Finding Vesta Uncovering the ‘Celestial Police’ and the asteroids they discovered
Asteroid 4 Vesta may hold clues about the early Solar System, but its discovery is just as intriguing. Emily Winterburn uncovers the Celestial Police, an elite group of planet-hunting astronomers
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Vesta, the brightest asteroid in our Solar System’s asteroid belt, will be at opposition on 4 March 2021. Although Vesta can be seen with the naked eye in locations with no light pollution, it can be a challenge to do so. With a magnitude of +5.9 it might just be possible, but it will be perfectly visible with binoculars or a moderate aperture telescope. You’ll be able to find it in the constellation of Leo, the Lion.
Vesta was one of the very first asteroids to be discovered and its story is one full of failure, jealously and discredited theories, as all good science history should be.
Today we know of around two million asteroids in our Solar System’s asteroid belt between Mars and Jupiter, but 250 years ago the existence of anything at all in that area of the sky was mere speculation. In 1766 a German astronomer and mathematician, Johann Daniel Titius formulated a mathematical law for explaining and predicting the spacing between planets in a solar system. This is the first discredited theory we will meet in this story. According to Titius’s law, astronomers should find a planet between
Mars and Jupiter as well as more, further out, yet none at this point had been found. Despite this lack of proof, six years after Titius first published his law it was adopted and popularised by German astronomer Johann Elert Bode. Then, on 13 March 1781, William Herschel discovered Uranus where the law predicted a planet should be.
Founding the ‘Celestial Police’
Following this success of what had now become the Titius-Bode law, interest in that region between Mars and Jupiter grew. Baron Franz Xaver von Zach and Johann Hieronymus Schröter, owner of an observatory at Lilienthal in Germany, both took it upon themselves to create and galvanise an active community of astronomers to search for missing planets. After a suggestion made by Joseph-Jérome de Lalande, Zach and Schröter, together with Karl Harding, Heinrich Olbers, Freiherr von Ende and Johann Gildemeister formed a group calling themselves the ‘Celestial Police’. Their aim was to organise and coordinate a large, international project to find the missing planet by assigning different sections of the sky to different astronomers across the world. In all, they wrote to 24 astronomers across Europe, inviting them to join the Celestial Police
to hunt for this missing object. They sent out their letters in 1800. Meanwhile, before even receiving his invitation, Italian priest turned astronomer Giuseppe Piazzi spotted Ceres in the exact place Titius-Bode’s law predicted on 1 January 1801.
Piazzi immediately announced his discovery in a letter to fellow Italian astronomer Barnaba Oriani, taking the precaution (as William Herschel had done before him) of claiming to have discovered a comet. Comet hunting by 1801 was a well-established and respectable astronomical activity; lots of people hunted for and discovered them. Planets were more unusual and to make such a claim could, if mistaken, have appeared arrogant and ungentlemanly. Piazzi did however hint at his suspicion, telling Oriani in his letter that while he thought he had found a comet, in fact, “it might be something better”. News of this discovery passed across Europe and astronomers across the continent observed, checked and calculated its orbit, until it was eventually agreed that this was indeed a new – if rather small – planet. Piazzi could now name it, choosing the title Ceres, after the Roman goddess of agriculture.
The search continues
Undeterred, the Celestial Police continued their search for unexplained objects along the ecliptic that were not stars. Then, over a year after they began, Heinrich Olbers, observing from the upper story of his home (which he’d turned into an observatory), discovered a second object, Pallas, on 28 March 1802. Olbers had actually been trying to take further observations of Ceres to help get a better understanding of this new planet when he came across Pallas. Once established as another planet (the term asteroid was still not in use), Olbers named it Pallas, after the Roman goddess of Wisdom.
Still the Celestial Police continued their searching and this time it was Karl Harding’s turn to make a discovery, finding yet another body within the asteroid belt in 1804. When Harding discovered his asteroid, he named it Juno, after the highest Roman goddess.
