Lesson 24 Navigating in Space

Voyages to other planets require enormous amounts of energy. The amount of energy expended can be minimized using the same force that moves the planets through the solar system. The force of the sun's gravitational field is used to navigate through space.

1973 - Mariner 10 was sent to Venus and Mars


1975 - Viking was sent to Mars


1977 - Voyager was sent to Jupiter, Saturn, Uranus, and Neptune.


For an object to move in a straight line in space it requires a lot of propulsion energy. Instead, scientists recognized Kepler's second law - planets move around the sun in orbitals shaped as ellipses - as an operting principle to allow spacecraft to follow the same path and be projected from around the sun into space.

The spacecraft is put in an independent orbit around the sun that intersects the orbits of Earth and Mars In order for the spacecraft to coast after launch to its destination.

To travel between two points in space the craft coasts to its destination in orbit around the sun, just as if it were an orbiting planet. The path the craft travels from one planet to another is known as the transfer orbit.

The Viking mission was limited in the selection of a transfer orbit because it was the most massive interplanetary craft ever launched. The Viking mission followed a Hohmann transfer, which is the classic method for travel between two planets. The craft was launched when the Earth was at its point in its orbit closest to the Sun and arrived at Mars when Mars was at its point in its orbit farthest from the Sun.

T^2 = 4pi^2/GM (a^3) - Kepler's third law dictates the length of the Earth year and also how long a spacecraft coasting in its own orbit will take to get from the Earth to the position of the orbit of Mars, which is about eight and half months.

A spacecraft must be launched when Mars is 44 degrees ahead of the Earth; this is kown as an opportunity.

Opporunity:

Venus - 54 degrees - every 19 months
Mars - 44 degrees - every 2 years
Jupiter - 97 degrees - every 13 months

There are only certain times each day that a spacecraft can be launched due to the Earth's rotation. Any increase or boost in speed creates a larger more eccentric orbit. To go from the Earth's orbit to a Mars tranfer orbit the craft must be launched at 2.9 km/s. However, to do this the craft must escape the parking orbit around the Earth. A rocket thrust will boost the rocket into a hyperbola orbit at 2.9 km/s; this must occur at 7:56 p.m. The Sun will then bend the hyperbola orbit into a Mars transfer orbit. In order to travel to Venus, the rocket thrust must be activated at 7:40 p.m. in order for the craft to travel inward.

The four outer planets line up for a craft to visit all four every 175 years. The craft must overcome gravity in order to leave the planet and utilizes gravity assist to travel to other planets.


300 years after the discovery of classical mechanics, discoveries are still to be made in space, physics, and every field of science.