I have a couple of questions about orbits. In particular, GSO.1) I understand that there are "slots" given to each country. Are this governed by ITU? How are they given?2) How wide are this "slots"? I've seen quoted integer degrees, but just Argentina has two or three slots, so they should be more.3) What's the optimum maneuver to "move" between two slots? The initial and final angular momentum and the relationship of potential and kinetic energy are the same. So I would assume that it can be made very cheaply if not very fast.4) Regarding Polar Orbits. It seems that USA launches southward, and I'm assuming Plesetsk, launches Northward. The Indians, launch also southward, but on the opposite side of USA. I don't know how the Chinese launch their polar orbit satellites. But the main question is: don't you end up with polar orbits going in opposite directions? I guess not opposite since that would leave the earth's orbit, but they should cross at a very steep angle. Of course since a lot of polar orbit satellites "don't exists" it's not like there's a lot of coordination. But isn't it dangerous?5) If it is dangerous, couldn't some country that's worried about another's spy satellite launch a "scientific" instrument in polar orbit that "just" happens to accidentally crash on one of those "satellites that don't exist"?
Quote from: baldusi on 01/27/2011 01:38 pmRegarding Polar Orbits. It seems that USA launches southward, and I'm assuming Plesetsk, launches Northward. The Indians, launch also southward, but on the opposite side of USA. I don't know how the Chinese launch their polar orbit satellites. But the main question is: don't you end up with polar orbits going in opposite directions? I guess not opposite since that would leave the earth's orbit, but they should cross at a very steep angle. Of course since a lot of polar orbit satellites "don't exists" it's not like there's a lot of coordination. But isn't it dangerous?1 No and no. It is all about timing. The orbit is fixed in space, relationship to the earth is not fixed by the launch site. Also think about this.2 On one hemisphere, the spacecraft is going north and on the other side it is going south.
Regarding Polar Orbits. It seems that USA launches southward, and I'm assuming Plesetsk, launches Northward. The Indians, launch also southward, but on the opposite side of USA. I don't know how the Chinese launch their polar orbit satellites. But the main question is: don't you end up with polar orbits going in opposite directions? I guess not opposite since that would leave the earth's orbit, but they should cross at a very steep angle. Of course since a lot of polar orbit satellites "don't exists" it's not like there's a lot of coordination. But isn't it dangerous?
1 So you basically have two polar orbits, one that goes North to South and another that goes South to North if looking from the Sun to Earth, right? And all polar orbit satellites are on those two orbits at different phase, right? They should cross at two points.2 I said that India launches southward and Russia and China northward.
What is the lower limit to the mass of the object providing the delta-v, ie could some useful amount of delta-V be gained from passing within 100 meters of a small asteroid?
Quote from: Danderman on 03/04/2011 03:37 pmWhat is the lower limit to the mass of the object providing the delta-v, ie could some useful amount of delta-V be gained from passing within 100 meters of a small asteroid?No. Roughly, the amount of delta-v benefit you could gain with a gravitational swingby is on the order of the escape velocity (at your minimum altitude). Escape velocity is proportional to the square root of mass divided by radius. So a big asteroid like Ceres has an escape velocity of 500m/s (significant), but an asteroid with a tenth the radius would have an escape velocity of only 50m/s (not significant).
How does proximity to the object figure into this. I can't seem to get any delta-V from Jupiter from my living room, so proximity must be a factor.
Quote from: Danderman on 03/04/2011 06:21 pmHow does proximity to the object figure into this. I can't seem to get any delta-V from Jupiter from my living room, so proximity must be a factor.Proximity determines escape velocity. At your current distance from Jupiter, escape velocity is hardly anything. Escape velocity is inversely proportional to the square root of your distance to the center of the object. Escape velocity is maximized by standing on the object's surface.
You can do this from a Lagrange point.
I would imagine that the delta-V required to go to a Lagrange point would be more than anything gained from the resulting Earth swingby, so in this case, the cost of having an absolutely positive departure date for Mars from Earth would be that extra Delta-V to hang around the Lagrange point.
Is there a "Free" trajectory where a spacecraft could just depart Earth, and return without a requirement for some additional maneuvering?
Quote from: mmeijeri on 03/06/2011 05:17 pmYou can do this from a Lagrange point.I would imagine that the delta-V required to go to a Lagrange point would be more than anything gained from the resulting Earth swingby, so in this case, the cost of having an absolutely positive departure date for Mars from Earth would be that extra Delta-V to hang around the Lagrange point.
Earth/Moon L1/L2 has various advantages as a staging point, such as being outside the van allen belts and close to earth C3=0. But in principle something like a highly elliptical earth orbit (a more extreme form of GTO) would do as well. But then you have the issue that your spacecraft has to pass through the van allen belts every few days, and has to cope with very different thermal environments.
Let's look at a mission like Rosetta or Cassini, where they flew a trajectory that returned to Earth after *years*, and used an Earth swingby to travel to their final destination.I am asking if its possible to duplicate this effort with shorter travel times, maybe not all the way out to Mars, but possible 50 or 100 days out, and then use an Earth swingby to travel to Mars.
Are there heliocentric orbits departing from Earth that can later intersect the Earth's orbit while the Earth is at the intersect point that would allow gravitational swingby maneuvers leading to other destinations, such as Mars or Jupiter?
The answer clearly is: yes, because its been done. In the case of Rosetta, the spacecraft entered a heliocentric orbit with an apohelion beyond Mars, returned to the vicinity of Earth (at perihelion), and then used a swingby manuever to gain velocity and change direction to intercept a comet.
This begs the question as to what is required to enter a heliocentric orbit that would re-intercept the Earth at perihelion, is the required duration some number of years or are there shorter trajectories? Are there trajectories between Earth departure and Earth intercept with a duration of days or weeks or months?