12
$\begingroup$

I think we could put satellite to orbit earth in such a way that it always see the sun. Which is orbiting along the path of earth orbiting the sun, like a wheel perpendicular to the sun

I don't know the specific name of this type of orbit so I can't find detail about it, are there any satellite orbit earth this way?

$\endgroup$
23
$\begingroup$

There are satellites that constantly observe the sun. Among them are the STEREO pair of satellites. These satellites don't orbit Earth, but orbit the Sun–one a little faster than Earth, one a little slower.

At first, I thought the Earth-centered orbit you suggest wouldn't work. I was thinking that if a satellite starts out in a polar orbit, the plane of the orbit would not rotate as the Earth orbited the Sun, leaving the orbit parallel to the sun a quarter year later and behind the Earth at times. However, it is possible. From Wikipedia on polar orbits:

To retain the Sun-synchronous orbit as Earth revolves around the Sun during the year, the orbit of the satellite must precess at the same rate, which is not possible if the satellite were to pass directly over the pole. Because of Earth's equatorial bulge, an orbit inclined at a slight angle is subject to a torque, which causes precession. An angle of about 8° from the pole produces the desired precession in a 100-minute orbit.

Another source cited by the wiki article.

These orbits are generally used for Earth-observing satellites that track Earth conditions (like atmospheric temperatures) at the same times of day.

Also, from a comment by cmaster - reinstate monica:

Those sun-synchronous orbits are also used for satellites that don't care about day and night on earth, but need an unceasing supply of energy. Examples are satellites that use radar to image the earth's surface. Radar is an active technology that needs lots of power to work over 800-km distances, and that power is supplied by the sun. The sun-synchronous orbits puts the satellite into eternal sunlight, allowing it to work 24/7 with a minimal battery and solar cell array size.

$\endgroup$
2
  • 7
    $\begingroup$ Those sun-synchronous orbits are also used for satellites that don't care about day/night on earth, but need an unceasing supply of energy. Examples are satellites that use radar to image the earth's surface. Radar is an active technology that needs lots of power to work over 800km distance, and that power is supplied by the sun. The sun-synchronous orbits puts the satellite into eternal sun light, allowing it to work 24/7 with a minimal battery and solar cell array size. $\endgroup$ Dec 25 '19 at 13:48
  • 1
    $\begingroup$ In the spirit of "always sees the sun" in the question, it could be mentioned that this sun synchronous orbit isn't the only option, when the mission requires it: The Solar Dynamics Observatory is a very data-rich mission that must stay in line-of-sight to its ground station. It has a highly inclined geosynchronous orbit, and is in full sunlight for all but about 3 hours a year (occluded by Earth for up to 15 minutes each time, during 2 periods that last 2 weeks each, plus occasional lunar occultations.) $\endgroup$
    – Ghedipunk
    Dec 26 '19 at 18:13
16
$\begingroup$

This is called a “sun synchronous” orbit.

If the Earth were a perfect sphere, orbits around it would have a fixed orientation in space. As the Earth goes around the Sun, an orbit would have the right orientation only twice a year.

But the Earth isn’t a perfect sphere: it bulges in the middle. The gravity of that bulge pulls inclined orbits toward the equator. That puts a torque on the angular momentum of the orbit. Much like an inclined top, this torque caused the orbit to precess, rotating in space.

With just the right inclination, this precession will happen once per year so that the satellite’s orbit stays properly aligned with the Sun.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.