# How many existing satellites have a continuously varying orbit because of human design decisions?

In the space community, there is an assumption that changes to an Earth satellite's orbit are infrequent (other than those due to atmospheric drag). Are there any known satellites designed to continuously change their orbits using solar sails or another inexpensive propulsion method?

By continuous, I do not refer to "station-keeping" maneuvers which are in fact designed to maintain a specific orbit. I refer instead to satellites which purposely change their orbits continuously, by design.

EDIT: By satellite, I mean an earth-orbiting object designed to avoid atmospheric re-entry for multiple years, doing something besides just adjusting its orbit during that time (i.e., its purpose is more than just to demonstrate the feasibility of a continuously changing orbit).

EDIT: Title changed to "How many existing satellites have a continuously varying orbit because of human design decisions?"

• I mean, there's the X-37b. Commented Jun 17, 2021 at 16:01
• secure.planetary.org/site/SPageNavigator/mission_control.html Commented Jun 17, 2021 at 17:35
• GOCE continuously thrusted to maintain, not change its orbit. space.stackexchange.com/a/33202/6944 Commented Jun 17, 2021 at 19:38
• Title says "powered" but question includes "using solar sails" which don't require power per se at least not from the spacecraft, though a little power is needed to maintain attitude. cf. 1, 2, 3. Should "powered" be removed from the title?
– uhoh
Commented Jun 18, 2021 at 2:52
• @user25876 once the X-37b reaches space and settles down from launch, does it undergo powered movement for any reason other than to maintain a semblance of stable orbit and/or re-enter Earth's atmosphere? Commented Jun 18, 2021 at 17:04

If:

• "powered" means "thrust via reaction mass of some kind being pushed via use of power by the spacecraft continuously, and
• "a continuously varying orbit" means continuously actively altering it's otherwise natural orbit (which already continuously varies due to several effects)

and it seems that they do, then the answer to "How many existing satellites..." is probably zero.

@OrganicMarble points out that:

GOCE continuously thrusted to maintain, not change its orbit.

I mentioned that:

...there are groups of satellites that have adjusted their attitude to make their solar panels interact with weak atmospheric drag to either drift slowly forward or backward over time compared to their nominal attitude in order to spread out within their orbital plane after being launched and deployed together. I'm still looking forward to a clarification from you on exactly will or won't count as "powered".

and now remember, I was thinking of the CYGNSS constellation. From How can the CYGNSS spacecrafts (actually) measure ocean roughness?:

above: screen shots from CYGNSS_FactSheet_October2014

They did use a little electrical power to perform attitude adjustments to increase or decrease atmospheric drag mostly from the solar panels, but it was not powered flight.

Here is an example of a simulation of the use of alternating between high and low drag configurations to achieve equal spaced phasing and then to maintain it using small station-keeping drag adjustments:

Figure 8: Time-discretized high-drag commands are assigned to achieve desired slots (b) with commands

Figure 9: Attitude modes of Dove satellite enable large drag area ratios (a) Orthographic projections with cross-sectional areas. (b) High-drag and low-drag attitudes

### But I could be wrong!

If someone thinks of something that meets your criteria, or something new is put in space that does, then new answers can be posted.

Not a complete answer nor can it be considered "powered", but this page from The University of Twente lists about 235 satellites that are in sun-synchronous orbits.

A sun-synchronous orbit is a nearly-polar orbit carefully calculated to use Earth's equatorial bulge to adjust the satellite's orbit by about one degree per day. Since the angle to the sun also changes by about one degree per day, this keeps the sun at the same angle to the satellite's orbit all year long.

This is useful for scientific satellites that perform long-term observations of the sun, or satellites that don't have any on-board batteries and therefore can't tolerate having Earth shadow their solar panels.

It represents a continuous change in the Keplerian orbital elements due to a non-central force (Earth's oblateness), albeit completely natural in origin.

• I consider changes to orbital parameters from things like gravitational field irregularities, atmospheric drag, etc. all things that designers could have predicted and accounted for. I am continually impressed by the ingenuity of the human race though, and the technological prowess referred to in this answer is one more convincing piece of evidence for me. Commented Jun 22, 2021 at 20:27
• Good point! I added a few finer points, please feel free to edit further.
– uhoh
Commented Jun 23, 2021 at 2:21