WAS Orbit Maker Lab
Learning objective: Use a computer model to investigate orbital mechanics, and how changing the
parameters of orbiting bodies effect the trajectory of a satellite.
Instructions:
Step One: Open the website: Astrodynamics Interactive Demo (alfonsogonzalez.github.io)
Here you will see two different ways to define the parameters of orbits and create a model:
1. Pink Orbit – State Vectors
The pink orbit is given in state vectors, which are the satellite’s position (r) and velocity (v).
• r = position
• v = velocity
2. Blue Orbit - Keplerian Orbital Elements
The blue orbit is given in Keplerian Orbital Elements, which define the semi-major axis,
eccentricity and inclination of the orbit.
• SMA - Semi-Major Axis
• ECC – Eccentricity (how elliptical the orbit is)
• INC – Inclination (the tilt of the orbit)
Step Two: Make sure that only the blue orbit is checked as “Active”. Click “Animate Orbits!”. Watch
how the blue path of the satellite around the Earth is represented on the position and velocity graphs
(Fig. 1) and the Groundtrack (Fig. 2).
Fig. 1 These graphs map the velocity, or speed, and the position, or distance from the center of Earth,
of the satellite over time.
Velocity
Position
In this example, the position is 9,020 km
from the center of the Earth.
Time
Fig. 2 The Groundtrack shows where the satellite is located above the surface of the Earth.
Step Three: Answer the following questions by experimenting with changing the orbital parameters.
Lab Report Questions:
1. The relationship between position and velocity:
a. Under the default parameters of the blue orbit, what is the fastest velocity (v) of the
satellite in its orbit? At what position (distance in km) from the planet do you observe
that velocity? Use the graphs to find your answer.
Velocity (km/sec) =
Position (km) =
b. What is the slowest velocity (v) of the spacecraft? At what position (distance in km)
from the planet do you observe that velocity? Use the graphs to find your answer.
Velocity (km/sec) =
Position (km) =
c. Is the satellite going the faster or slower when it is close to the planet?
2. Orbital Inclination & Latitude: Change the inclination (INC) of the blue orbit to be 0 degrees and
the Period to 1. Click “Animate Orbits!”
a. What degree Latitude does the satellite pass over as shown on the Groundtrack? Write
your answer and draw the shape of the Groundtrack below:
b. Change the inclination(INC) of the blue orbit to 45 degrees, and click “Animate Orbits!”.
Draw the shape of the Groundtrack below:
c. Where on Earth do you think the satellite will pass over if you change the inclination of
the blue orbit to 90 degrees? Write your prediction below:
I predict the satellite will pass over _______________
d. Now test your prediction! Change the inclination(INC) to 90 degrees and click “Animate
Orbits”. Was your result what you expected; why or why not?
3. Finding escape velocity: Spacecraft that have enough velocity will escape the gravitational orbit
around the planet and head out into space. You will know that you have passed escape velocity
when the orbit is no longer closed, but is a curved line going off the grid.
a. What is the escape velocity of the orbiting satellite from Earth?
Uncheck the blue box and check the pink box so that only the pink orbit is “Active”. Click
“Propagate Orbits!” (Note: this may glitch out if you use the Animate button. Refresh
the page if necessary.)
Slowly increase the velocity (Vy) of the pink orbit, clicking “Propagate Orbits!” for each
new velocity. You will not be able to find the EXACT escape velocity, within .1 km/sec is
close enough!
We suggest starting with increments of 1, and then moving to smaller increments when
you think you are close or go over. Create a table of the velocities that you try:
Vy (km/sec)
Example: 7.62
8.62
Escape? (yes/no)
no
b. What is the escape velocity of the orbiting satellite from the Moon?
To find this, change the central body to the Moon by clicking the “Central Body” drop
down, and then repeat the above steps with the pink orbit. Remember to click
“Propagate Orbit!” after each new velocity.
Vy (km/sec)
Example: 1.8
2.0
Escape? (yes/no)
no
c. What the difference between the escape velocity from the Earth and from the Moon?
Why do you think this difference exists?