# Tag Info

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An orbit has an angular momentum that is conserved. That angular momentum vector keeps the orbit at a fixed orientation in inertial space. As a planet orbits the Sun, the local time of the ascending node moves around the clock over that planet's year. The only way for an orbit to be Sun-synchronous is for there to be a torque applied to the orbit to ...

12

Sun-synchronous orbits take advantage of the precession of the ascending node due to orbit perturbations (primarily the $J_2$ spherical harmonic due to the slight bulge in Earth's shape at the equator). By choosing the right combination of orbit altitude and inclination, that precession can be set to take 1 year so that the local time remains the same at ...

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Sun-synchronous orbits are those whose orbital plane makes a constant angle with the radial from the Sun. For that to occur, the orbital plane of geocentric satellites must rotate in inertial space with the angular velocity of the Earth in its orbit around the Sun, which is $360^\circ$ per $365.26$ days, or $0.9856$ degrees per day. With the orbital plane ...

8

A dogleg maneuver is done to change the inclination of a certain payload, and the reason it limits the payload capacity is most likely due to the cosine losses. In order to perform such a maneuver a rocket has to yaw in a certain direction in order to change the direction of flight. However, this yaw comes at a cost, as the more you stray the direction your ...

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No. First, the matter of oblateness which introduces the necessary precession. In some cases it will force the sun-synchronous orbit altitude below the body's surface (obviously impossible). In other cases other bodies will disturb the orbital motion too strongly, destabilizing the orbit.

7

From the way the question is phrased, it would be referring to the eclipse of the spacecraft by the Earth (i.e. the sun is blocked by the Earth), not of the spacecraft by the Moon. Eclipse can refer to any body eclipsing any other body, not just the Moon. LTAN is Local Time of the Ascending Node. The spacecraft crosses the equatorial plane of the Earth and, ...

7

You are correct in your understanding, Once it reaches the vicinity of the South Pole, specifically the most southern point in its orbit it will start going north. The reason they said north to south is to differentiate it from the orbits going west to east (which by the way stay west to east). This is also very neatly demonstrated by the satellite's ...

6

Sun-synchronous orbits are possible because the Earth is not a perfect sphere. The Earth is best described as an oblate spheroid. This non-spherical shape and the resulting gravity field can be used to alter the orbit in a predictable way ― specifically to precess the orbit with a period of one year.

6

Could someone explain it like I was 5? TL;DR: A true "terminator-riding" satellite doesn't exist. The fuel maintenance costs for such an orbit would be ridiculously high. Sun synchronous satellite orbits are not quite Keplerian. The right ascension of ascending node is constant in a Keplerian orbit. The right ascension of ascending node instead precesses ...

6

The satellite does not run parallel to the terminator, it crosses the terminator twice on each orbit. So on each orbit, half the path underneath the orbit is in shadow, the other half is in daylight. The satellite's inclination is measured relative to the equator (98.2°) and poles (8.2°), and does not change with the seasons. The northernmost point of the ...

6

Physically, you can think of a sun-synchronous orbit as having its orbital plane precessing once a year. Like a toy gyroscope’s plane of rotation precesses under the torque due to gravity, so does the satellite’s plane due to the torque of being attracted to the tidal bulge. Now think of a prolate Earth as having a “negative tidal bulge” (to first order, ...

5

These are called "Sun-synchronous orbits" and extensively used for remote sensing, where satellites are looking down at the Earth's surface. One of the main reasons for these orbits is that the shadows on the ground are always the same (i.e. the illumination angle is constant between each flyby). This is especially useful for imaging since the changes in ...

5

It's not needed, so no "global treaty organizations" would be interested in spending even a minute thinking about it. In the meantime (so to speak), each landed mission on Mars plans its operations to the Local Mean Solar Time, which is defined by a fixed longitude related to the targeted or actual landing site.

5

The Planet Labs Flock 3p constellation is in LEO at 500 km and doing an effective job of imaging all the land area of the Earth daily, albeit at around 5 meter resolution. The best angular resolution achievable for a given telescope diameter is determined by the diffraction limit. For visible light at 500 km distance this works out to a minimum aperture ...

4

The planet calculator lists a J2 of $0.001960454$ for Mars, higher than the Earth's $0.001082627$. That makes sense, as Mars has a smaller mass, but still approximately the same rotation rate. That means achieving a sun-synchronous around Mars is slightly easier than around Earth. However, MOM is not in such an orbit.

