APOD: 2024 October 10 - Five Bright Comets from SOHO

Good morning, Rallypoint, and welcome to the October 10th edition of Astronomy Picture of the Day (APOD). The Solar and Heliospheric Observatory (SOHO) is a joint project of the European Space Agency (ESA) and NASA, and was launched on December 2, 1995. Its primary mission is to study the Sun; but it has also helped to discover over 5,000 comets. Today's APOD is a collage of five of those discoveries.

SOHO orbits the Sun (with the Earth) at one of the five Lagrange points relative to the Sun and the Earth. What's a Lagrange point? First, you need two large bodies (e.g. the Sun and Earth). Next, you need a smaller object (e.g. a satellite, like SOHO). Normally, the two larger bodies would exert a proportional, unbalanced gravitational force at any point, altering the orbit of the smaller object at that point. At a Lagrange points, the gravity of the two large bodies and the centrifugal force of the smaller orbiting object balance each other. This makes Lagrange points an excellent location for satellites like SOHO, as orbit corrections (which will require fuel, adding extra weight to the satellite) required to maintain a desired orbit are kept at a minimum.

The three Lagrange points (L1, L2, L3) were discovered by the Swiss mathematician Leonhard Euler around 1750, a decade before the Italian-born Joseph-Louis Lagrange discovered the remaining two (L4, L5). Lagrange published his essay on orbital mechanics (describing all five) in 1772.