Synchronous orbit

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

A synchronous orbit is an orbit where the orbital period equals the rotation rate of the orbited body. The eccentricity and inclination are not bound to specific values, although to be synchronous the orbit must not intersect with the atmosphere or surface of the orbited body, causing the orbit to change. Satellites in synchronous orbits have a ground track forming an analemma.

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:

a1f=1f23a1

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

a12=1223a1=143a1

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 120 or 2π3. The longitudes of the ascending nodes can also be spaced out by 120 to make the clover appearance.

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

Tundra orbit

A Tundra orbit is a highly elliptical geosynchronous orbit with a high inclination (approximately 63.4°), an orbital period of one sidereal day, and a typical eccentricity between 0.2 and 0.3. A craft placed in this orbit spends most of its time over a chosen area of planet, a phenomenon known as apogee dwell, which makes them particularly well suited for communications satellites serving high-latitude regions.

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.

Body Synchronous orbit Semi-synchronous orbit Tidally
locked
Altitude Semi-major axis Altitude Semi-major axis
Kerbol 1,508,045.2864911 km 1,769,645.2864911 km 853,206.67364869 km 1,114,806.6736487 km
Moho 18,173.165095464 km † 18,423.165095464 km † 11,355.866754732 km † 11,605.866754732 km † No
Eve 10,328.472087012 km 11,028.472087012 km 6,247.5020653022 km 6,947.5020653022 km No
Gilly 42.13815091369 km 55.13815091369 km 21.734858494219 km 34.734858494219 km No
Kerbin 2,863.3340594888 km 3,463.3340594888 km 1,581.7637421839 km 2,181.7637421839 km No
Mun 2,970.5633375712 km † 3,170.5633375712 km † 1,797.3297445154 km 1,997.3297445154 km Yes
Minmus 357.9408652496 km 417.9408652496 km 203.28624686962 km 263.28624686962 km No
Duna 2,879.9999101376 km ‡ 3,199.9999101376 km 1,695.873623222 km 2,015.873623222 km No
Ike 1,133.8954459646 km † 1,263.8954459646 km † 666.20423861853 km 796.20423861853 km Yes
Dres 732.24443996802 km 870.24443996802 km 410.21964423484 km 548.21964423484 km No
Jool 15,010.461350651 km 21,010.461350651 km 7,235.7612618439 km 13,235.761261844 km No
Laythe 4,686.318661595 km † 5,186.318661595 km † 2,767.1760266031 km 3,267.1760266031 km Yes
Vall 3,593.2015526243 km † 3,893.2015526243 km † 2,152.5632938174 km † 2,452.5632938174 km † Yes
Tylo 14,157.877626718 km † 14,757.877626718 km † 8,696.8803368373 km 9,296.8803368373 km Yes
Bop 2,588.1692033103 km † 2,653.1692033103 km † 1,606.3918640917 km † 1,671.3918640917 km † Yes
Pol 2,415.0791464797 km † 2,459.0791464797 km † 1,505.1227900036 km † 1,549.1227900036 km † Yes
Eeloo 683.69089065809 km 893.69089065809 km 352.98998261971 km 562.98998261971 km No
  • † indicates that the altitude resides outside the SOI
  • ‡ indicates that the altitude is the same as the orbit of another object

Trivia

  • The Tundra orbit, like the Molniya orbit, was developed by Soviet scientists. The Molniya orbit was specifically designed in the 1960s to provide better communication coverage for high-latitude regions, which geostationary satellites struggled to cover effectively. The Tundra orbit, while similar in its high inclination and elliptical shape, was developed later to offer continuous coverage over specific areas by having satellites spend most of their time over a chosen region. Both orbits were innovative solutions to the unique challenges posed by the Soviet Union's geographical location and the need for reliable communication and surveillance capabilities.

See also