43

A very good question! The reason is essentially to do with tides. And a slightly over-simplified summary is: If the moon orbits more slowly than the rotation of the parent body (as our Moon does, 12 degrees per day while the Earth rotates about 360 degrees per day) then the moon will gradually orbit further and further away. If the moon orbits faster than ...


25

Orbits at the altitude of GEO are stable for very long times (millions of years). There is no significant decay of the orbital height due to some kind of drag, so the risk of these satellites interfering with working ones is close to zero. On the other hand, there are good reasons to store them above the belt and not below: The region below is used for ...


21

Orbital decay due to aerobraking/drag typically acts to first circularise an orbit, then slowly spiral the satellite into reentry. This is for one main reason:you get more drag at perigee. Where a force acts on an orbit collinear with the velocity vector the satellites current altitude remains unchanged, all other parts of the orbit are changed. So the ...


20

Actually, it makes a lot of sense to raise the orbit of end-of-life geostationary satellites: Coming from Earth you have to cross through a lower orbit to transfer from low earth orbit to a geostationary orbit but you don't have to go farther out than that (some transfer orbits do, but it's not a requirement). That means that a higher orbit has less risk of ...


18

Yes. It is considered (sorry I don't know exactly how much, but probably a LOT - they track the torque on individual bolts during construction for example). They had an accident a while ago where a thruster started firing and oscillating wildly, causing the whole station to wobble and shake, a bit like they do in KSP. Although the forces applied exceeded ...


17

Mercury's orbit isn't "decaying" so much as it is changing shape. The primary cause is perturbations from the Sun's gravity field. That MESSENGER is subject to a good deal of solar radiation pressure and that Mercury's gravity field is a bit lumpy add to this. The primary mechanism is the Kozai effect. This effect applies to an object in a highly eccentric ...


16

Progress-M 27M was (on date of writing this answer, see updates below) expected to decay around Thursday, May 7, 2015 at 23:01:00 ±8 hours UTC. This predicted reentry time will get more precise as it experiences atmospheric decay, additional radar tracking measurements are taken and object's TLE are updated. Eventually, it should be possible to predict ...


14

In practice, the g force applied for orbital corrections is very small. The satellite operator has plenty of time to make the correction, and if you are capable of accelerating the satellite at more than a small fraction of a g, it suggests that you brought too much mass along in the form of a rarely used, over-powered engine. For the ISS in particular, ...


12

This paper describes tests done in 2010, where thrusters on the ISS were fired specifically to analyze the resulting vibrations and compare them with the theoretical models. Modal analyses, model validations and correlations are performed for the different configurations of the International Space Station (ISS). Three Dedicated Thruster Firings (DTF) ...


12

Density of the proton wind coming from the Sun is few ten particles per cubic centimeter (atmospheric density is orders of magnitude higher in LEO), thus not influencing a satellite by drag. Actually, the proton flow could also add tiny amounts of speed to the satellite when hitting it from the back. The solar storm is accompanied by a tiny magnetic field (...


12

The orbital mechanics of satellites are independent from the mass of the satellite. As long as the sats mass is tiny compared to the mass of Earth. The total mass of the ISS is much larger than the mass of the dragon capsule itself, the same is true for the volume and surface of both. So the atmospheric drag of both changes only very little after docking.


10

Orbits beneath synchronous orbits have a higher angular velocity than their planets rotation, orbits above have a slower angular velocity. Drag (atmospheric or tidal) would try to match the angular velocity to the planets rotation. So below a synchronous orbit objects get slower, above it they would speed up (and slow down the rotation of the body they are ...


8

Probably not. To control the point of reentry, you need to be able to adjust from a perigee high enough to not promptly reenter (i.e. above 200km) to one low enough to promptly reenter (i.e. below 80km) in significantly less than the time it takes to complete a single orbit -- otherwise, the unpredictable effects of drag in the variable density upper ...


8

It is an indirect effect; increased solar activity affects the Earth's atmosphere in such a way to cause a net density increase at a given orbital height. This increases the drag on objects in LEO. See the entry section of this article.


8

According to Spaceflight Insider: ... After that, Volkov was the first to leave the module. He had with him a flash drive containing messages from last year’s 70th anniversary of Russia’s Victory Day. It was thrown retrograde from the space station so as to ensure that it will not collide with the station in future orbits before its orbit ...


8

It will burn up in the atmosphere. Without and source of internal thrust it will decay into a lower orbit due to micro-drag forces, and it is too small to survive re-entry. There is no way it could land back on earth without a form of heat shield at the very least. Also from the video it appears that the Cosmonaut threw it against the direction of travel ...


7

In this supplementary answer I've crudely processed all TLEs for Vanguard 1 and plotted the trends. I used the mean motion (revs/day) to get a period, divided by 0.9975 (estimating from this answer) to undo the effects of $J_2$, then used $a^3=GM (T/2 \pi)^2$ to estimate a semimajor axis, periapsis and apoapsis based on the TLE's eccentricity value. This is ...


6

This really depends on your altitude first, and second your aerodynamic properties, the point in the solar cycle, and the mass of the object. The peak of a solar cycle increases drag on satellites as the upper atmosphere grows during that period of time. LEO varies dramatically, the altitude of the ISS is only stable for at most maybe a year without any kind ...


5

It depends entirely on the orbit and the "aerodynamic" properties of the satellite. For example, the ISS is often quoted as descending between 70 to 100 metres per day and needs frequent boosts. (As geoffc has pointed out, it is an exceptional case due to the large area it covers.) Another interesting case was GOCE. This earth observation satellite was ...


5

Yes, MGS's orbit is in the process of decaying, but very slowly. I can only provide a lower limit on the lifetime, since for planetary protection MGS was required to assure at least a 50-year orbital lifetime from when it entered orbit. So no less than 32 years from now. I'm sure that the actual lifetime is much more than that.


5

Why are deorbited satellites allowed a slow orbit decay instead of burning them up rapidly? The ultimate reason is simple: There are no international rules or regulations placing limitations on orbital debris. The only thing that comes close is the Space Liability Convention. That convention is akin to having no speed limits on roadways, including school ...


4

Probably some tens of millions of years. Billions of years appears unlikely. Even in the next million years or so, there is a significant chance of colliding with Earth or Venus: Research quoted by BBC news reports: That's the conclusion of an analysis by Czech and Canadian researchers. They calculated that the roadster has a 6% chance of colliding with ...


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 ...


2

Using a very strong engine for high accelaration in a very short time requires additional mass for the rocket engine and the structure. Therefore additional fuel is necessary to accelerate the heavy engine itself. This method is inefficient. To raise a circular orbit, the most efficient method is a Hohmann transfer using two short burns. But the Hohmann ...


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 ...


1

This additional answer is just to show the effect of the solar activity on the decay rate. The graphs were obtained from 15279 TLEs downloaded from https://celestrak.com/NORAD/archives/request.php processed with the CSpOC's SGP4 library freely downloadable from www.space-track.org. The following graph shows the mean radius vector and the mean air density (...


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