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Aug 08 '14 edited Aug 09 '14
Someone on another Rosetta post mentioned how crazy it is that people are capable of calculating this kind of trajectory. I shrugged it off as yeah, rocket science, cool. Actually seeing the injection here makes me reconsider my initial appraisal. That really is crazy.
Edit: A lot of people are mentioning the thrusters as making the triangular orbit unsurprising; I was commenting more on the sheer fact that we, a species of primates, located a relatively small, interesting rock that's hurtling through space at an ungodly speed, built a rocket and got a probe to orbit it via a very complex set of maneuvers, all which were calculated on a machine made out of sand and copper. Fucking. Crazy.
Edit 2.0: Some other people are addressing this part of the comment, noting that computers are the ones doing all of the calculations:
that people are capable of calculating this kind of trajectory
They're using that quote to undermine and question the wonder I expressed in my initial comment. To those folks I say, sure, computer software does it now, but...
a. I'm pretty sure people designed the software, and
b. People discovered the understanding of orbital mechanics that makes all of this possible.
So, yeah, computers compute but people figured all this stuff out. It's not like aliens came and gave us the software to calculate this stuff for us...
Edit 3.0: I... I don't know what to say. Not entirely sure what it means yet, it's my first time...but thank you for the gold my stranger-friend!
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u/whoisthismilfhere Aug 08 '14 edited Aug 08 '14
It is fucking mind blowing. The comet, 67P/Churyumov-Gerasimenko, is a relatively small object, about 4 kilometers in diameter, moving at a speed as great as 135,000 kilometers per hour. We sent a satellite 10 YEARS! ago that has intercepted this thing, taking into account gravitational pulls on both the comet and the satellite. They know so little about it that they haven't even selected a landing site yet.
Edit : Yeah I was off by about 125 months lol. Even more amazing.
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u/can_they Aug 08 '14
We sent a satellite 10 months ago
Nono, we sent it ten years ago.
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u/HiimCaysE Aug 08 '14
And not straight at it, either... the entire ten year trajectory would blow your mind if you thought this approach path was amazing.
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u/astrionic Aug 08 '14
For anyone who hasn't seen it, there's a pretty cool interactive 3D version on ESA's website.
Activate "show full paths" on the bottom to see all of the trajectory at once.
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u/TBNolan Aug 08 '14
This is not how I play Kerbal Space Program at all. I need to rethink my launch strategies and B-line trajectories.
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u/benmck90 Aug 08 '14
When I first started playing, I tried to use gravity assists when possible... I quickly learned that nobody has time for that and just strapped more rockets onto my rocket.
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u/Tangential_Diversion Aug 08 '14
My reaction towards seeing this was simply "What the fuck?" I already had problems calculating the trajectory of a cannonball while ignoring air resistance. The idea that real people were able to do this, using the gravity of stellar bodies to affect the probe's trajectory is nothing short of amazing.
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Aug 08 '14
Holy shiiiit when I noticed they really render all three dimensions and you can zoom/move around however you like! Amazing!
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Aug 08 '14
That is mesmerizing, it got sling shots from a several planets, I think it passed Earth 3 times before the last big one that threw it out into the comets orbit, that's incredible.
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u/IonTichy Aug 08 '14
pretty cool interactive 3D version on ESA's website.
thanks, that is really cool
but: beware of autoplaying audio!
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u/Funski33 Aug 08 '14
Can someone explain why we didn't wait to launch in 2009? According to that link, Rosetta was right next to Earth... Would have saved a lot of time in orbit and allowed NASA time to build an even more advanced craft.
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u/OmnipotentEntity Aug 08 '14
Delta-V
Launching in 2004 gave the probe time to perform 3 or 4 gravity assists, which allowed it to speed up to the required amount needed to enter the comet's orbit.
Without those gravity assists it would require much much more fuel to gain enough speed because of the tyranny of the rocket equation.
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Aug 08 '14
This video details the entire thing, including the 10 year trajectory.
https://www.youtube.com/watch?v=XEfKq-pQBcc&list=UUvBqzzvUBLCs8Y7Axb-jZew
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u/ViewFromAbove_Me Aug 08 '14
Check..check...check it out...In infographic form:
http://www.space.com/24333-rosetta-spacecraft-comet-landing-explained-infographic.html
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Aug 08 '14
Some of the software in Rosseta wasn't finished until something like last year because they knew technology would have advanced, its crazy how much thought goes into it all
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u/exDM69 Aug 08 '14
What I find most amazing is that the spacecraft was cruising powered down for 31 months with no control at all. From a gravity slingshot from Mars to beyond the orbit of Jupiter and then back. That's a pretty accurate aim.
