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Posted: |
Oct 30, 2016 - 8:49 AM
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By: |
Metryq
(Member)
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They send the spacecraft on a rare, fact-finding mission, lasting almost a decade and with almost no possibility of a follow up for a long time to come. You'd think they'd work out methodologies to scrape every single last scrap of information out of the instrumentation, wouldn't you?
I wonder what solutions have been proposed for this kind of thing? Short of equipping the probe with an advanced engine, or loading it down with enough fuel for orbital braking, both of which would increase the cost of the mission significantly, what other options are there?
Would a two-part probe be feasible? Before high speed passage by the target, could the probe split—sending one half to pass over the opposite side of the body—using a tiny engine? The secondary probe might be extremely small, perhaps with a transmitter sufficient to reach only the main probe as a buffer and relay.
"Give me a ping, Vasili. One ping only."
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Posted: |
Oct 30, 2016 - 9:48 AM
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By: |
Grecchus
(Member)
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The two-probe thing has obviously been done - most missions to Mars capitalize by piggy-backing small landers on larger vehicles intended to orbit the planet. The landers detach when they get close enough and use heat shields to attempt atmospheric passage before touching down on the planetary surface.
As far as I'm concerned, Pluto is interesting because it is the largest KBO, and also carries with it a very heavy object from the KBO population. How these two similar-sized objects ended up in close proximity to one another, and in such an elevated orbit to the Sun compared with the rest, is a question that has got to be answered. It's the mechanics of how they came to be where they are that gets me. The blob on Pluto is kind of interesting, but, that's just a consequence of the even more interesting orbital mechanics that determined the system in the first place, so is of secondary importance.
How to orbit Pluto? Best guess is to use a similar propulsion system to that of Dawn. In fact, it would probably have to be a hybrid. Use conventional rocket systems to push off from earth and hurtle towards Pluto. Use a detachable chemical rocket pack to adjust the inbound trajectory, then jettison the chemical rocket straight-jacket when spent and revert to ion propulsion for a long, spiral coast to the planet. That's one complicated method and probably long-winded way to have any kind of hope of getting to orbit Pluto.
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Posted: |
Oct 31, 2016 - 4:09 AM
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By: |
Metryq
(Member)
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The smaller craft would need enough fuel on board to slow it's (sic) trajectory and make a course correction.
We seem to have a communication problem here. I understand all about unpowered "cruise" and gravity assists. I'm talking about a simple deviation here: could a secondary probe be detached, which would thrust itself away so that it passed the opposite side of the body from the main probe? No slowing down, no orbiting, no landing, no big engines—no "opposing direction." In fact, the secondary would have no big anything. Its cameras and other sensors might have a feeble transmitter capable of reaching the main probe, only, for a data dump.
I don't know the details of piloting probes, but I assume a probe is on a fixed course long before encounter. Once that approach course is established, a secondary probe might deviate with a very small thrust and have time to build up the distance before fly-by.
Given the cost in time and money for such a mission, a "7-10 split" maneuver like this might be worth the risk.
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Posted: |
Oct 31, 2016 - 9:25 AM
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By: |
Grecchus
(Member)
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Can a secondary probe going a gazillion miles per hour simply fire maneuvering thrusters and go in another direction? I don't know the anwer to that.
Generally speaking, you need an accurate inertial navigation system on board that can keep track of the probe's relative position to all large planets and other objects in the vicinity (ie. the Solar system), as well as the stars fixed to the Celestial Sphere. The probe will be going generally either away from or toward the Sun. Probes don't travel at a constant speed as they move about between the massive bodies in the Solar system. Their speed will be constantly changing as they move further from one planet and then closer to another. They will accelerate towards the nearest planet because at a certain distance it's gravity will have the dominating sphere of influence within the immediate surrounding space. Even after achieving escape velocity from earth, and after turning off their rocket engines so as to enable coasting, any spacecraft will still be in earth's gravitational hold (and every other planet/moon in varying degrees of relative strength) and that constant tug of gravity will always be there. It's just that the further radially away you get, the weaker the earth's direct pull of gravity becomes. This is one of the mind-boggling consequences of all the mass in the Universe. Everything is pulling on everything else.
So, if you fire your spacecraft rocket engine, where it goes from then on is dependant on which direction it's pointing and what force is being applied, as well as the duration of the burn. If the spacecraft fires it's engine so that the thrust keeps it going in the same direction it was going before the engine turned on, it will still go in the same direction except that it will have speeded up. If the spacecraft turns so that the rocket engine is pointing against the direction in which it is moving, then the net result of firing the engine will be to slow it down relative to a nearby object directly ahead.
