China looked at putting a monitoring satellite in retrograde geostationary orbit via the moon

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HELSINKI — China appears to have considered boosting its space situational awareness capabilities by placing a satellite in a retrograde orbit out at the geostationary belt.

A paper published in Nature Scientific Reports by authors from the Xi’an Satellite Control Center looks at using a lunar swingby to insert a satellite into a retrograde orbit out at the geostationary belt (GEO) for monitoring activities and debris warning.

Satellites in GEO orbit at around 35,786 kilometers above the Earth,  at zero degrees inclination and an altitude at which the orbit of satellites matches the Earth’s rotational period. This belt is thus very useful for applications such as telecommunications and weather, with the satellites appearing to be in a fixed spot in the sky for transmissions and making observations. 

Monitoring satellites out at GEO such as the U.S. Geosynchronous Space Situational Awareness Program (GSSAP) are typically in prograde orbits tens of kilometers above or below the belt. This allows them to drift either west or east respectively and sweep past satellites and objects in the belt over time at a low relative velocity, moving a degree or so a day, and allowing close approaches for inspection.

An orbital informant satellite in retro-GEO however would be able to pass by all assets in GEO every 12 hours. 

This would allow a rapid cataloging of debris as well as movements of other satellites, providing important information on a highly-valued orbit.

Satellites in geosynchronous orbit (GEO) are of great value for government and commercial purposes and monitoring of this critical orbit is vital. China has greatly increased in its launch rate and presence in GEO over the last decade, and thus has an increasing need of situational awareness capabilities to understand activities and changes.

Getting a satellite into retrograde geostationary orbit (retro-GEO) would be a challenge, however, as the spacecraft needs to be inclined by 180 degrees, traveling in the opposite direction to everything else in GEO. Spacecraft heading to GEO are launched to the east, using the rotational speed of the Earth to help them achieve orbit.

Launching to the west incurs high energy penalties for launch, leading the authors, citing earlier Western papers on the topic, to consider a path to retro-GEO via the moon. So far, the highest inclination of a satellite in geosynchronous orbit is 60.3 degrees, an orbit used by Japan’s Quasi Zenith Satellite System for regional GPS enhancement. China similarly has Beidou-IGSO satellites inclined by 55.2 degrees.

Heading to the moon and using it for a gravity assist could put a spacecraft into a transfer orbit for entering retro-GEO incurs a lesser fuel cost.

The concept has been around since the 1980s at least, but has not been utilized. One reason is that a spacecraft in such an orbit could add to the debris dangers it would aim to assess.

“I think it could be dangerous because you’re moving through the GEO region at a much higher relative velocity to everything else,” Brian Weeden, Director of Program Planning for the Secure World Foundation, told SpaceNews.

“If you’re doing this at exactly the same altitude as GEO, it would be like a car driving the wrong way down the freeway. Hopefully, you’d still have it above or below GEO to avoid collisions with the geostationary belt, but you’d still have to do some collision avoidance and you’d have less time to make decisions,” Weeden says. This is due to the higher relative velocities of the objects involved.

Another downside would be that monitoring satellites in retro-GEO would also not be able to conduct Rendezvous and Proximity Operations (RPOs) to inspect a country’s own assets or spy on those of others because of the high relative velocities. 

“I don’t really see much value in doing it,” Weeden says. “If the goal is monitoring objects in the GEO region, I think you’re better off with a constellation of satellites in normal drift orbits. And I think the international reaction would be one of significant concern.” 

“It might be slightly better for doing up-close characterization because you can cover the belt faster, but at the expense of shorter times observing any one particular object or region.”

China appears to have chosen a path similar to GSSAP, sending pairs of satellites to sweep above and below the GEO belt. It launched the Shiyan-12-01 and Shiyan-12-02 satellites in December 2021 which could be the first of a series of GEO inspector satellites.

A game of orbital cat and mouse between Shiyan-12-01 and 02 and U.S. surveillance USA 270 earlier this year highlighted that China had exquisite SSA, tactics and counterspace capabilities.