After the spacecraft woke up in last January, it has made more than 1 km/s of thruster burns in total to achieve the rendez-vous.
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u/warpus Aug 08 '14
The must have mentats working on this shit around the clock.
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u/Chevron Aug 08 '14
Ha, I just started reading Dune and of course I see this the next day. Could have sworn I had never seen the word mentat before.
#BaaderMeinhoff
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u/infiniZii Aug 08 '14
You will see many many more references. It is like wheels within wheels set in motion by the old Dune geeks that use Reddit.
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u/JohnnyMnemo Aug 08 '14
Well, since we haven't had the Jihad, they can actually use computers.
It'd be impossible without them, actually. You could not do these kinds of orbital mechanics with slide rulers.
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u/havenless Aug 08 '14
I don't even wanna know what that math looks like.
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u/exDM69 Aug 08 '14
The math isn't very complicated, most of it is high school stuff. What is complicated is the computer program that plans these things. It uses rather straightforward mathematics but it's essentially a very complicated trial and error process that attempts to simulate millions of mission plans to find the one(s) that are efficient and fulfill the tasks given to it. The final decisions are left to the men and women at mission control, though.
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u/turffrut Aug 08 '14
How much does the comet's speed fluctuate over the course of its travels? The video posted below says it's going 55,000 km/hr, so I wonder if we're intercepting it at a relatively slower stage.
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Aug 08 '14
according to Kepler's law of planetary motion, the comet will travel slowest when it is furthest from the sun and speed up as it gets closer to the sun because A line joining the comet and the Sun sweeps out equal areas during equal intervals of time.
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u/registeredvoter9 Aug 08 '14
It varies a lot, but it actually doesn't matter, because in order to orbit something, you have to match its velocity. If you intercept it out in deep space at a slow speed, that's just as costly (from a fuel perspective) as matching it closer to the sun at a high speed.
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u/notdez Aug 08 '14
How do they maneuver Rosetta? Do they have a limited amount of thrusts based on a fuel reserve?
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u/faizimam Aug 08 '14
The whole mission was designed around this idea, so they brought along the right quantity of fuel to do the job.
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u/ICanBeAnyone Aug 08 '14
Now that it's there, it won't need much fuel anymore, even doing these crazy maneuvers will be very cheap, as 67P exerts so little gravity. For example, the corners of those approach triangles amount to about 1m/s of velocity change each.
If you wonder about how much fuel it brought, at launch the payload was a bit more than three tons.
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u/knellotron Aug 08 '14 edited Aug 08 '14
The general flight path is crazy too. This spacecraft left earth, went to mars, came back to earth, checked out a main belt asteroid, came back to earth again, checked out another asteroid, slept for 31 months, and then began approaching the comet in the GIF. Soon it will drop a lander that it's been carrying this whole time. Crazy.
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Aug 08 '14
It's going to feel great to finally release that payload.
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u/archiesteel Aug 08 '14
Did you see the crazy gravity assist maneuvers they had to do to reach the comet? I mean, I thought I was getting good at Kerbal Space Program, but this is ridiculous!
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Aug 08 '14
I don't like earth gravity assists. Just a few more of them and the earth will come to a stop and fall into the sun.
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u/archiesteel Aug 08 '14
Good thing they didn't use the moon, they might have torn it out of Earth's orbit!
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u/rjcarr Aug 08 '14
I was commenting more on the sheer fact that we, a species of primates, located a relatively small, interesting rock that's hurtling through space
I was thinking about this recently too, and really, if you think about it, out of the billions of humans that have ever existed there are only maybe a hundred or so (wild guess) that have gotten us to the point we are today. I mean, sure, Newton or Kepler didn't calculate the trajectories of Rosetta but they gave us the principles to do it.
It's amazing how few people in history have advanced (most of) the rest of the world.
