In this activity students use a computer model to study the orbit of a comet.

What do the orbits of comets look like?

Our Solar System consists of many types of objects circling around the Sun, held in their orbits by gravity. Comets are special kinds of objects. Write down everything you know about comets. For example: where are they in the Solar System? When do we see them? What do they look like, and why?

This activity will look specifically at the orbits of comets.

The PhET model Orbits is opened in a separate window.

This activity addresses NSES standards for earth and space science and inquiry at grades 5-8 for earth in the solar system
(http://books.nap.edu/readingroom/books/nses/6d.html#es).

1. Open the PhET model Orbits This picture shows how to adjust the model.
   
2. Click START and observe the model. Let it run for several revolutions. Describe what happens.
3. How are the orbits of the comet and the planet different?
4. RESET the model and give the comet the following velocity:
5. Vx = +70
6. Vy = -80
7. Run the model and wait until the comet has made a full revolution. Describe what happened. How many planet years equal one comet revolution? If the comet were small, do you think it could be seen from Earth during its full orbit?
8. RESET the model, which will keep these values. Measure the following, using the timer:
9. One planet revolution =
10. One comet revolution =
11. Time the comet spends inside the planet’s orbit =
12. The last measurement is a little hard; get the best value you can. It is significant because that’s the part of the comet’s orbit when someone on the planet could probably see it.
13. RESET the model and give the comet the following velocity:
14. Vx = +120
15. Vy = -120
16. In this case, the comet’s velocity is so great that it will never come back. This curve, called a hyperbola, is an open-ended ellipse. It is the path of an object coming from outside the Solar System.
17. As this model demonstrates, comets can be affected by the gravitational pull of planets as well as the Sun. Scientists use this “slingshot” effect to send space probes to the outer planets and beyond.
18. To observe this, open the “slingshot” model from the drop-down menu. Run the model. What happens?

1. Suppose the comet were only visible when it was inside the planet’s orbit. For what percentage of its time would it be visible?
2. Why do you think the comet goes so much faster when it is close to the Sun?
3. What is the difference between a hyperbola and an ellipse?
4. Explain how the “slingshot” effect could use a large planet to “throw” a small object, such as a space probe, far from the Sun.

In 1705 Edmond Halley, using Newton’s newly formulated laws of motion, predicted that the comet seen in 1531, 1607, and 1682 would return in 1758 (after his death). The comet did indeed return as predicted and was later named in his honor.

• What is the average period of Halley’s Comet? It was last seen in 1986. When will it be seen next?
• Based on what you saw in this model, suggest a reason why the period of Halley’s Comet is not exactly constant.

Look up other famous comets and their periods.

For other PhET models, go to the Physics Education Technology website.