Lets start with a little background. When we launch a satellite (such as the Hubble) into orbit around the Earth, not only do we need to know its precise position, we also need to know where it (antennas, sensors, solar arrays, etc.) is pointing.
First lets talk a bit about “where it is”. An orbit is a nothing more than an object falling around another object. Both Kepler and Newton came up with a set of laws that describe this phenomenon.
Kepler’s 3 laws of planetary motion:
1) The orbit of a planet is an ellipse with the sun at one of the foci.
2) The line drawn between a planet and the sun sweep out equal areas in equal times.
3) The square of the periods of the planets is proportional to the cubes of their mean distance from the sun.
So what is that telling us? In a nutshell, all orbits are ellipses, the close to the body you are orbiting the faster you go (e.g. if you have a highly elliptical orbit the satellites velocity will increase as it approaches the object being orbited and decrease as it get further away), and the further away an orbit is, the slower the object moves.
These laws not only apply to planets, but to any orbiting body.
Note: Super geek alert #1: (took that term from Physicist hehehe)
For an orbiting body this is not entirely correct. It turns out that both bodies end up orbiting a common center of mass of the two-body system. However, for satellites, the mass of the Earth is so much greater than the mass of the satellite, the effective center of mass is the center of the Earth.
Newton’s three laws (and law of gravitation)
1) The first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. (Commonly known as inertia)
2) The second law states that force is equal to the change in momentum (MV) per change in time. (For a constant mass, force equals mass times acceleration F=ma)
3) The third law states that for every action there is an equal and opposite reaction. In other words, if an object exerts a force on another object, a resulting equal force is exerted back on the original object.
Newton’s law of gravitation states that any two bodies attract one another with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.
Note: Super geek alert #2:
Actual observed positions did not quite match the predictions under classical Newtonian physics. Albert Einstein later solved this discrepancy with his “General Theory of Relativity”. In November of 1919, using a solar eclipse, experimental verification of his theory was performed by measuring the apparent change in a stars position due to the bending of the light buy the sun’s gravity.
So what is all this trying to tell us? Planets, satellites, etc orbit their parents in predictable trajectories allowing us to “know” where they will be at any given time. A set of coordinates showing the location of these objects over a period of time is called its ephemeris.
From here out let us stick to satellites in orbit about the Earth. Since the Earth is not a perfect sphere (its an Oblate Spheroid), satellites drift from their predicted position due to the Earth’s non-spherical shape. Also at low Earth orbits, the atmosphere creates a drag on the satellite also causing a drift (perturbation) in its orbit. At higher altitudes, such as a geosynchronous orbit, the solar wind and effects from the moon are more pronounced.
This requires us to update the ephemeris periodically.
Now that we have a better understanding of its orbital position, we need to concentrate on its pointing (Attitude Control).
Why do we need to worry about pointing? If the satellite has solar panels (arrays), they need to point towards the sun to provide power. Sensors need to point at their respective targets, such as a star sensor, sun sensor etc. Thermal and possible contamination consideration must be taken into effect when pointing also.
Remember for every action there is an equal and opposite reaction. So if I spew mass (jet of gas out of a thruster nozzle), the satellite will move in the opposite direction. Also if I spin a wheel onboard the satellite, the result will be the satellite spins in the opposite direction.
Satellites (and spacecraft) are incredibly precise machines with exquisite craftsmanship. The life of a satellite is often computed by the onboard fuel requirements. For geostationary satellites, periodic maneuvers (delta-Vs) must be accomplished to keep them on station. This is also required for many lower orbiting satellites as well. For an orbit plane change (move it into a different orbit), mass must be ejected to move the satellite.
Note: Super geek alert #3:
The Hohmann transfer orbit is the most energy efficient (minimum energy solution) way of getting from one circular orbit to a higher or lower circular orbit. This type of transfer orbit is used by the interplanetary spacecraft to travel to the other planets in our solar system.
Since fuel is precious and usually cannot be replenished (called consumables), other methods of pointing were devised that did not require mass ejecta from the satellite. Spinning reaction wheels were one. If you have orthogonal reaction wheels, just by spinning them you can provide precise pointing. Unfortunately, external forced (perturbations) adds unwanted momentum to the wheels. To compensate (unload momentum from the wheels) for this, I have seen both low-level monopropellant jets or torque rods used for this purpose.
Note: Super geek alert #4:
A monopropellant is one that does not require an oxidizer to function. Usually monopropellants are composed of a liquid compound called Hydrazine (N2H4). When this liquid comes in contact with a platinum catalyst, it is decomposed into gaseous ammonia (Nh3), nitrogen and hydrogen. This gas is then ejected (fired thru a nozzle) out a jet to providing motion for the satellite.