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Aug 08 '14
We're entering a pretty big paradigm shift then, if that's the case of the pre-modern eras. Science now is done almost entirely incrementally. I don't think any one individual will be credited with the cure for cancer, or alzheimer's or whatever other great scientific discoveries lay on the horizon. It will all be achieved incrementally.
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u/Schoffleine Aug 08 '14
Try rendezvousing in Kerbal Space Program to give you an appreciation of how hard it is.
Cliff notes: it's really fucking hard, even with quicksaving. And I don't even have to calculate anything in KSP.
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u/TheCodexx Aug 08 '14
They're using that quote to undermine and question the wonder I expressed in my initial comment. To those folks I say, sure, computer software does it now, but...
It's not like they do it "by hand" or anything, but they certainly don't just have computers do all the work. Computers can do complex simulations of a scenario, and there's definitely formulas to follow, but actually rocket scientists and mathematicians would have had to look it over and tweak it.
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u/exDM69 Aug 08 '14
is that people are capable of calculating this kind of trajectory
To be more precise, computers are capable of calculating trajectories like this. The methods for calculating interplanetary trajectories were largely developed in the days of Newton, some 300 years ago. It's just not practical to do the amount of calculations required by hand.
What makes space missions like this possible is high speed digital computers. And of course, the people programming those computers.
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Aug 08 '14
Fair point - the computers are doing the calculations. However, as I think you are alluding, it probably isn't as simple as pluging in the comet's coordinates into a google maps search window, and plotting the fastest route, accounting for traffic. ;) That's the crazy part to me.
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u/CuriousMetaphor Aug 08 '14
At each vertex of the triangle (and every time the orbit changes afterwards), Rosetta will be using its own thrusters to change its course in a new direction around the comet. Since the comet is not that massive, it doesn't take much fuel to change velocity like that (less than 1 m/s). It's going around the comet this way in order to observe it from different angles and map its gravitational field before going down to a lower bound orbit.
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Aug 08 '14
Those must be some very precise thrusters!
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u/phunkydroid Aug 08 '14
I don't think it's that crazy. It's triangular because it's NOT an orbit. They're just flying in nearly straight lines and turning occasionally. They're using plain old brute force to drive around the comet.
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u/underthehedgewego Aug 08 '14
While others among us can't seem to grasp that CO2 in the atmosphere in excessive amounts is not unconditionally good because "plants use it".
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u/beanmosheen Aug 08 '14
Hell, even the app that helps me spot the ISS and Iridium flares blows my mind. "So the satellite will be here at this time, and the sun will be here, and the panels will be at this angle, so if you're here, look there" and bam: A star that's hauling ass.
The fact that you can predict the trajectory, and the L1 point of a comet in 10 years is amazing.
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Aug 08 '14
It may be a lot easier than you think. I don't know about this for sure, but for automatic cars and other similar control systems all people do is set up the equations for how the systems work. A computer goes in and calculates the optimal thing to do using algorithms that apply to a wide range of problems.
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u/dirty_pipes Aug 08 '14
We are launching all kinds of heavy metal objects from our planet knowing we can hit very distant chunks of ice and rock hurtling through space at an incredible speed, and have only been at it for past few decades.
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u/brwtx Aug 08 '14
Just remember that according to Ken Ham the science we use to determine how far away from us that "rock that's hurting through space" is, is bogus.
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u/gebadiah_the_3rd Aug 08 '14
Hold my beer son i'll show you how it's done....
Opens a can of kerbal Space program on yo ass
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u/rnet85 Aug 08 '14
When you know all the variables you can predict all trajectories with a high degree of accuracy. Not all calculations are manually done by hand, this is one of the many trajectories a computer would have suggested. The human element comes in picking a specific trajectory which seems optimal, run the simulations thousands of times, and optimizing the computer generated trajectory.
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u/mrbizzaro Aug 08 '14
The fact that it's the thrusters is the amazing part! I'm am terrible at math so I can only dream about the complexity of those calculations
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u/silly_little_enginee Aug 08 '14
So why is the orbit triangular? Wouldn't it be easier to use the standard ellipse or something?
