Difference between revisions of "Synchronous orbit"

Latest revision as of 03:15, 23 February 2019

A time-warped animation of a small satellite in synchronous orbit around Kerbin.

Important! You need to match your orbital period with sidereal rotation period not the solar day. So, for Kerbin it will be 5h 59m 9.425s instead of 6h that a lot of people go for.

Stationary orbits

Stationary orbits are a special kind of synchronous orbit. Its 0° inclination and its eccentricity of 0 cause its ground track to be only a point: a satellite in this orbit has no motion relative to the body's surface. Since it is impossible to get all orbital values exact for a stationary orbit, satellites in stationary orbits form small analemmata.

Some celestial bodies don't allow for synchronous orbits because the altitude required to synchronously orbit is beyond the body's sphere of influence. The body's slow rotation rate causes this effect: a very high altitude is necessary to allow for such a long orbital period. Tidally locked moons don't have synchronous orbit possibilities either because of their slow rotation. Moho is the only planet without any possibilities for a craft to achieve a synchronous orbit because of its very slow rotational period; Moho completes approximately two rotations during the time it takes for an object in the highest possible orbit to complete a revolution.

Semi-synchronous and similar orbits

When the orbital period is half as long as the rotational period, the orbit is usually described as semi-synchronous. It is possible to calculate the semi-major axis of a semi-synchronous orbit using Kepler's third law of planetary motion. With the knowledge about the semi-major axis of a synchronous orbit and the ratio between the two orbits:

${\displaystyle a_{\frac {1}{f}}={\frac {1}{\sqrt[{3}]{f^{2}}}}\cdot a_{1}}$

The fraction f is the quotient of the period of the synchronous orbit (a₁) and second orbit (a_1/f). When the second orbit is a semi-synchronous orbit this quotient is 2:

${\displaystyle a_{\frac {1}{2}}={\frac {1}{\sqrt[{3}]{2^{2}}}}\cdot a_{1}={\frac {1}{\sqrt[{3}]{4}}}\cdot a_{1}}$

An orbit where the orbital period is lower than the rotational period has some advantages, as some bodies don't allow synchronous orbits but opportunities for semi-synchronous orbits.

When dropping numerous payloads that should land nearby each other, the orbit should be an integer multiple of the celestial body's sidereal day. This way, the body stays the same relative to the orbit and has the same descent route, if each payload is detached at the same point in the orbit (e.g. apoapsis). The inverse factor f (= 1/f) defines how many days are between two detachments. For example, a super-synchronous orbit has f=1/2 so a payload could be dropped every two sidereal days or, when orbiting Kerbin, one every twelve hours.

An example of a semi-synchronous orbit for real world scientific applications is a Molniya orbit.

Sun-synchronous orbit

→ See also: Sun-synchronous orbit on Wikipedia

In the real world, there exists a sun-synchronous orbit. It's important to note that, although the name implies it, the orbit is not synchronous around the Sun. Instead, it describes an orbit around Earth which itself rotates, such that it appears the orbiting object is motionless relative to the Sun. Since it requires objects to have uneven gravitational fields, it is impossible to simulate in KSP.

Molniya orbit

A Molniya orbit is a semi-synchronous, highly elliptical orbit. The eccentricity should be as high as the central body permits. A three-satellite constellation in Molniya orbits can provide constant coverage to the high attitude regions. To set up such constellation, the mean anomalies of these three satellites should be spaced out by ${\displaystyle 120^{\circ }}$ or ${\displaystyle {\frac {2\pi }{3}}}$. The longitudes of the ascending nodes can also be spaced out by ${\displaystyle 120^{\circ }}$ to make the clover appearance.

For Kerbin, that equate to 70k for PE, 3117k for AP, and around 63 degree inclination.

Advantages of synchronous orbits

One advantage of a synchronous orbit is that they allow dropping multiple payloads from one craft, because the orbit will periodically travel above the same point on the body's surface. Usually, the orbit has a large eccentricity so that the payload has to complete a minimal amount of maneuvers to reach the surface. In this case the payload is detached at the apoapsis and decelerated such that it lands on the celestial body. After the payload has successfully landed, the next payload can be dropped as soon as the craft reaches the apoapsis again.

Stationary orbits

Communication to a satellite in a stationary orbit is easier than if it was in another orbit, as the ground based antennae do not have to move to account for the satellite's motion relative to the orbited body.

Orbital altitudes and semi-major axes of Kerbal's major bodies

The following table contains the altitudes for circular, synchronous orbits around all of Kerbal's celestial bodies, even when the altitude resides outside the SOI. The altitudes are relative to the body's surface, while the semi-major axes are measured from the body's center.

• † indicates that the altitude resides outside the SOI
• ‡ indicates that the altitude is the same as the orbit of another object

See also

• KEO, the stationary orbit around Kerbin
• Geosynchronous Orbit (Math), some math on how to calculate a synchronous orbit
• Synchronous orbit on Wikipedia
• Stationary orbit on Wikipedia
• Geostationary orbit on Wikipedia
• Geosynchronous orbit on Wikipedia