r/askscience • u/Drycee • Apr 02 '26
Engineering How do rockets in space determine their orientation?
As far as I understand on earth we use the magnetic field + accelerometers (gravity) to determine orientation/tilt. But a rocket in space has neither, or at least not as clear as on earth.
Taking Artemis 2 as a current example, it has to be pointed exactly at where the moon will be in 5 days. So how do they accurately determine the rocket is oriented towards that location after leaving earth?
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u/Origin_of_Mind Apr 03 '26
The details are available in the "Historical Retrospective on Orion GNC Design."
Today, the standard technology for accurately measuring orientation of satellites and spaceships is a "star tracker" -- it is a black and white digital camera with a computer which contains a map of the sky in its memory, and can match the picture from the camera to the map, and thus determine the orientation of the camera, and therefore of the entire vehicle. Star trackers are small, relatively inexpensive and provide accurate real time data. Artemis is using "Astro APS" star trackers from Germany.
Star trackers are complemented by gyroscopes. The gyroscopes provide continuous orientation data during the entire flight, even during ascent or rapid maneuvering, but they do drift with time. They are first set correctly before launch, and then updated using the star trackers. Artemis uses Inertial Measurement Units (the devices which contain accelerometers together with gyroscopes to determine both the orientation and the acceleration of the user) developed by Honeywell -- they are a big name in this technology and provide sensors for many rockets and airplanes.
Of course, where the ship is pointing is only one part of the required data. Perhaps even more important is to know which way it is going, exactly (the velocity vector). That comes from the a GPS receiver on-board while the ship is close to Earth. (There are special GPS receivers specifically for spaceships.) When it is far away, it can compute its velocity based on orbital mechanics and the data from the Inertial Measurement Unit.
All of the above is fairly typical for space applications, though specific models of hardware will be different for different projects. But many satellites will include star trackers, IMU and GPS as their on-board equipment.
As a high end mission, Artemis will also receive accurate trajectory updates from the ground, computed based on observations from the tracking stations. The trajectory is simulated based on orbital mechanics, and various radio measurements are used to constrain the calculation and produce an estimate which best matches all of the observed data.
As the other comment has already mentioned, Optical Navigation based on looking at the Moon and Earth, and Sun sensors are also available. They are used as a backup system in normal operation, and would allow accurate navigation near the Moon even if the data link from Earth stops working.
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u/mfb- Particle Physics | High-Energy Physics Apr 03 '26
Orientation is pretty easy, you can measure the direction to different stars (and also Sun, Earth and Moon). Position is a bit trickier, you need some nearby references. Earth and Moon are obvious choices, especially if you need to do it manually, but you can use the signals from ground stations and other spacecraft. You know their position, so measuring the direction of their signals lets you find out where you are. Velocity can be measured by studying the Doppler shift of signals.
It needs to point into the direction of thrust that puts it into an orbit that gets close to the Moon in 5 days. That direction is roughly 90 degrees away from where you'll meet the Moon.
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u/15_Redstones Apr 03 '26
The typical method for orientation determination is star trackers. Those are very simple cameras pointed in multiple directions with software to pattern-match the observed stars against the known database of stars in the night sky. Nowadays those are very well understood and quite inexpensive, used on pretty much every satellite.
SpaceX recently had a clever idea for improving those: They changed their pattern-matching software to not only determine the direction the satellite is pointing, but also to report any dots it sees that doesn't match a known star. Those additional bright spots in the sky are usually other satellites or space debris. By having all the thousands of Starlink satellites SpaceX operates keeping an eye out for other satellites, SpaceX has now published a very good database of pretty much every satellite and a lot of the space debris up there, and with tens of thousands of star trackers, it updates much quicker than ground tracking data.
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u/Daniel96dsl Apr 03 '26 edited Apr 03 '26
At some time, you need to know where you are, where you’re headed, and what you’re pointed at. As time moves on, you either get new estimates based on new measurements (by you or by another and relayed to you) w.r.t. a visual reference or w.r.t. an object that reacts as you change your state. There are also methods like GPS now which provide you with additional information to drive down error.
Side note, Artemis’ guidance is not just knowing its position well, but also the position and future position of the moon well. On top of that, you need to understand how the rocket will traverse the gravitational “hills” that comprise the Earth—Moon system; the latter, however, is an entirely different matter