All spacecraft undertaking journeys within the Solar system have very carefully prepared missions, and every single evolution made will have been very carefully planned for either in the case of fulfilling the mission as designed, and any contingencies that may arise. In any case, the propellant tanks are the bottom line. Because their fuel capacity is limited, the rocket engine/s can only fire for as long as the fuel exists to allow them to do so. Between being full and being empty, the total amount of acceleration the rocket engine can impart is known as the total Delta-v. If you were to fire the rocket engine in a certain direction until the fuel is exhausted, the spacecraft will not be able to alter it's course or speed thereafter. No more Delta-v.
https://www.wired.com/2016/10/15-months-new-horizons-finally-transmitted-6-25-gigs-pluto-data/
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Posted: |
Oct 31, 2016 - 8:04 PM
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By: |
Metryq
(Member)
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Thanks for the remedial dynamics lesson.
Even after achieving escape velocity from earth, and after turning off their rocket engines so as to enable coasting, any spacecraft will still be in earth's gravitational hold
Wrong. After achieving escape velocity from Earth, a probe may still be bound to the Sun, but Earth no longer has influence. When a probe makes a gravity assist maneuver past a planet, it cannot come out the other side traveling faster relative to the planet—unless the spacecraft also fires its engines during the maneuver. However, the probe can steal some momentum from the planet relative to the Sun, which is the whole point of such maneuvers. Gravity assists help a spacecraft climb to a higher orbit around the Sun.
Orbital mechanics seems counter-intuitive because we're accustomed to the movement of objects at the Earth's surface, deep inside its sphere of influence. Confusion can also result from explaining orbital mechanics with poor analogies. For example, the Woody Woodpecker tutorial in the movie DESTINATION MOON graphically depicted the Moon's gravity as a magnet. Without the intervention of a third body or some other influence, capture of one body by another is essentially impossible. (That's one of the flaws in the nebular hypothesis. A planet is very unlikely to "sweep out" material in its own orbit. And bodies crossing a planet's orbit are also unlikely to be captured, short of a direct impact.)
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Posted: |
Nov 3, 2016 - 8:19 AM
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By: |
Grecchus
(Member)
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Wrong. After achieving escape velocity from Earth, a probe may still be bound to the Sun, but Earth no longer has influence. When a probe makes a gravity assist maneuver past a planet, it cannot come out the other side traveling faster relative to the planet—unless the spacecraft also fires its engines during the maneuver. However, the probe can steal some momentum from the planet relative to the Sun, which is the whole point of such maneuvers. Gravity assists help a spacecraft climb to a higher orbit around the Sun.
Sure, I understand Jupiter is rotating that much slower after so many earth probes flew past and robbed it of some angular momentum. Pretty amazing . . . no physical contact required.
I was referring to the n-body problem, where the gravitational pull between multiple bodies and their resultant motions, herein restricted to discussion of the Solar system, is ever present no matter how far away the probe is from one of them, or however close it is to another. As a matter of fact, the radial pull of planetary bodies that are in-line through to the centre of the Sun may have had major effects on the evolution of the Solar System. And relevant to this thread is the proximity of Pluto and Charon to each other before they were tugged and flipped not only together, but with other KBOs that may have been flung out of their incipient orbits by the mass of whatever-it-was that led to what we see today.
I must admit the patched conics method used to design planetary flybys is a specialist subject, but I do have a book or two to fill in the details if absolutely necessary.
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Posted: |
Dec 15, 2018 - 2:03 PM
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By: |
Solium
(Member)
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I think piggy back probes is the way of the future. It costs so much to get into space and takes so long to get to these destinations, it makes a lot more sense to combine missions.
There's any number of combo missions, orbiters, fly byes, landers, rovers, hoppers, winged crafts, and sample return.
We're starting to see this. the Japanese probe that recently orbited an asteroid, had two hoppers a lander and a sample/return vehicle.
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Posted: |
Dec 19, 2018 - 8:02 AM
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By: |
Grecchus
(Member)
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We're starting to see this. the Japanese probe that recently orbited an asteroid, had two hoppers a lander and a sample/return vehicle.
Isn't that also the Osiris Rex mission at Bennu? This particular news item, for instance, is stunning. Can you spot the difference?
https://www.sciencenews.org/article/asteroids-bennu-ryugu-strange-diamond-shapes
This probably requires its own new post, however, since New Horizons is about to play its second game of 'catch,' I don't suppose it would hurt to include this snippet of info, too:
https://www.sciencenews.org/article/new-horizons-ultima-thule-preview?tgt=nr
It's a bit unfortunate that the story with New Horizons is, "now you see it, now you don't," but that's the nature of the biz.
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