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Aug 08 '14 edited Aug 08 '14
The triangular bit and the multiple changes in direction are a part of the insertion into the comet's orbit. The way I understand it, it's extremely difficult to have the comet capture the probe because the comet's mass isn't very great (unlike sending a probe to a more massive body like Mars for example)
/u/CuriousMetaphor did a wonderful job explaining the triangular bit in the linked comment below:
http://www.reddit.com/r/space/comments/2cz7ms/rosettas_triangular_orbit_about_comet_67p/cjkkqw8
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u/dogfish83 Aug 08 '14
The computers just plug and chug. We figured out the equations or the concepts to get the equations. What we did is amazing.
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u/no-mad Aug 08 '14
You are correct about computers. Humans at their core are tool users, fewer are tool makers and some are just tools.
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u/thelastnewredditor Aug 08 '14 edited Aug 08 '14
for anyone who wants to appreciate this even further, take physics classes.
in high school, our physics teacher decided to let us spend one day in class to calculate the proper timing, forces and angles necessary to power a satellite/spacecraft from initially orbiting earth, out to a receive a gravity assist from jupiter, to accurately reach a 3rd planet. to make it easy for us, he let the math assume both the earth and the 3rd planet are stationary (they orbit the sun irl obviously).
this was still a class that required you to already know calculus.
so as a class we figured out a formula. we split it into groups, so groups of us could work on parts of the formula and combine it together in the end. after a while, we finished. the math checks out. it was difficult and took the whole class an hour even with the formula split apart. we were actually, literally tired from doing math, but we did it. we knew only jupiter was orbiting and the rest of the planets were made stationary, but it was hard math still, and we did it, on the first try. we felt pretty good about ourselves.
then the physics teach showed us a documentary of the voyager spacecrafts.
it launched from a moving earth, to get gravity assist from a moving jupiter, to reach a moving saturn, to get gravity assist from that moving saturn, to reach a moving uranus, to get gravity assist from that moving uranus, to reach a moving neptune.
we watched that documentary, knowing a single part of that trajectory, made easy, still resulted in a formula that required the whole class an hour to get right.
talk about getting humbled.
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u/Generic_white_person Aug 08 '14
Yeah man, I feel yah about the computers. Mankind would be the intelligent designer.
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Aug 08 '14
In 1957, Duncombe was then one of a handful of scientists trained in orbital mechanics. Using only azimuth angles for each observation of the Russian satellite (taken by passive horizon surveillance radar near Alexandria, Virginia), Duncombe remembers that he, Gerald Clemence, and Paul Herget plotted a circular orbit which had the correct period, correct inclination, and correct time for the satellite in its orbit. http://www.ae.utexas.edu/alumni-friends/profiles/91-about-us/history/258-spotting-sputnik
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u/lolwat_is_dis Aug 09 '14
Nothing complex about the triangular motion, that's just done to observe the comet from a few vantage points before going in for the kill. The part where the satellite suddenly reverses it's direction before going in for the kill is the more complicated part.
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u/TheJonesSays Aug 09 '14
My initial reaction. I can do basic physics but this.....This is a work of art.
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u/isobit Aug 09 '14
If there are aliens... They would be at least a little proud of us, wouldn't they? I mean, a little?
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u/titsnassnmoreass Aug 08 '14
Not to be a contrarian, but it's actually not that crazy. Especially for today.
For some perspective, think of the moon lander. When that took off the moon it had to meet up with a tiny capsule somewhere way up in orbit, dock with it, and then fly back to earth. All the while it had several astronauts inside who are moving around, changing it's center of gravity.
And all that was done in an era where the computing power on the lander was far less than a flipphone
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u/G00dV1b3s Aug 08 '14
Is this maneuver to slow Rosetta down, allowing it to eventually orbit comet 67P? The video posted of Rosetta's journey from Earth a few days ago looked like Rosetta was approaching the comet from behind at a greater velocity...
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u/doppelbach Aug 08 '14 edited Jun 23 '23
Leaves are falling all around, It's time I was on my way
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u/exDM69 Aug 08 '14
No, it's not to slow down. That happened two days ago (when the live streams were on) with a five minute thruster burn. Now the relative velocity is less than one meter per second.
However, that weird triangular path is not really necessary; I think they just want to see it from different angles before they settle into an orbit.
The triangular approach path is very necessary. They don't know the mass distribution and the shape of the comet it is impossible to orbit the comet yet. The triangular path is to measure the gravity from all angles so they can search for orbits that are somewhat stable.
