Asteroids

The first asteroid discovery occurred twenty years after Herschel discovered Uranus — and, like the discovery of Neptune that came later, it was no accident. Prior to Uranus’s discovery, Johann Titius and Johann Bode had noted an intriguing numerical pattern, now known as the Titius–Bode law. In units where the distance between Earth and the Sun is 10, the series

a = 4 + x,

where x = 0, 3, 6, 12, 24, 48, 96 … gives the approximate distances of the planets from the Sun, except at the position corresponding to x = 24, where no planet was observed. In these same units, Mercury’s distance from the Sun is 4, Venus’s is 7, Earth’s is 10, Mars’s is 16, and Jupiter’s and Saturn’s are 52 and 100, respectively. No planet existed at 28 units from the Sun — but that’s where the pattern suggested one should be.

Dividing by 10 converts these to astronomical units (AU), which can be compared with the actual measured values:

  • Mercury: 0.4 (0.387 AU)
  • Venus: 0.7 (0.723 AU)
  • Earth: 1.0 (1.000 AU)
  • Mars: 1.6 (1.524 AU)
  • Missing planet: 2.8
  • Jupiter: 5.2 (5.203 AU)
  • Saturn: 10 (9.55 AU)

The coincidence was intriguing but not initially taken seriously. When Herschel discovered Uranus in 1781 and its distance (19.2 AU) fit almost perfectly into the next predicted position (19.6 AU), Bode urged astronomers to search for the “missing planet” near 2.8 AU. On January 1, 1801, Giuseppe Piazzi discovered a star-like “asteroid” — later named Ceres — orbiting at 2.77 AU from the Sun, right where the Titius–Bode law had predicted.

Despite this early success, the later discoveries of Neptune and Pluto did not fit the sequence, and the Titius–Bode law is now regarded as an interesting coincidence rather than a true physical rule.

Learning Activity

The video begins with details about Piazzi’s discovery, the origin of the term “asteroid,” and the identification of the asteroid belt. As you watch this part of the video, consider the question:

  • What similarities and differences are there in the discoveries of Ceres and Pluto?

After discussing the early history of asteroid discoveries, the video describes the different types of asteroids observed and why these compositional differences exist. As you watch, pay special attention to:

  • the spacing and distribution of asteroids within the belt,
  • NASA’s Dawn mission, which orbited Vesta (2011–2012) and Ceres (2015–2018), revealing their geology and composition (mission website),
  • the Mars-crossing asteroids, near-Earth objects (NEOs), Trojan asteroids, and co-orbital asteroids found beyond the main belt, and
  • asteroid naming conventions.

As you continue through the video, look for the answers to these questions:

  • How did the asteroid belt form, and why does it have gaps similar to those in Saturn’s rings?
  • Why do different types of asteroids exist?
  • Why is Mars so small?

Note: Since the video was produced, NASA has launched new missions to study additional populations of asteroids — including Lucy (to the Jupiter Trojans) and Psyche (to a metal-rich asteroid that may be the exposed core of a protoplanet). These missions build on Dawn’s legacy to further reveal how the Solar System’s small bodies formed and evolved.