Spaceflight 101I made this thread (-mostly cut&paste job on Wiki articles-) to clear up the terms and methodology spaceflight and space travel.
I also intend this thread to serve as a guide for mission designers for orbital, reentry, lauch or scientifically accurate missions.
Planetary orbitIn physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity.
History and basic theoryOrbits were first analysed mathematically by Kepler who formulated his results in his laws of planetary motion:
- Kepler's first law (1609): The orbit of a planet about a star is an ellipse with the star at one focus.
- Kepler's second law (1609): A line joining a planet and its star sweeps out equal areas during equal intervals of time.
- Kepler's third law (1618): The square of the sidereal period (the time that it takes the object to make one full orbit around the Sun)(T) of an orbiting planet is directly proportional to the cube of the orbit's semimajor axis (one half the major axis)(a).
a^3/T^2 = constant
Isaac Newton demonstrated that Kepler's laws were derivable from his theory of gravitation.
- F is the magnitude of the gravitational force between two objects
- m1 is the mass of first object
- m2 is the mass of second object
- r is the distance between the objects
- G is the gravitational constant, that is approximately : G = 6.67 × 10^−11 N m^2 kg^-2
Understanding orbits- As the object moves, it falls toward the orbited object. However it moves so quickly that the curvature of the orbited object will fall away beneath it.
- A force, such as gravity, pulls the object into a curved path as it attempts to fly off in a straight line.
- As the object falls, it moves sideways fast enough (has enough tangential velocity) to miss the orbited object. This understanding is particularly useful for mathematical analysis, because the object's motion can be described as the sum of three one-dimensional orbits around a gravitational center.
Orbiting and reaching spaceFrom a spaceflight perspective, the definition of space usually used is that space begins 100 km (62 miles) above Earth's surface. The United States sometimes uses a 50 mile definition.
(See boundary to space.)*This is a legal definition, there is no solid boundary where the atmosphere begins.
Achieving orbit is a prerequisite for going anywhere else, such as to the Moon or Mars.
The first successful orbital launch was of the Soviet unmanned Sputnik I mission on October 4, 1957.
The first orbital flight made by a human being was Vostok 1, carrying Yuri Gagarin on April 12, 1961.
One can distinguish the sub-orbital spaceflight and the orbital spaceflight
(cf. Difference between sub-orbital and orbital spaceflights).Sub-orbital spaceflightA sub-orbital spaceflight (or sub-orbital flight) is a spaceflight that does not involve putting a vehicle into orbit.
The sub-orbital spaceflight should not be confused with a partial orbital spaceflight: a low Earth orbit, with deorbiting after less than one full orbit.
If the objective is just to reach space, sub-orbital flights are appealing because this is very much easier (it simply means going higher than the edge of space) than to achieve orbit (which requires a velocity of about 8 km/s (18,000 mph)). A dedicated sub-orbital spacecraft can therefore be built and operated much more cheaply than an orbital spacecraft. Less powerful sub-orbital craft may not reach speeds much higher than around 1.1 km/s to 1.3 km/s (2,500-3,000 mph).
Orbital spaceflightAn orbital spaceflight (or orbital flight) in the general sense is a spaceflight where the trajectory of a spacecraft reaches the height of, and through having an appropriate velocity enters into, orbit around an astronomical body
Note too that the edge of space (100 km) is much lower than the altitude where a vehicle can circle the earth even once without reentering due to atmospheric drag. Also note that the required speed to "go orbital" (to achieve orbit with known methods) requires atleast 9 km/s (18,000 mph) delta-v.
(EDIT - this is the begining I while write more to this, and feel free to add your opinion and knowledge on the issue)