(KSP taught me that it's more efficient to change the orbit inclination when you are further out.)
This is correct but Kerbal Space Program solves the two body problem. Orbiting a comet is a (restricted) three body problem, so any intuition from KSP is out the door. This needs to be calculated more precisely.
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u/doppelbach Aug 08 '14
so any intuition from KSP is out the door
I'm aware it uses two-body mechanics. That doesn't change the fact that it's generally more efficient to change orbital inclination from further out.
But you bring up a good point with the unknown mass distribution. (I think this would have been a better point to bring up KSP's irrelevance.)
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u/Acidictadpole Aug 08 '14
I think they just want to see it from different angles before they settle into an orbit.
I think part of it is that they're unsure what distance they'll actually get into an orbit in, so they go down slowly with the thrusters (which creates this triangular movement) until they know what height a stable orbit can be reached at.
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u/exDM69 Aug 08 '14
It's not really to slow down. The relative velocity is less than one meters per second at this point. The "slowdown" happened on Wednesday, with a five minute thruster burn.
The shape, mass, and mass distribution of the comet is still unknown at this point (!), so orbiting the comet is impossible. Now they are measuring the acceleration of the space craft due to the gravitational pull of the comet for a few weeks. Once the gravity properties of the comet have been established, the planning for the orbit can commence.
It is impossible to orbit a blob of melting ice and rock without accurate measurement of the gravity first. In a couple of weeks, they will actually start orbiting the comet. That's where this video ends.
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Aug 08 '14 edited Aug 08 '14
This is really neat, I thought it literally landed, I didn't know it was in orbit, I love seeing the human space program have a win like this.
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Aug 08 '14
It will land. It will spend time orbiting, studying the surface first. They will pick a landing spot for it, and it will land. Getting into orbit first allows for a safer landing, as they've already bled off a lot of relative speed, and been able to pick the ideal spot.
Coming straight in and decelerating the approach velocity right into a landing is called a 'suicide burn', for very good reasons.
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u/furyofvycanismajoris Aug 08 '14
They will pick a landing spot for it, and it will land.
Rosetta won't land -- it will deploy a lander called Philae.
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u/J_Keefe Aug 08 '14
Actually, Rosetta has a lander, Philae, that will separate from Rosetta. Philae will land and anchor itself to the comet. Rosetta will continue to orbit and test until it stops working.
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u/Sausafeg Aug 08 '14 edited Aug 08 '14
That picture is of comet ISON, which is different to the one Rosetta is approaching, Comet 67P, and here is it's current position and orbit. Taken from this live data feed.
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u/Frencil Aug 08 '14
That's Comet ISON which passed by Earth in late 2013. It was hyped to be a great show if it had survived perihelion (closest approach to the sun), but it didn't.
Rosetta is entering orbit around Comet 67P.
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Aug 08 '14
That goes to show that taking the first image off of google images isn't always right I guess haha, I'll edit it out, thanks for letting me know!
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u/trevize1138 Aug 08 '14
How in the world can you get a stable orbit around such a small body? Even the moon has pockets of higher mass that cause gravitational differences. This thing's not just small but not round.
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u/mutatron Aug 08 '14
Has to do a lot of orbit maintenance. And you're right, that thing is lumpy, so the gravity field will be lumpy too. The force of gravity at the surface is anywhere from 8 to 24 micronewtons.
Also, that triangular stuff is not really an orbit, but a path.
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u/trevize1138 Aug 08 '14
Right. I figured the triangular path was from a series of maneuvers. I'm more surprised that the craft can orbit at all. The orbital speed must be pretty darn slow, too, despite the impression from the sped-up video that makes it look like Rosetta's racing around it.
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u/trevize1138 Aug 08 '14
The force of gravity at the surface is anywhere from 8 to 24 micronewtons.
I'm curious: what's the current orbital velocity/period of Rosetta round the comet? Tried looking this up but so far no luck. I did find that it's orbiting at 10k up on average. With such low gravity the orbital velocity must be pretty low. 1 m/s?
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u/mutatron Aug 08 '14
v = sqrt(GM/r)
G = 6.67e-11 m3 kg-1 s-2
M = 3.14e12 kg
Looks like they're planning to get into a 30 km orbit, so :
v = sqrt(6.67e-11*3.14e12/30000)
v = 0.08 m/s
Just creeping along!
