First Asteroid hunters: Dramatic Minor Planet Discoveries of 19th Century
Astronomers knew of only the visible planets in the Solar System when the first asteroids were discovered. While these mysterious objects were moving across the fixed background of stars, astronomers were cautious about calling them planets. This is the story of an exciting time when telescopes were revealing a dynamic and complex universe.
In 1766, the German Mathematician Johann Daniel Titius discovered a pattern in the distances of the known planets from the Sun. The pattern was that each subsequent planet was approximately twice the distance from the Sun as the previous one. This formula was refined and publicised by the German Mathematician Johann Elert Bode, and became widely known as the Titius-Bode law, in 1772. The pattern correctly predicted the location of Uranus, which was discovered by the German-British astronomer and composer William Herschel in 1771.
The law sparked excitement and influenced the hunt for a missing planet between the orbits of Mars and Jupiter. Despite around two decades of intensive searches by newfangled telescopes, no such planet was discovered. In 1880, a group of astronomers calling themselves the ‘Celestial Police’ led by Austrian astronomer Franz Xaver von Zach and the German astronomer Johann Hieronymus Schröter divided up the sky for a more focused hunt.
The discovery of the minor planets
The Italian mathematician Giuseppe Piazzi was not part of the Celestial Police but spotted a faint, moving object in the constellation of Taurus on 1 January 1981. He tracked the object for weeks, assuming it to be a comet, but it lacked a tail. He suspected that it was a planet, but was cautious in claiming such a dramatic discovery. Subsequent observations indicated that it was indeed orbiting the Sun between Earth and Jupiter. This was Ceres.
The largest asteroids were discovered first, unsurprisingly. (Image Credit: The Planetary Society).
The asteroids are numbered serially in the order in which their discovery is confirmed. Astronomers discovered a number of objects between Mars and Jupiter in short order, including Pallas by Heinrich Olbers in 1802, Juno by Karl Ludwig Harding, and then Vesta by Olbers again in 1807. Harding and Olbers were both German astronomers. After the discovery of 1 Ceres, 2 Pallas, 3 Juno and 4 Vesta, there was a lull in new discoveries till 1945.
The amateur German astronomer Karl Ludwig Hencke discovered 5 Adrastea in December 1945, reigniting interest in worlds between Mars and Jupiter, and opened the floodgates. Over the next seven years, 15 new asteroids would be discovered in what is known as the main belt. Newer telescopes had opened the floodgates. By 1952, 20 asteroids were discovered and more were piling up fast, and continue to do so till this day. Ryugu is 162173 and Bennu is 101955.
What are asteroids?
Around that time, the term minor planet was used in scientific literature for the inner, terrestrial worlds, Mercury, Venus, Earth and Mars. The newly discovered asteroids were initially called new planets, and were considered full-fledged members of the Solar System. Ceres, Pallas, Juno and Vesta were all demoted long before Pluto. Newspapers and journals at that time trumpeted the new discoveries as planets, but follow-up observations revealed their diminutive sizes, all smaller than the 940 kilometre wide Ceres.
By the 1850s, the term ‘asteroid’ meaning star-like originally used by William Herschel gained traction because of their point-like appearance. While the scientists resisted the use, preferring the term minor planets and even planetoids, the general public took to the word asteroid. Officially, asteroids are still considered minor planets. Olbers theorised that the asteroids were fragments of a shattered planet, while others suggested that the asteroid belt consisted of material that never coalesced into a world.
The sport of asteroid hunting
By the 1850s, asteroid hunting had become a sport. Newspapers and journals eagerly tracked new discoveries, but information spread slowly, initially as correspondences between scientists, then as reports in scientific magazines, and finally reaching the general public in the form of news articles and star charts. Asteroid hunting became a craze, as there were thousands of small rocky bodies between Mars and Jupiter.
Astronomers bemoaned that the advent of photography had killed the sport of asteroid hunting. It was possible to merely photograph portions of the sky with long duration exposure, with the streaks revealing the presence of asteroids. Despite centuries of searches, with newer and more sophisticated instruments, crowdsourcing discoveries, and using machine learning, humans have still not discovered half the asteroids in the Solar System. By the end of another decade, humans hope to have discovered at least 90 percent of the asteroids larger than 140 metres wide, that pose a significant threat to the Earth.