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The International Space Station as it appeared during its construction in orbit around the Earth on 2001-08-20

An orbital spaceflight (or orbital flight) is a spaceflight in which a spacecraft is placed on a trajectory where it could remain in space for at least one orbit. To do this around the Earth, it must be on a free trajectory which has an altitude at perigee (altitude at closest approach) above 100 kilometers (62 mi) (this is by convention the boundary of space).

The expression "orbital spaceflight" is mostly used to distinguish from sub-orbital spaceflights, which are flights where apogee of a spacecraft reaches space but perigee is too low.

Orbital spaceflight from Earth has only been achieved by large rockets that are capable of accelerating from rest to about 10 km/s. This figure allows for atmospheric drag (approximately 300 m/s with the ballistic coefficient of a 20 m long dense fuelled vehicle), gravity losses (depending on burn time and details of the trajectory and launch vehicle), gaining altitude, and the horizontal acceleration needed to reach orbital speed (~7.8km/s depending on altitude).

The only proven technique involves launching nearly vertically for a few kilometers while performing a gravity turn, and then progressively flattening the trajectory out at an altitude of 170+ km and accelerating on a horizontal trajectory (with the rocket angled upwards to fight gravity and maintain altitude) for a 5-8 minute burn until orbital velocity is achieved. Frequently 2-3 stages are needed to achieve sufficient delta-v.

Project HARP was a failed attempt, and a ram accelerator is another design, to launch an object into orbit with a gun, possibly with additional propulsion by a rocket.

Any object in orbit at an altitude of less than roughly 200 km is considered unstable due to the amount friction caused by the atmosphere. For a satellite to be in a stable orbit (i.e. sustainable for more than a few months), 350 km is a more standard altitude for low Earth orbit.

There are three main 'bands' of orbit: low Earth orbit, intermediate circular orbit and geostationary orbit.

Due to the high speeds of orbital spaceflight, atmospheric reentry is much more difficult compared to sub-orbital flights.

Even if the vehicle is a satellite that is ultimately expendable, most space authorities are pushing towards controlled re-entry techniques to avoid issues of space debris reaching the ground and causing a hazard to lives and property. In addition, this minimises the creation of orbital space junk.

Returning craft though (including all potentially manned craft), have to find a way of slowing down as much as possible while still in higher atmospheric layers and avoid hitting the ground (lithobraking) or burning up. The problem of deceleration from orbital speeds is solved through using atmospheric drag (aerobraking) to lose nearly all of the speed. On an orbital space flight initial deceleration is provided by the retrofiring of the craft's rocket engines, perturbing the orbit (by lowering perigee down into the atmosphere) onto a suborbital trajectory.

Aerobraking is achieved by orienting the returning space craft to fly so as to present the heat shields forwards towards the atmosphere so as to protect against the high temperatures generated by atmospheric compression and friction caused by passing through the atmosphere at hypersonic speeds. The thermal energy is dissipated mainly by compression heating the air in a shockwave ahead of the vehicle using a blunt heat shield shape, with the aim of minimising the heat entering the vehicle. Sub-orbital space flights, being at a much lower speed, do not generate anywhere near as much heat upon re-entry.

This has allowed maverick aircraft designer Burt Rutan recently (July 2004) to demonstrate an alternative or complementary approach to heat shield dependent reentry with the suborbital SpaceShipOne. It may be possible that the concepts utilized in SpaceShipOne's design can be applied to orbital space craft design and result in intrinsic stability of the vehicle through reentry (as opposed to the active stability used on the Space Shuttle.) Currently however, the need for an ultra high-performance and ultra reliable heat shield is a major difference between crafts designed for orbital flights (as opposed to sub-orbital ones), demonstrated in the Mercury program wherein the orbital flights used a typical ablative heat shield while the sub-orbital flights relied simply on a large metal heat-sink.

• List of orbits

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