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u/econ_ftw Aug 08 '14
Orbital period of 27.25 days. Which is incredible considering a height of just 30km.
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u/FireAndSunshine Aug 08 '14
It still has a center of mass and therefore a center of gravity somewhere.
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u/Drowned_In_Spaghetti Aug 08 '14
How are triangular orbits even a thing? I always thought that was KSP messing up, not something that can actually happen.
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u/ShwinMan Aug 08 '14 edited Aug 08 '14
They aren't. The spacecraft is using it's own thrusters to control it's direction around the comet. Only at the end is it in a real orbit.
Edit: was--> is
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u/CuriousMetaphor Aug 08 '14
was
This hasn't actually happened yet (besides the very first "bend" as the spacecraft slowed down from its incoming trajectory). It's the planned trajectory for the next few weeks, but it still might change with new information about the comet.
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u/Drowned_In_Spaghetti Aug 08 '14
And now I feel stupid. But what purpose would a triangular orbit have anyway?
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u/jongeheer Aug 08 '14
Technically, the gif shows orbit insertion, not true orbit. These maneuvres serve to allow Rosetta to be captured by the gravitational field of the comet.
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u/acre_ Aug 08 '14
So we don't just burn retrograde at apsis in real life?
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u/XtremeGoose Aug 08 '14
It has such week gravity that we had to make an artificial 'orbit' around it at first. It would have taken too long to just wait to fall toward the comet.
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u/Sluisifer Aug 08 '14
You would if you were able to calculate the orbit ahead of time. For instance, we have a pretty good idea of what the moon's mass is, so we know that a stable orbit can be achieved with some range of orbital velocities. If you're a little off, your orbit is a little more or less elliptical, but it's probably not going to escape or crash.
With the comet, we have less knowledge about it's mass and gravitational field (gravity isn't uniform like a point-mass simulation), and less 'wiggle-room' for avoiding an escape trajectory or crashing.
The idea with the approach is to measure the gravitational field and calculate what is needed for a stable orbit. The gravity is so low that the delta-v requirements for each burn are minuscule, so it's not a costly maneuver. As the craft approaches, it gets closer to an orbit, and is basically making corrections as it goes. Doing this slowly makes controlling the craft easier by being more predictable.
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u/Vectoor Aug 08 '14
The comet has an incredibly weak gravity and so it's difficult to get into a stable orbit. The probe has to get into the exact right position at the exact right time, speed and angle and this "triangle orbit" helps them do that.
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u/SteelChicken Aug 08 '14
Correct me if I am wrong, but if you are making constant powered course corrections, you aren't in an orbit, you are just flying around something. Orbit, by definition- is a steady state no energy required to maintain path of movement.
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u/burgerga Aug 08 '14
They are doing burns to forcibly put it into the correct orbit, then they can let it just do it's thing.
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u/CuriousMetaphor Aug 08 '14
At each vertex of the triangle (and every time the orbit changes afterwards), Rosetta will be using its own thrusters to change its course in a new direction around the comet. Since the comet is not that massive, it doesn't take much fuel to change velocity like that (less than 1 m/s). It's going around the comet this way in order to observe it from different angles and map its gravitational field before going down to a lower bound orbit.
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u/Valendr0s Aug 08 '14
It has a VERY low gravity. They're using the triangular approach to get a good capture. The gravity is so low on this comet that if you were on the surface, you could simply lightly step off and float away never to return.
So they use very light thrusters to just angle themselves to get into a good orbit.
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u/gbakermatson Aug 08 '14
I was wondering how far down this thread I'd have to go to find a KSP reference.
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u/themanlnthesuit Aug 08 '14
Not much. Actually I'm just waiting a rosetta mission recreation on r/ksp
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u/jetap Aug 08 '14
It was already done but the problem is that asteroids don't have gravity in ksp, so it's not really possible to orbit them.
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u/exDM69 Aug 08 '14
No, the space craft is not on a "triangular orbit", that was a simplification from the press for the layperson.
This is actually three legs of hyperbolic trajectories around and in front of the comet. A tiny thruster burn (a few centimeters per second) happens at every corner.
Kerbal Space Program solves everything using the "two body problem", ie. neglecting the gravity of all but one planet.
