Daniel Kirkwood
The astronomer who discovered in 1866 the gaps in the asteroid belt made by Jupiter's gravity, now called the Kirkwood gaps.
Key facts
- Born
- 1814
- Died
- 1895
- Nationality
- American
- Field
- Astronomer and mathematician
Daniel Kirkwood grew up on a farm in Maryland and taught himself algebra before becoming a mathematics teacher and later a professor at Indiana University for many years.
By the 1860s, nearly a hundred asteroids had been found. Kirkwood looked at how long each took to go around the Sun and noticed something odd. There were almost none with periods that were simple fractions of Jupiter's period, such as one-third or one-half. In 1866 he announced that these empty zones, now called the Kirkwood gaps, were caused by Jupiter.
An asteroid in such a zone would line up with Jupiter in the same place again and again. Those repeated tugs, called a resonance, add up over time and stretch its orbit until it is thrown out of that zone. The effect is like pushing a child on a swing at just the right moment each time.
The gaps matter greatly for meteorites. Asteroid fragments that drift into a resonance can have their orbits stretched until they cross the paths of Mars and Earth. This is one of the main routes by which pieces of the asteroid belt are delivered to Earth as meteorites. Kirkwood also argued that meteor showers are made of debris from comets, an idea confirmed by Schiaparelli in the same years.