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Earth to Orbit (ETO) is placing a payload into an orbit around the Earth.[1][2] Traditionally this has used rockets, however ETO can also include other theoretical forms of transportation such as orbiting skyhooks, railguns, hypersonic cannons, and space elevators.

Methods

Earth to orbit transportation can be roughly broken up into four groups, these are:
Rocket based transportation
Titan II Rocket Launch

A series of suborbital rockets are used in order to accelerate a payload to orbital velocities. This is the only method of placing a payload into an earth orbit which has been successfully used. It involves constructing a series of rockets which are usually stacked on one another, imparting their change in momentum to the payload before dropping away, although a series of suborbital rockets are usually used. Other schemes involving SSTO or Single Stage To Orbit vehicles have been proposed. However they all suffer from a low mass fraction, reducing their effectiveness for transporting payloads into orbit.
Ballistic methods

A stationary platform at least partly in Earth's atmosphere that delivers the required momentum by using the Earth to react against. Examples of this may be a railgun, coil guns, or a hypervelocity gun. Because the payload is accelerated to velocities greater than required for orbital velocity by the time the payload has left the device, either very high accelerations are required, or very long structures are needed.
Orbital transportation

This method uses a satellite, already in Earth orbit to impart part of the required momentum to accelerate the payload to orbital velocities, a concept known as moment exchange tether. The end of the tether dips in the atmosphere, however because it has zero relative velocity to the Earth's surface. It is not affected by atmospheric heating that would otherwise destroy an object entering Earth's atmosphere without a heat shield. As momentum is transferred from the momentum exchange tether to the payload it loses orbital energy, decreasing in altitude. The loss in altitude of the tether must be recovered, otherwise the tether itself may reenter the Earth's atmosphere. One way to avoid this may be to use a conductive tether and pass electricity through it, forming an electrodynamic tether to generate a force to accelerate the tether after a loss of momentum.
Relay transportation

This method uses a combination of the above methods to cover the various parts of the journey into orbit.

Orbital spaceflight
Spacecraft propulsion
Space elevator
Space fountain
Tether propulsion

References

Highly Resuable Space Transportation: Approaches for Reducing ETO Launch Costs to $100-$200 per Pound of Payload
Nowicki, Andrew. "Earth-to-Orbit Transportation Bibliography". Retrieved 2005-02-18.

Space Encyclopedia