4

Wikipedia is correct. To have a sun synchronous orbit the satellite's orbital plane needs to precess by at a rate equal to the planet's mean orbital rate about the sun. The Keplerian orbital plane doesn't change. Some perturbing force is needed to make the plane precess. IN the case of sun synchronous satellites, the source of the perturbation is the planet'...

4

Let me try. I'm not sure if I'm right so take everything with a grain of salt, but that's how I'd approach it. Instead of trying to find the precise eclipses that will occur (based on LTAN) and trying to adapt to their duration, I'd choose a worst scenario eclipse: the satellite moving in the same direction as the Moon, as result staying in the shadow the ...

4

There is no official universal time system for Mars. However, NASA has proposed an analog for Universal Time on Earth, that they refer to as Coordinated Mars Time, or MTC: The prime meridian of Mars is defined by the location of the crater Airy-0 (De Vaucoulers et al., 1973), named in honor of the British astronomer George Biddel Airy, who built the ...

4

The special property of sun-synchronous satellites is the precession, at pace equal to Earth's orbital period around the Sun (1 year). That means, the satellite's RAAN drifts by a full 360 degrees over the year, and as result, local solar time at the moment of crossing the equator remains constant. if the satellite is injected on any other day from the ...

4

There shouldn't be a difference in whether the mission is technically feasible. The flight plan would be altered as deflection would be mirrored, but I can't see any reason engineering aspects would differ. That said, I can't imagine that it would ever launch north.The reason the launch site was chosen is because there's nothing south of it until Antarctica....

4

The north-south motion (or south-north) itself doesn't produce a sun-synchronous orbit. It's actually the deviation from straight north-south, coupled with the primary's (the planet or other body the satellite is orbiting) oblateness, that allows sun-synchronous orbits. The wikipedia article on nodal precession is a good general source on this topic. If a ...

3

Yes, it would be possible to have a figure-8 shaped orbit around the Earth/Moon system. In a two body system like the Earth/Moon or Sun/Earth system, there are five known points where the gravity of the two objects balances out. These are known as Lagrange points. It is possible for a spacecraft to orbit these points. Orbits around Lagrange points are ...

3

As @OrganicMarble suggests a nice Sun-synchronous orbit is probably the answer to both "least" and "most". Sun-synchronous orbits around the Earth are often used to keep satellites over areas of the Earth that happen to be in constant sunlight for photographic reasons, and a side benefit can be that there is constant sunlight available for the satellite's ...

3

For: Do sun-synchronous orbit pass any spot around Earth... (emphasis added) Sun-synchronous orbits use the strong $J_2$ component of the Earth's gravity to precess their ascending node all the way around the Earth's axis once per year. Since the Earth's axis is tipped by ~23 degrees with respect to the ecliptic, it is important to remember that this ...

3

It isn't that risky, if the proper precautions are taken. The key item, as it is in most cases with spaceflight, is redundancy. There is a larger chance of single event upsets during a geomagnetic storm. This can be mitigated by hardware redundancies. In addition, geomagnetic storms only really become an issue for most spacecraft when they are beyond LEO. ...

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Two classical examples of spacecraft which "take advantage" of being (almost) constantly insolated: 1- Sun Observation Missions: Spacecraft in Lagrange's points such as SOHO or in dawn-dusk orbit such as PROBA-2 want to look at the sun, because that is what their payloads were made for. No eclipse time means no interruption of the data flow. 2- Synthetic ...

3

In addition to Hans' answer, these are the terms used in the spaceflight community to describe the orbits you're referring to. Satellites that orbit "north to south" are called polar orbits. These are all orbits that place the satellite over the poles. These orbits have an inclination (angle between the orbit's plane and the equator) of about 90º. A sun-...

2

@Hohmannfan's answer states MOM is not in a sun-synchronous orbit. I just though I'd add some math to that. Using the equation for rotation of ascending node posted by @MarkAdler, I calculated the delta_OMEGA - how much the ascending node rotates in one year, for two cases: earth LEO and mars MOM. For earth LEO (400km) I get a little more than 97 degrees, ...

2

The 800 km orbit is much more important than the LTAN. At 800 km orbit, the satellite is going to have a orbital period of roughly 100 minutes. For a few of those successive 100 minute orbits, it's going to spend close to 1/2 of the orbit in eclipse. Without designing for any safety margin, the power system needs to be able to fully charge the batteries ...

2

If you are asking if a free-return trajectory is a stable orbit, the answer is no. A circumlunar free-return trajectory is one such that you will return in a "figure eight with one lobe around the moon and one lobe around the earth", but it is a trajectory and not an orbit because the same path cannot be taken again without dropping back into a ...

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