When we think of how fast we're going (or the rate at which we're traveling) in a car, we usually express it in "miles per hour". This rate (miles/hr) is really reflecting the average distance that we've gone in the time it takes to travel that distance. For example, a car that travels 60 miles in 1 hour is traveling at a rate of 60 miles/hr. Alternately, a car that has a speed of 60 miles/hr will have traveled 60 miles in 1 hour. At this rate, in 2 hours it will have traveled a distance of 120 miles.

Therefore, given some basic information, like how fast an object is going (its speed or rate) and how long it took to get to where it is now ( time), it is possible to determine the distance that object has traveled. This concept can be expressed as an equation that has the following form:

D = R x T

where D is the distance, R is the rate or speed, and T is the elapsed time.

This basic idea can be applied to many situations. In the following exercises, you will calculate the distance to the moon ( TASK A), and the speed or rate of the Galileo Spacecraft on its way to Jupiter (TASK B ).


For the calculations we will perform in this lesson, we will be using the metric system. That is, we'll express distance in terms of kilometers and not miles .

Some useful relationships:

TASK A: Distance to the Moon (Apollo Data)

Apollo 11: Lunar Landing Mission

The Apollo 11 spacecraft was launched from Cape Kennedy at 13:31:01 GMT on July 16, 1969. After 2 hr and 33 min in Earth orbit, the S-IVB rocket engine was reignited for acceleration of the spacecraft to the velocity required to escape Earth's gravity. Although at times the spacecraft reached speeds near 40,000 km/hr, the average speed was about 5500 km/hr .

Lunar-orbit insertion began at 75:50 (75 hours and 50 minutes) ground elapsed time (GET). The spacecraft was placed in an elliptical orbit (61 by 169 nautical miles), inclined 1.25 degrees to the lunar equatorial plane. At 80:12 GET, the service module propulsion system was reignited, and the orbit was made nearly circular (66 by 54 nautical miles) above the surface of the Moon. Each orbit took two hours.

The lunar module (LM), with Astronauts Armstrong and Aldrin aboard, was undocked from the command-service module (CSM) at 100:14 GET, following a thorough check of all the LM systems. At 101:36 GET, the LM descent engine was fired for approximately 29 seconds, and the descent to the lunar surface began. At 102:33 GET, the LM descent engine was started for the last time and burned until touchdown on the lunar surface. Eagle landed on the Moon 102 hr, 45 min and 40 sec after launch.

ACTIVITIES: Using the above information and our D = R x T equation, can you calculated the distance to the moon?


The moon is actually 384,400 km away from the earth . If you multiplied the rate (5500 km/hr) by the time (73.45 hours), your answer should be = 403,975 km which is pretty close to the actual lunar distance. We didn't get the exact distance because the Apollo spacecraft speeds up and slows down on the course of its journey to the moon, and we had to approximate the rate with an average speed.

If you would like to learn more about the Apollo 11 mission to the moon, try Apollo Facts .

TASK B: Galileo Spacecraft Navigation

Like Voyager and some other previous interplanetary missions, Galileo used planetary gravitational fields at certain stages for extra propulsion (called a "gravity-assist"). The spacecraft used the gravitational fields of Venus and Earth to pick up enough velocity to get to Jupiter. The total trip time was longer than six years.

Path of Spacecraft

On arrival day (December 7, 1995), the Orbiter skims 1,000 km above Io, picking up a gravity-assist, and then subsequently flies over the descending Probe so that the Probe can relay its data to the Orbiter. About an hour after the Relay, the engine will burn fuel for nearly an hour to place Galileo into orbit around Jupiter.

If you'd like to learn more about navigating Galileo to Jupiter, check out: JPL-Trajectory

Galileo Spacecraft Navigation

The following table lists statistical information for Galileo in January, 1995. Remember, at this point in time, the Galileo Spacecraft has reached its destination (It arrived in December.) and is in a stable orbit around Jupiter.

Date Elapsed Time
Distance from the Earth
(millions of km)
Distance Traveled
(millions of km)
Starting Point 1-1-95 0883.7------
Position 1 1-5-95 96880.43.39.6
Position 2 1-13-95 288872.211.511.1
Position 3 1-18-95 408866.117.7---
Position 4 1-25-95 576856.327.413.2
Position 5 1-31-95 720846.836.9---

ACTIVITIES: Using the above table, which tracks the distance of the Galileo Spacecraft from the earth in January of last year, make two graphs:

Sample Plot:


Back to Activity 1 or Forward to Activity 3

This module was written by Eileen V. Ryan
NOAO/ Planetary Sciences, University of Arizona, Tucson AZ.

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Galileo Solid State Imaging Team Leader: Dr. Michael J. S. Belton

The SSI Education and Public Outreach webpages were originally created and managed by Matthew Fishburn and Elizabeth Alvarez with significant assistance from Kelly Bender, Ross Beyer, Detrick Branston, Stephanie Lyons, Eileen Ryan, and Nalin Samarasinha.

Last updated: September 17, 1999, by Matthew Fishburn

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