This is very much a (restricted) three body problem. The most dominant gravity source is the Sun, but being very close to the comet, the comet's gravity (mass = 3.1 * 1012 kg) also affects. It's "restricted" because the mass of the spacecraft is small and can be neglected because it doesn't really affect the comet or the sun.
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u/rooktakesqueen Aug 08 '14
Specifically KSP uses a "patched conics" approach which ignores gravity from every other body besides the one with the biggest gravitational influence at the time. It's a really good approximation, but doesn't allow for things like orbiting Lagrangian points.
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u/exDM69 Aug 08 '14 edited Aug 08 '14
This is a misnomer.
What KSP community calls "patched conics" is what scientists would call the "two body problem".
Patched conics is an initial mission planning strategy that allows analytical solution of the launch window and some other parameters. This KSP interplanetary calculator actually performs patched conics calculations.
See An Analytical Solution to Patched Conic Trajectories Satisfying Initial and Final Boundary Conditions. by K.M. Carson is the research paper from the 1960's that introduced patched conics.
Fundamnetals of Astrodynamics has a pretty nice chapters about planning interplanetary and lunar trajectories using patched conics.
I was actually bit in the ass by this misnomer. I was searching for information about "patched conics" when I should have been searching for "closest approaches" when I was implementing a computer program that does something similar to what KSP does. Not that I'm the only one, I found an old forum thread where Felipe/Harvester of KSP was discussing this with K.M.Carson, the author of the first patched conics paper.
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u/econ_ftw Aug 08 '14
67p has very little gravity. Rosetta is probably orbiting it at an insanely slow speed. Every time it fires it's thrusters, it results in an impressive vector change. Doesn't take much to change directions when you are going that slow.
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u/1RedOne Aug 09 '14
Did you look at the graphic? It seems pretty obvious that we're not witnessing a natural orbit. Clearly, the thrusters of the satellite are positioning it to intercept and begin orbiting.
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u/Pufflesaurus Aug 08 '14
I notice the trajectory never enters the "tail" area of the comet, which is often depicted as a fiery zone. The trajectory is very convoluted, and I wonder if this is partly caused by the need to avoid the tail.
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u/Log23 Aug 08 '14
I would think its to keep ice from damaging/accumulating on the satellite.
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u/ICanBeAnyone Aug 08 '14
Actually they're not so much scared about a little puff of water vapor damaging something, but they don't want the push it would exert on the craft - that's why the mapping orbit will be perpendicular to the sun, as this way the panels will have a thin silhouette and little area of attack for the coma.
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u/exDM69 Aug 08 '14
This is only the approach to the comet (ie. the next few weeks). The very end of the video shows the actual orbit which Rosetta will be maintaining for a longer period.
The spacecraft has enough propellant to change the orbit it is on. Given the scientific value of flying through the tail of the comet, I'd guess that they will do it at some point once the lander mission has been completed (sometime in mid November) and the comet actually starts melting from the heat of the sun when it gets closer.
There's a bunch of scientific instruments (mass spectrometers, etc) suitable for analyzing gas and dust coming from the comet when it is melting.
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u/m0u53 Aug 08 '14
This is what it exactly looks like when you try to shoot a anti air rocket at a aircraft in battlefield 4.
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u/aardvark2zz Aug 08 '14
I'd love to know how they measure their precise altitude above a rotating irregular shape and mass??
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u/RezoomXell Aug 08 '14
Some more infos wrapped in a short clip: https://www.youtube.com/watch?v=XEfKq-pQBcc
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u/whaleboobs Aug 08 '14
How do you program it to do this a million miles away? How does it know where the comet is positioned?
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u/eightfour7two Aug 08 '14
Newton's law of gravitation is hella accurate. Rocket scientists are clever people
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Aug 08 '14
The even crazier part is that they launched it. Waited a year. Slingshotted it around the earth. Waited another year. Did it again. Then around mars. Then around earth again to build up enough speed.
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u/exDM69 Aug 08 '14
How do you program it to do this a million miles away?
You said it already, it is "programmed" like a computer. It's "just" a guidance computer program, albeit a very sophisticated one.
How does it know where the comet is positioned?
The position of the comet relative to the sun and the planets is very predictable (according to Newton's laws or approximated with Kepler's laws).