“The Celestial Police continued to search for asteroids for eight years after Vesta was discovered in 1807, but with no luck”
Three years later, in March 1807, Heinrich Olbers made his second discovery in the asteroid belt: Vesta. He chose the name, in keeping with the tradition so far established, of selecting the name of a Roman goddess. This time he chose Vesta, Roman goddess of the hearth, the home and of family. His search began when he started to hypothesise that perhaps Ceres and Juno were fragments of a former planet. This is the second now discredited theory in this story. Both Ceres and Juno, Olbers noted, were rather small to be the planet predicted by the Titius-Bode law. What if, he suggested in a letter to Carl Friedrich Gauss in 1802, “…Ceres and Pallas were fragments of a former larger planet that had been destroyed by colliding with a comet?” He went on, “Might we then still discover more pieces of this previously existing planet at its appropriate position?” The discovery of Juno seemed to confirm his suspicions, as did his own discovery of Vesta. (Today it’s believed that the asteroid belt is more likely to be pieces of a planet that never formed thanks to the disruption of the formation process by Jupiter’s gravitational pull.)
Meanwhile, as all these discoveries were being made and discussed in letters and society meetings across Europe, William Herschel was beginning to voice concerns. After all, his had been the first planet to be discovered since antiquity, and the first by a named discoverer. This small flurry of additional findings seemed (in his eyes) to rather dilute this achievement. In a rather long article in the Philosophical Transactions of the Royal Society (May 1802), he set out to analyse these new discoveries, determinedly not referring to them as planets. They were, he explained in his abstract, too small for that name, and rather oddly he preferred to call them stars. Towards the end of the paper, he proposed a new name, “asteroids”, declaring that, “These bodies will hold a middle rank, between the two species that were known before,” by which he meant planets and comets. It took a while, but by the mid-19th century that name had stuck.
The Celestial Police continued to search for asteroids for another eight years after Vesta’s discovery, but with no luck. By 1815 they reluctantly concluded there were no more planets or their parts to be found in that part of the sky. It was to be 30 years before Karl Ludwig Hencke (and in time, others) would prove them wrong.
Of all the asteroids discovered both before and since, Vesta is the brightest and the only one bright enough to be seen with the naked eye thanks to its size and the nature of its surface. Over the years, astronomers have continued to study these asteroids, steadily getting closer and closer to measuring their various properties, including size and mass. In the 1880s, Edward Charles Pickering and his team at Harvard College Observatory measured Vesta’s mean diameter to be approximately 513km, those measurements were refined in the 1980s and again in the 1990s, before being finally confirmed by NASA’s Dawn mission (see box, opposite).
In 1966 Vesta became the first asteroid to have its mass determined, by using observations of its gravitational perturbations on fellow asteroid Arete. It was measured by Hans G Hertz from NASA’s Goddard Space Center, who estimated Vesta’s mass to be approximately 1.2x10–10 solar masses, an estimate that was further refined to be closer to 1.3x10-10 solar masses by the Dawn mission.
Vesta also has interesting physical characteristics, which again have been further studied and better understood thanks to NASA’s recent Dawn mission. Vesta is the second most massive body in the asteroid belt (after Ceres) and has a 500km-wide crater on its surface (almost the width of the entire asteroid’s diameter). The Dawn mission named this crater Rheasilvia after the mother of Romulus and Remus in Roman mythology.
NASA’s Dawn mission was the result of many years of planning and a series of false starts. Proposals had been made since the early 1980s to send a spacecraft to Vesta to study it in more detail, not just by NASA but by ESA and the Soviet Union too, but it wasn’t until the Dawn mission was launched in 2007 that those ambitions were finally realised. The mission set out to visit both Vesta and Ceres to learn more about these objects, the asteroid belt and the formation of our Solar System. In the process they were able to confirm or improve on earlier studies of these space rocks.
A people’s story
The story of Vesta is one that allows us to see science for the human process that it is. It is fallible, it makes mistakes, but keeps going, and in the end finds something wonderful. Titius and Bode might not have been right about their law (it falls down at Neptune whose orbit doesn’t fit its rules) and Olbers might not have been right about how he thought the asteroid belt was formed. When he proposed the term ‘asteroid’, Herschel may have
“Vesta is the only asteroid that is bright enough to be seen with the naked eye thanks to its size and the nature of its surface”
only thinly disguised his jealousy that others could also discover planets. And more recently, since the 1980s astronomers might have tried and failed for nearly four decades before finally getting a space mission to visit Vesta. But rather than diminishing the achievements, all these setbacks only make our knowledge and appreciation of this celestial body greater. On 4 March, as we go out to take a look at this bright asteroid, spare a thought for all those failures: taken together they have made it possible for us to know what we’re looking at.