An ingenious method of unloading momentum without the use of fuel was devised using simple electromagnets. Remember the Earth is surrounded by a magnetic field (why your compass works). If you attach orthogonal electromagnets on your satellite and turn them on, the resultant field interacts with the Earth’s field causing a torque on the satellite. These are what are known as Torque Rods.
Since the reaction wheels, gyros, and torque rods all work using electricity and the solar arrays provide that electricity, theoretically the life of the satellite is indefinite. Unfortunately, there are degradations of the thermal coatings, blankets, sensors, and failures of both the gyros and reaction wheels that ultimately limit the life of any satellite.
Over a period of time, these degrade to the point that the satellites can no longer function within design spec. At some point, you either have to replace the satellite, repair it, or say farewell.
For the Hubble, it was designed to be serviced by the space shuttle fleet. The tradeoff is when does the cost and risk of service out weigh the science benefit. This is what the scientific and engineering communities are wrestling with per the original post.
Actual observed positions did not quite match the predictions under classical Newtonian physics. Albert Einstein later solved this discrepancy with his “General Theory of Relativity”. In November of 1919, using a solar eclipse, experimental verification of his theory was performed by measuring the apparent change in a stars position due to the bending of the light buy the sun’s gravity.
It is a little appreciated fact that gravity bends light even under Newtonian gravity. The quantitative predictions of the two models, however, differ by a factor of two.
[Geek alert: Why does gravity bend light under Newton? According to Sir Isaac, the force of gravity on an object is proportional to the mass of the object. But for a given force, the acceleration of the object is inversely proportional to its mass, so the object's mass drops out of the equation. Acceleration is thus independent of the object's mass, for small masses--even for massless photons! The speed of light is large, but it's finite, so the bend angle is nonzero.]
5.56mm
When the space telescope is in orbit, where do these external forces come from? How much of a net torque is exerted by the solar wind? How long does it take for the reaction wheel angular momentum to build up to a level that is unacceptable, or is there some reason the wheels must be absolutely still? Also, once you start adding angular momentum to a wheel, isn't it hard to subtract exactly the same amount of momentum? (Hmmm... maybe that is why the wheels need to be normally still, so you can just apply a brake to stop a turn of the telescope).
A monopropellant is one that does not require an oxidizer to function. Usually monopropellants are composed of a liquid compound called Hydrazine (N2H4). When this liquid comes in contact with a platinum catalyst, it is decomposed into gaseous ammonia (Nh3), nitrogen and hydrogen. This gas is then ejected (fired thru a nozzle) out a jet to providing motion for the satellite. otherwise known as an "EXOTIC FUEL"
An ingenious method of unloading momentum without the use of fuel was devised using simple electromagnets. Remember the Earth is surrounded by a magnetic field (why your compass works). If you attach orthogonal electromagnets on your satellite and turn them on, the resultant field interacts with the Earth’s field causing a torque on the satellite. These are what are known as Torque Rods.
Since the reaction wheels, gyros, and torque rods all work using electricity and the solar arrays provide that electricity, theoretically the life of the satellite is indefinite. Unfortunately, there are degradations of the thermal coatings, blankets, sensors, and failures of both the gyros and reaction wheels that ultimately limit the life of any satellite.
Over a period of time, these degrade to the point that the satellites can no longer function within design spec. At some point, you either have to replace the satellite, repair it, or say farewell.
For the Hubble, it was designed to be serviced by the space shuttle fleet. The tradeoff is when does the cost and risk of service out weigh the science benefit. This is what the scientific and engineering communities are wrestling with per the original post.
Thanks...For the review in the Newtonian laws, will LaGrange Points help in this matter, or are they too far out?...wrong positions...they'er still on same elipic (sp?) plane...aren't they?... :|
Now if you want to get really picky ...
The earth has a non-trivial moon, which makes it wobble slightly in its solar orbit. Thus, the real center of mass of the earth-moon system is a bit off from the geographical center of the earth. Since the satellite also feels the tug of the moon, it is basically orbiting the earth-moon center of gravity.
If the satellite has solar panels (arrays), they need to point towards the sun to provide power. Sensors need to point at their respective targets, such as a star sensor, sun sensor etc.
Couldn't these be made in spherical form, so that they'd always be "pointed" in the right direction? I guess a flat panel has more surface area exposed to the target than a small sphere, but a larger sphere should do just as well.
LOL, thanks for the additional info, RA.