Space continues in all directions,and has no known limits. Space continues in all directions,and has no known limits.The moon moves through space around the earth.The earth and the other planets circle in space around the sun.The sun and billions of other stars make up a giant galaxy whirling through space.Countless other galaxies are scttered throughout space as far as we can see with the largest telescope . Space begins where the earthís atmosphere is too thin to affect objects moving through it.Near the earthís surface,air is plentyful.But higher above the earth,the air becomes thinner and thinner until it fades to almost nothing,and space begins.Space usually is said to begin about 160 km. above the earth.At this height,a satellite may continue circling the earths for months.But,even there,enough air is still present to slow a satellite and cause it to fall.Solar storms in the upper atmosphere may also cause satellites to fall sooner then expected. Space between the earth and moon is called cislunar spacer as the moon is approached through cislunar space,earthís gravity becomes weaker and the moonís gravity becomes stronger.The combined gravities of earth and moon are effective to about 1.600.000 km. from earth and this distance is sometimes called translunar space. Space between the planets is called interplanetary space.The sunís gravity controls interplanetary space. But each planet and moon also has it own gravity. Vast distance separate the bodies that move in interplanetary space. The sun is about 150 million kilometers from earth.Venus,the closest planet to the earth,approaches only to within about 40 million kilometers of earth.Interplanetary space reaches for beyond pluto,the planet most distance from earth.It ends where the sunís gravity is no longer effective-perhaps 80 billion kilometers from earth. We find greater distances in interstellar space (space between the stars).Proxima Centauri,the nearest star outside the solar system,is over 40 trillion kilometers away.To cover suvh great distances,a spacecraft would have to travel almost as fast as light.Even than,a round trip to a star could take a space travelerís whole lifetime.Interstellar space reaches unimaginable distances.Then inter galactic space (space between the galaxies) begins. A spacecraft may make several kinds of trips into space.It may be launched into orbit around the earth,rocketed to the moon,or sent past a planet.For each trip,the spacecraft must be launched at a partivular velocity.The job of the launch vehicle is to give the spacecraft,this velocity.If the spacecraft carries a crew,the spacecraft itself must be able to slow down and land safely on the earth. Overcoming gravity is the biggest problem in getting into space.Gravity pulls everything to earth and gives object their weight.A rocket overcomes gravity by producing thrust (a pushing force).To lift a spacecraft,a rocket must have a thrust greater than its own weight of the spacecraft.The extra thrust accelerates the spacecraft.That is,it makes the spacecraft go faster and faster until it reaches the velocity needed for its journey. Rocket enginees create thrust by burning large amounts of fuel.As the fuel burns,it be cornes a hot gas.The heat creates extremelly high pressure in the gas.This pressure does two things: a)it pushes the flaming gas backward and out through the rocket nozzle b)it pushes the rocket forward.This toward push on the rocket is the thrust. Rocket fuels are called propellants.Liquid-propellant rockets work by combining a fuel,such as krosene or liquid hydrogen,with anoxidizer,such as liquid oxygen.The fuel and oxidizer burn violently when mixed.Solid fuel rockets use dry chemicals as propellants. The primary vehicle for research and exploration in the United States space program is the space Shuttle.The space Shuttle takes of like a rocket,orbits the earth like a spacecraft,and l [÷dev ›ndir]