The orbit that keeps a satellite in step with the Sun.

What a sun-synchronous orbit is, why satellites like Landsat 9 fly slightly backwards over the poles at 705 km, and why consistent lighting matters for comparing images over years.

Look at two satellite photographs of the same field taken a year apart, and you might want to know whether the crops changed, whether a river moved or whether a forest was cleared. That comparison only works if the two images were taken in similar light. If one was captured in the morning and the other in the late afternoon, shadows point in different directions and the colors shift, and it becomes hard to tell real change from a trick of the light.

Many Earth-observation satellites solve this problem with a clever choice of orbit. It is called sun-synchronous, and it is one of the most widely used orbits in the sky. In Orbit Atlas, search for Landsat or a weather satellite and you will see one.

What sun-synchronous means

A satellite in a sun-synchronous orbit crosses the equator at the same local solar time on every pass. If it crosses heading south at about 10 in the morning today, it will cross at about 10 in the morning tomorrow, next month and next year, wherever on Earth that crossing happens.

The United States Geological Survey gives Landsat 9 as an example. It flies in a sun-synchronous, near-polar orbit at an altitude of 705 km, inclined at 98.2 degrees, and crosses the equator at about 10:12 a.m. local time, give or take five minutes. It completes an orbit every 99 minutes and returns to the same path over the ground every 16 days.

The problem: Earth moves around the Sun

Imagine a satellite in an ordinary polar orbit. Its orbital plane stays roughly fixed in space. But Earth travels around the Sun once a year, so over the months, the angle between that fixed plane and the direction of the Sun slowly changes. A satellite that crossed the equator at mid-morning in January would be crossing at a quite different local time by April, and at the opposite time of day by July.

To keep its crossing time constant, the orbital plane itself needs to turn by about one degree per day, completing a full turn in a year, in step with Earth's journey around the Sun.

The solution: use Earth's bulge

Earth is not a perfect sphere. It bulges slightly at the equator because of its rotation. That extra mass gives a gentle sideways tug on any satellite whose orbit is tilted relative to the equator, and over time the tug causes the orbital plane to slowly rotate, an effect called nodal precession.

The rate and direction of that rotation depend on the orbit's height and tilt. For an orbit tilted a little more than 90 degrees, meaning it travels slightly backwards relative to Earth's rotation, the plane turns eastward. Choose the height and the tilt carefully and the plane turns at exactly the rate needed: roughly one degree a day. That is why sun-synchronous satellites have inclinations around 96 to 99 degrees, such as Landsat 9's 98.2 degrees, rather than exactly 90.

No extra fuel is needed for this. The satellite rides Earth's natural gravitational quirks to stay in step with the Sun. Only small corrections are needed over its lifetime.

Why so many satellites use it

Consistent lighting

Every image of a given place is captured at about the same local time, so shadows and lighting angles are similar from pass to pass. Seasonal changes in the Sun's height still occur, but the daily time is fixed. This makes long records like Landsat's, which stretch back to 1972, much more valuable for spotting real change on the ground.

Global coverage

Because the orbit is nearly polar, Earth rotates beneath it, and the satellite eventually passes over almost every part of the planet. USGS notes that the overlap between neighboring paths ranges from 7 percent at the equator to about 85 percent at extreme latitudes, because the paths converge toward the poles.

Choosing the time of day

Different missions pick different crossing times. Mid-morning crossings are common for land imaging. A frequently given reason is that in many regions clouds tend to build up later in the day, while by mid-morning the Sun is already high enough to light the surface well. Other satellites choose a dawn-dusk orbit that rides along the boundary between day and night, keeping their solar panels almost always in sunlight.

Working in pairs

A single satellite on a 16-day repeat cycle sees each place only about twice a month, and clouds can spoil many of those chances. Landsat 9 flies in the same orbit as Landsat 8 but offset by 8 days, according to USGS. Together they revisit each place every 8 days, and USGS says the two satellites add nearly 1,500 new scenes to the archive every day.

Limits of the orbit

Sun-synchronous orbits are generally in low Earth orbit, often between about 600 and 800 km. That is close enough for detailed imaging, but it means each satellite sees only a strip of Earth at a time. It also means the satellite passes a given place only briefly, so continuous watching of a single region needs a different approach, such as the geostationary weather satellites described in our guide to orbit heights.

These popular altitudes are also shared with a great deal of space debris. Our guide to space debris explains why some of the busiest orbits are getting crowded.

See it for yourself

Open Orbit Atlas, search for Landsat 9 and select it. Notice how its orbit runs nearly over the poles, tilted slightly back from vertical, and how the ground track shifts westward with each pass as Earth turns. Then compare it with the space station, whose orbit is tilted at 51.6 degrees. Our inclination guide explains why the station's orbit looks so different.