2011
A comet's fall from the Oort Cloud
- Round
- Theory
- Points
- 10
- Level
- Bronze
- Taught in
- Section 3.4
Adapted from IOAA 2011 (Katowice), theory problem 1. We have restated it in our own words and kept the original data. The solution was last checked on 21 October 2026.
Most comets that pass through the inner Solar System only once come straight in from the Oort Cloud. Estimate how long a comet takes to make this journey, if it was at aphelion in the Oort Cloud, 35 000 au from the Sun.
A comet that ends up close to the Sun after starting so far away moves on an ellipse that is almost a straight line.
- The perihelion is tiny next to 35 000 au, so the orbit is a very long ellipse with
- By (3.2) in years and astronomical units, its period is
- The journey from aphelion to perihelion is half the period.
AnswerThe journey takes about 1.16 million years.
This problem also appears among the olympiad problems of Section 3.4, Where a body is on its orbit, which teaches the ideas it uses. Equation and figure numbers refer to the course.