In order to launch a spaceship or rocket off the ground to orbit the Earth or to travel, there are certain basic principles and laws that must be followed and understood.
Sir Isaac Newton came up with some laws that still hold true today. He was born on December 25, 1642, the same year that Galileo Galilei died. Newton’s Law of Universal Gravitation states that: the force of gravitational attraction between two point masses (m1 and m2) is proportional to the product of the masses divided by the square of the distance between them. In the following equation, G is a gravitational constant, r is the distance between the centers of the masses and F represents the force:
____
mlm2
In other words, if the distance between the objects is doubled, the attraction between them is diminished by a factor of four, and if the distance is tripled, the attraction is only oneninth as much. Due to gravity, a satellite needs more speed to stay in a lower orbit, than in a higher one. (NASA, 1994)
Newton’s First Law—An object continues in a state of rest or uniform motion unless acted on by an external force. (NASA, 1994) This is also known as the Law of Inertia. An example of this is as follows: In zero G or microgravity, like in the Space Shuttle, objects stay where put. You can leave a pen in midair and come back for it hours later. If an astronaut is sitting in the middle of a cabin, too far to push off a wall, they’re stuck until someone comes to push them into motion. (NASA, 1995)
Newton’s Second Law—The resultant force acting on an object is proportional to the rate of change of momentum of the object. The change of momentum being in the same direction as the force. Force= mass x acceleration. (NASA, 1995)
Newton’s Third Law—To every force or action, there is an equal and opposite reaction. (NASA, 1995) An example: If you stand on a skateboard and throw a basketball, the board rolls in the direction opposite the way you throw. (The Astronomy Society of the Pacific, 1996)
Put Newton’s three laws together like this: An unbalanced force must be exerted for a rocket to lift off from a launch pad or for a craft in space to change speed or direction (first law). The amount of thrust (force) produced by a rocket engine will be determined by the mass of the rocket fuel that is burned, and how fast the gas escapes the rocket (second law). The reaction, or motion of the rocket is equal to and in the opposite direction of the action, or thrust from the engine (third law). (NASA, 1995)
A bit of trivia—during a 1960 baseball game between the New York Yankees and Detroit Tigers, Mickey Mantle hit a home run out of the stadium. It is in the Guiness Book of World Records as the longest homerun ever. If Mickey Mantle had swung his bat 150 times faster, he could have hit the ball into orbit. (The Astronomy Society of the Pacific, 1996)
The Space Shuttle is able to stay in orbit because it is launched in a trajectory that arcs above the Earth, so that it is travelling at the right speed to keep it falling while maintaining a constant altitude above our surface. For instance, if the Space Shuttle climbs to a 320 kilometer high orbit, it must travel at a speed of about 27,740 km/hr. to achieve a stable orbit. At this speed and altitude, the Shuttle’s falling path will be parallel to the curvature of the Earth. (NASA, 1995)
Zero gravity is a term commonly misused when people talk about space travel. The correct term is microgravity. Let’s start at the surface of the Earth. Here the acceleration of an object acted upon by our gravity is 1G or 9.8 meters per second squared. If you fell off a roof that was 5 meters high, it would take you just one second to reach the ground. In 1% of Earth’s gravity (microgravity), the same drop would take 10 seconds. If the microgravity environment was equal to one millionth of Earth’s gravitational pull, the same drop would take 1000 seconds or about 17 minutes. (NASA, 1995)
One way to create a microgravity situation is to free fall. If you were travelling in an elevator and the cables break, both you and the elevator would be travelling at the same rate downward. Another way to create this is to travel far away from the Earth. (NASA, 1995)
Weightlessness in outer space, is another commonly misused synonym for microgravity. Microgravity results from giving a spacecraft enough forward velocity to counterbalance the downward pull of gravity. A spacecraft on a long voyage between planets is actually in orbit around the sun. Here its forward velocity counters the pull of gravity in the manner as a satellite that orbits the Earth. (NASA, 1995)
Two terms often confuse students and even a number of adults. They are rotation and revolution. Rotation means to spin on an imaginary axis. A period of rotation is the time it takes for an object to turn once on its axis. This is also known as one day. Revolution has to do with orbiting. A period of revolution is the time it takes for one object to orbit another. The Earth orbiting the sun. We term this a year. (Coble, Rice, Walla, Murray, 1991)
Two interrelated concepts are difficult for many students to grasp. They are distances in space and the time it takes to travel to different places in the Universe.
The Earth is constantly in motion. It orbits the Sun at a speed of 20km/second. It also orbits the Galaxy, completing one orbit every 200 million years. The Earth has gone around the center of the Milky Way 100 times in the last 4.6 billion years. (Brown, 1996)
When students are asked to define what a billion means, they have a hard time. The following is from Project Spica and is worth putting on an overhead or poster.
Number


Equals

Equals

1


1 drop

1 drop

10


10 drops

10 drops

100


5ml

teaspoon

1000


50m1

graduated cylinder

10,000


500ml

water bottle

100,000


5 liters

2 1/2 liter bottle

1,000,000

50 liters

35 gallon can

10,000,000

500 liters

approx. 2 barrels

100,000,000

5000 liters

approx. 20 barrels

1,000,000,000

50,000liters

1 1/2 (semi) gas truck

(Braile, 1990)
How long is a billion seconds?
1


1 second

10


10 seconds

100


12/3 minutes

1000


approx. 16.7 minutes

10,000


approx. 2.8 hours

100,000


approx. 28 hours

1,000,000

approx. 11.6 days

10,000,000

approx. 116 days

100,000,000

approx. 3 years

1,000,000,000

approx. 31 years

(Braile, 1990)
Nothing in the Universe travels faster than light. All forms of light travel at the same speed. The speed of light is 299,800,000 meters per second, and is usually rounded off to 300,000,000 meters per second. The speed of sound is only 340 meters per second. (Smith, 1992)
When talking about distance, one term that comes up is an astronomical unit. One astronomical unit is the distance between the center mass of the sun and the center of mass of the EarthMoon system. (Smith, 1992)
Due to the fact that distances in the Universe are so great, they are on many occasions, expressed in terms of “light years.” One light year is equal to the distance light travels in one year. Do this calculation with students:

60 seconds/minute x 60 minutes/hour x 24 hours/day x 365 days/year = 31,536,000 seconds/year. 31,536,000 sec./year x 300,000km/sec. =
9,461 trillion kilometers
= the distance light travels in one year.
This is also equal to approx. 6 trillion miles per year. (Smith, 1992)
It takes sunlight 8.5 minutes to travel to Earth. In other words, the sunlight that is shining on Earth right now is 8 1/2 minutes old. Our closest star—Proxima Centauri is 4.2 light years away. The brightest star, excluding the Sun is 9 light years away. Looking at these stars is like looking back in time. For instance, if an explosion occurred on Proxima Centauri today, we wouldn’t know about it for over 4 years. (Smith, 1992)
The diameter of the Milky Way Galaxy is 100,000 light years. Earth is 30,000 light years from the center. We’re about 2.2 million light years from the nearest galaxy. (Brown, 1996)
Some additional comparisons of distance and time are as follows: the Moon is 250,000 miles from Earth and it takes light 1.2 seconds to travel there. Mars is 40,000,000 miles away and it takes light 4 minutes to reach there from Earth. Pluto is 3.6 billion miles away and it would take light 5.4 hours to travel from the Earth and reach the farthest planet. (U.S. Space Foundation, 1996)
In the lesson plan section of this unit, I have included a couple of activities relating relative size and distance.