Lesson Plan on Aspects of Launching A Spacecraft

Objectives: Students will be able to

1. explain the reasons for time constraints on spacecraft launch.
2. explain why gravity assists are used.
3. research space missions that have used gravity assists.

Grade Level: 8 - 12

Background Information:

A precise time and day for the launch is necessary so that the rocket can take advantage of the earth's position in the solar system and its rotation on its axis. Exact knowledge of orbital motion and planetary positions is necessary in deciding on the launch date. It must be certain that the object of destination is going to be at the right place at the right time when the craft arrives. A launch window is the span of time in which the launch can take place. This window is limited to a number of days or weeks. The launch window is also limited to a number of minutes each day in order to take advantage of the Earth's rotation. It can save a lot of time, energy, and money if the Earth's natural rotation can be utilized for the launch. The direction the spacecraft is going should be coordinated with the rotation of the Earth on its axis so that they are both going the same way. The location of the launch can also be a factor in "helping" the rocket. For instance, areas closer to the equator go around once every 24 hours, just like all the other parts of the world, but have to cover more ground. So it seems that places nearer the equator are moving faster, about 1600 kilometers per hour to be more specific. If the rocket is launched closer to the equator, like at Cape Canaveral, and in the direction the Earth is turning, the rocket can utilize that "boost" from the Earth's rotation and add it to its own exerted velocity. This way the craft uses less propellant to achieve its orbital speed. It is kind of like jumping off of a moving merry-go-round.

It is important to calculate the optimum time and direction of the launch in order to achieve the proper trajectory. The spacecraft's trajectory is the path it takes to its destination. It is also during the launch and entrance into its trajectory that the main thrust of propulsion for the entire mission is utilized to establish the desired orbit. The type of rocket used for the launch depends on how heavy the spacecraft is and how much change in its velocity is needed to accomplish its mission. Basically the spacecraft needs to be launched to a high enough altitude, no less than 150 km, so that it rises above the Earth's atmosphere. There the air will be thinner and cause less resistance. It will then be accelerated to a velocity (about 30,000 km/hr) to escape the Earth's gravitational pull and enter a virtual "free fall." Newton's first law states that once an object is in motion it will stay in motion until acted upon by another force. So, the spacecraft will continue to fall freely in the path of its orbit until another thrust of propulsion may be necessary to change its course.

In space, there are many forces to change an object's motion. However, these forces acting on an object in space are very small in comparison with the forces of atmospheric drag and Earth's gravity. And, according to Newton's second law, when a force acts on an object, its speed, direction, or both will change. Some space missions use a gravity assist to change the velocity or direction of the spacecraft. A gravity assist means to use the gravity of one celestial body to change the spacecraft's energy. Some missions, like Galileo and both Voyager missions, used gravity assists around planets like Earth, Venus, and Jupiter to help get them to where they are going.

Gravity assists can be described in a simplistic way by basic vector addition (see diagram below). Planets orbiting around the Sun have a great deal of angular momentum. An object (like a spacecraft) traveling close to a planet is attracted toward the planet when it enter its gravitational field, and exchanges  some of this angular momentum. That is, the speed and direction of both the spacecraft and  the planet change. The spacecraft's increased speed and change in direction, however, is much more significant than the planet's decreased speed because of the obvious mass difference between the two.

So, in reference to the Sun, the distance between the spacecraft and the planet, as well as the vector velocity and mass of the planet, act upon the spacecraft vector velocity . Adding these vectors together results in a greater spacecraft velocity vector as it leaves the planet's gravity field. This change in momentum accelerates the spacecraft, causing a slingshot effect, where the planet whips the craft around and projects it back into solar orbit.

The NEAR spacecraft will be using the Earth in January, 1998 for a gravity assist to get it to its final destination: Eros. For NEAR, the gravity assist is further used to decrease the spacecraft's maximum distance from the Sun, and to increase the spacecraft orbit's tilt (inclination) to nearly match the inclination of Eros.

Activities:

No Frames Table of Contents

Contact Karen Krupinsky (kgurley@gsfc.nasa.gov) or Tammy Seergae (tseergae@umd.edu) for further information.