Cassini fly-by of Enceladus

Wow, a 30-mile-high fly-by. Are we not lucky to live in the time of the Cassini mission? I can't wait to see what they find out next.
 
Andy44 said:
Wow, a 30-mile-high fly-by. Are we not lucky to live in the time of the Cassini mission? I can't wait to see what they find out next.
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What is even more amazing is that this is the 2nd time they have done a 50 km fly-by this year. Last one was on 3/12/2008.:cheers:
 
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It is just amazing at how precise the navigation can be done :speakcool: ... now all that is missing is an online instant of Orbiter running with realtime simulations.
 
NukeET said:
What is even more amazing is that this is the 2nd time they have done a 50 km fly-by this year. Last one was on 3/12/208.:cheers:
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And the next one on October 9th is to be 25 km (15.5 miles)! Imagine whipping along at 17 km/s at that altitude!
 
SiberianTiger said:
Those guys in Cassini team are dashing people. 25 km altitude this time! Hope they won't end up smashing their toy into a moon... :blink:
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It continues to amaze me at how accurate they are at determining the orbit of a spacecraft in the Saturn system, which is so far away as to be a mere point in the sky from Earth. I have never heard a good explanation how they do this. I imagine that you can do ranging and doppler range rate from an Earth surface antenna, but the azimuth and elevation of the antenna are effectively just tracking Saturn, no?

Apparently they are confident in their system if they are going to buzz a moon at 25 km! I hope the vehicle has windshield wipers, since it will be flying through the geysers.
 
Andy44 said:
It continues to amaze me at how accurate they are at determining the orbit of a spacecraft in the Saturn system, which is so far away as to be a mere point in the sky from Earth. I have never heard a good explanation how they do this. I imagine that you can do ranging and doppler range rate from an Earth surface antenna, but the azimuth and elevation of the antenna are effectively just tracking Saturn, no?
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The angular position is determined using VBLI (very large baseline interferometry) between the DSN antennas.
 
Also, Cassini has some of the finest laser gyros on board.
 
tblaxland said:
The angular position is determined using VBLI (very large baseline interferometry) between the DSN antennas.
Click to expand...

An question that has just come to me: will launching of several navigation beacon heliocentric satellites help future interplanetary probes and (maybe) manned ships to navigate? A sort of interplanetary GPS/GLONASS/Galileo, with greater output energy.
 
SiberianTiger said:
An question that has just come to me: will launching of several navigation beacon heliocentric satellites help future interplanetary probes and (maybe) manned ships to navigate? A sort of interplanetary GPS/GLONASS/Galileo, with greater output energy.
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Only, if you can estimate their position better as the position of the probe. GPS only works because the position of the satellites is known.
 
Urwumpe said:
Only, if you can estimate their position better as the position of the probe. GPS only works because the position of the satellites is known.
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At least the radial distance to the Sun can be found with great precision, isn't it? The same way they measure the Sun quakes. The question is whether the two other coordinates of the "space beacon" are determinable. Let's suppose we aren't limited in payload very much (in contrast with the probe which is stuffed with propellant and scientific instruments). You have enough room for any sane navigation device on board. What would you suggest?
 
SiberianTiger said:
You have enough room for any sane navigation device on board. What would you suggest?
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I expect they would need an antenna capable of tracking quasars since I believe these are the reference points DSN uses.

As a matter of interest, I found these two documents:
 Cassini Orbit Determination Performance During The First Eight Orbits Of The Saturn Satellite Tour
Use Of Very Long Baseline Array Interferometric Data For Spacecraft Navigation


The first document shows Cassini flyby errors in the range 0.5km to 73.5km. It also states the biggest error was due to errors in the estimates of the natural satellite orbits and masses, evidenced by the trend of errors getting smaller with each flyby. According to the second one, DSN VLBI resolution is in the order of 2nrad which translates to about 5km at Saturn.


Further to Andy's post above, if I am reading the charts in the first document correctly, range is accurate to about 2m.
 
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WOW. Amazing at that distance! I've said it before and it bears repeating: Cassini is one of the coolest space missions we've seen to date. The science, the photos, and the engineering involved, it's all fantastic. And the Saturn system is surely one of the most majestic natural features of the Solar system. I really envy those who are flying this vehicle right now.

I'll have to dig into those pdfs when I get a chance.
 
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