today, the spacecraft remains in stable orbit around Ceres and will remain there as a "monument" to human curiosity and ingenuity for years to come.    

Reference

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Asteroids, by Clifford J Cunningham

Vesta - by NASA