Artificial satellites in low inclination orbits are rarely placed in retrograde orbit.[1][2] This is partly due to the extra velocity (and propellant[3]) required to launch into orbit against the direction of the Earth's rotation.

‘Numerous small, distant satellites in both prograde and retrograde orbits have been discovered recently.’ ‘These are all in circular prograde orbits near Neptune's equatorial plane, and they probably formed in place.’ ‘Two of the new moons have prograde orbits - that is, they orbit in the same direction as the planet turns.’.

Most commercial Earth-observing satellites use retrograde sun-synchronous orbits to ensure that observations are performed at the same local time each pass of any given location,[4] while almost all communication satellites use prograde orbits.[5]

1. If a captured moon has prograde, circular and equatorial orbit, can we know it is captured? Is the Sun prograde or retrograde with respect to the rotation of the Milky Way? Are there any natural satellites in retrograde Sun orbit? How did “oddball” Valetudo, Jupiter's new prograde moon, end up in a wider orbit.
2. Orbital motion of an object in terms of retrograde and prograde is described with respect to the movement of another object. In this case, the object which it’s orbiting. A retrograde orbit is one where a satellite revolves opposite to its parent.
3. Problem 3 (40%) A satellite is in a prograde circular orbit about the Earth at an altitude of 500 km, and needs to be placed into a prograde circular orbit with an altitude of 16 000 km. Solve for: (a) the Av required to place the spacecraft on an elliptical transfer orbit that will allow it to reach the higher altitude orbit, (b) the Av required for the transfer from the elliptical transfer.
4. A prograde orbit is its opposite—a satellite revolves in the direction of rotation of the parent object. The Moon orbiting the Earth is an example of a prograde orbit. Israeli satellites orbiting Earth is an example of retrograde orbit.

## Examples

Israel has successfully launched seven Ofeq satellites in retrograde orbit aboard a Shavit launcher. These reconnaissance satellites complete one Earth orbit every 90 minutes and initially make about six daylight passes per day over Israel and the surrounding countries, though this optimal Sun-synchronized orbit degrades after several months. They were launched in retrograde orbit so that launch debris would land in the Mediterranean Sea, and not on populated neighboring countries on an eastward flight path.[6][7]

The United States launched two Future Imagery Architecture (FIA) radar satellites into 122° inclined retrograde orbits in 2010 and 2012. The use of a retrograde orbit suggest that these satellites use synthetic aperture radar.[3]

Earth-observing satellites may also be launched into a sun-synchronous orbit, which is slightly retrograde.[8] This is typically done in order to keep a constant surface illumination angle, which is useful for observations in the visible or infrared spectrum's. SEASAT and ERS-1 are examples of satellites launched into sun-synchronous orbits for this reason.

## Space warfare and accidents

Arthur C. Clarke wrote an article called 'War and Peace in the Space Age', in which he suggested that an artificial satellite in retrograde orbit could use 'a bucket of nails' to destroy an SDI (anti-warhead) satellite. This premise was questioned[citation needed] on account of the vastness of space and the low probability of an encounter.

Nevertheless, a satellite in retrograde orbit could pose a major hazard to other satellites, especially if it were placed in the Clarke belt, where geostationary satellites orbit. This risk highlights the fragility of communication satellites and the importance of international cooperation in preventing space collisions due to negligence or malice.

• USA 205 – an example of a retrograde satellite

## References

1. ^http://www.wseas.us/e-library/conferences/2009/istanbul/TELE-INFO/TELE-INFO-08.pdf 'Most satellites are launched in a prograde orbit because the Earth's rotational velocity provides part of the orbital velocity with a consequent saving '
2. ^Ippolito, L.J. (2008). Satellite Communications Systems Engineering: Atmospheric Effects, Satellite Link Design and System Performance. Wiley. p. 23. ISBN9780470754450. Retrieved 2014-11-30.CS1 maint: discouraged parameter (link)
3. ^ abAllen Thomson. 'SeeSat-L Oct-10 : Reason for FIA Radar 1/USA 215 retrograde orb'. satobs.org. SeeSat-L. Retrieved 2014-11-30.CS1 maint: discouraged parameter (link)
4. ^http://www.ioccg.org/training/turkey/DrLynch_lectures2.pdf 'Most Earth observing satellites are launched so as to have retrograde orbits.'
5. ^http://www.sac.gov.in/Satcom_Overview.doc[permanent dead link] 'Orbits of almost all communication satellites are prograde orbits, as it takes less propellant to achieve the final velocity of the satellite in prograde orbit by taking advantage of the earth's rotational'
6. ^'Shavit (Israeli launch vehicle) -- Encyclopædia Britannica'. britannica.com. Retrieved 2014-11-30.CS1 maint: discouraged parameter (link)
7. ^'Shavit'. deagel.com. Retrieved 2014-11-30.CS1 maint: discouraged parameter (link)
8. ^Timothy A Warner; Giles M Foody; M Duane Nellis (2009). The SAGE Handbook of Remote Sensing. SAGE Publications. p. 109. ISBN9781412936163. Retrieved 2014-11-30.CS1 maint: discouraged parameter (link)

## The Prograde Orbit of Exoplanet TrES-2b

#### Abstract

We monitored the Doppler shift of the G0 V star TrES-2 throughout a transit of its giant planet. The anomalous Doppler shift due to stellar rotation (the Rossiter-McLaughlin effect) is discernible in the data, with a signal-to-noise ratio of 2.9, even though the star is a slow rotator. By modeling this effect we find that the planet's trajectory across the face of the star is tilted by -9° ± 12° relative to the projected stellar equator. With 98% confidence, the orbit is prograde.

Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration, and was made possible by the generous financial support of the W. M. Keck Foundation.

Publication:
Pub Date:

August 2008

DOI:
10.1086/589235
arXiv:
arXiv:0804.2259
Bibcode:
2008ApJ...682.1283W
Keywords:

• planetary systems;
• planetary systems: formation;
• stars: individual: GSC 03549–02811 TrES-2;
• stars: rotation;
• Astrophysics