Overview
This lecture covered the shapes of orbits in the solar system, with a focus on comets, how their tails form and point, and why comets appear stationary compared to shooting stars.
Orbit Shapes and Ellipses
- All planetary and small-body orbits are ellipses, not perfect circles.
- Most orbits in the Kuiper belt, asteroid belt, and for planets are nearly circular.
- The Sun is not at the center of any orbit; it is at one of the ellipse's foci.
- Highly stretched (eccentric) orbits can occur if objects are influenced by collisions or gravitational interactions.
Kuiper Belt Objects and Melting
- Kuiper belt objects contain ice, unlike asteroids.
- When such an object moves into a highly eccentric orbit, it can approach the Sun closely.
- Close approaches to the Sun cause these icy objects to partially melt.
- Most melting occurs quickly, since the comet moves fastest when near the Sun (Kepler's Laws).
Comets and Tails
- A comet is an icy object, usually from the Kuiper belt or Oort cloud, with an orbit that brings it close to the Sun.
- Melting material forms a visible tail as the comet nears the Sun.
- The comet's tail always points away from the Sun due to the force of the solar wind, not in the direction opposite its motion.
Observing Comets vs. Shooting Stars
- Comets are different from shooting stars (meteors).
- Comets appear almost stationary in the sky despite moving very fast, due to their great distance from Earth.
- In contrast, shooting stars zip quickly across the sky and are visible for only a brief moment.
- The slow apparent movement of comets is due to their large distance from Earth.
Key Terms & Definitions
- Ellipse — An elongated circle; the shape of most orbits in the solar system.
- Focus (Foci) — One of two points inside an ellipse; the Sun sits at one focus of planetary orbits.
- Kuiper Belt — A region beyond Neptune filled with icy objects.
- Comet — An icy body from the outer solar system whose orbit brings it close enough to the Sun to form a visible tail.
- Solar Wind — A stream of charged particles released from the Sun that blows comet tails away from the Sun.
- Shooting Star (Meteor) — A small piece of debris burning brightly as it enters Earth's atmosphere.
Action Items / Next Steps
- Review the distinction between comets, meteors, and asteroids.
- Be prepared to explain why comet tails always point away from the Sun.
- Read more on the effects of eccentric orbits and Kepler's laws.