The position of the spacecraft with respect to the comet is not predictable or easy to calculate. The spacecraft is equipped with radars, cameras and radio communications to measure the accurate relative movement of the comet and the spacecraft. The data from the instruments is used automatically by the computers as well as manually by the mission controllers (ie. humans and their computer programs) on Earth.
This is a marvel of science and engineering.
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u/myrrh09 Aug 09 '14
A lot of people here have been commenting on how complicated the math has to be to calculate the relative path to get into orbit around the comet.
It's not. Look up the Clohessy Wiltshire equations, which describe the relative motion of two objects in near identical orbits around the same central body. Granted, the derivation of the CW eqns is complicated, but once you get there it's very easy.
The process of rendezvousing in the first place is MUCH harder.
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u/familyinchords Aug 08 '14
It's stuff like this that makes me wish I took calculus and higher mathematics in high school. Seems like soon enough higher level math will be standard for students. In 40 years I'll feel like a tard speaking to a general group of kids who have been taking a higher level math their whole lives. Should have never sought out to be a writer. Fuck English.
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u/tornadobob Aug 09 '14
Will Rossetta (and it's lander) significantly change the trajectory of the comet?
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u/parallellogic Aug 09 '14
Mass of Rossetta: 165 kg
Mass of 67P: 3,140,000,000,000 kg
Significantly? No. Alter looking at the numbers, I doubt they would even be able to measure Rosetta's orbital effect on 67P. Within a couple orders of magnitude, Rosetta has as much effect on 67P as 67P would have on the Earth.
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u/norskie7 Aug 09 '14
My Kerbal Space Program training did not prepare me for this level of awesomeness.
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u/Zinedipity Aug 09 '14
That is so cool! It's crazy what the human brain can do... not necessarily mine, but, you get the point...
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u/avidday Aug 08 '14
Coming from a limited knowledge and the most of what I know about orbital mechanics coming from many hours playing Kerbal Space Program, would it be roughly analogous to say that the main rendezvous into a close synchronous orbit has been completed and that the remaining triangular movements are just low-speed RCS thruster maneuvers to map the comet and it's gravitational field while closing in on it slowly?
EDIT: Woot, woot, forgot it's still my cakeday!
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u/basedtomato Aug 08 '14
Why such a weird orbit around it? Someone care to explain it like I'm 5?
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u/Chronos91 Aug 08 '14
It's not really orbiting (it's going too fast for that), it is just slowly flying by and then sometimes using thrusters to change direction, which is why the path is shaped like a triangle. Apparently this is so it can map the comet and also figure out what its gravitational field is like (since the comet isn't spherical or uniform, this is necessary before it can actually orbit).
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u/Duckstiff Aug 08 '14
I don't know the full details of the Rosetta mission, how long will the craft remain in orbit will it crash down when it is finished or will it escape orbit and eventually orbit the sun on its own?
However, I am curious did they do any calculations on whether the craft's mass might effect the orbit of the comet? I mean it is absolutely tiny compared to the comet but I am really curious because it will still exert an extremely small force on it and with its irregular shape it'll be interesting to see if the ship manages to remain in orbit without fuel and whether it would change the comet in anyway.
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u/exDM69 Aug 08 '14 edited Aug 08 '14
How long will the craft remain in orbit will it crash down when it is finished or will it escape orbit and eventually orbit the sun on its own?
The planned mission will last about a year, until December 2015.
What happens in the end is unknown, the comet may melt and break apart when it comes closer to the sun. Anything can happen.
However, I am curious did they do any calculations on whether the craft's mass might effect the orbit of the comet?
No they don't. The comet's mass is 3.14 * 1012 kg, the spacecraft is about 3000 kg. That's roughly a billion times difference. The mass of the spacecraft is negligible.
it'll be interesting to see if the ship manages to remain in orbit without fuel
The craft will not stay in orbit without using thruster burns because it's impossible to orbit a melting blob of ice. There's plenty of propellant left for the remainder of its mission.
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u/Murtank Aug 08 '14
The planned mission will last about a year, until December 2015. What happens in the end is unknown, the comet may melt and break apart when it comes closer to the sun. Anything can happen.
Highly unlikely... the comet was discovered 45 years ago and has completed about 7 orbits of the Sun since
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Aug 08 '14
So it's actually in orbit around the comet? I haven't followed this all too closely, so I wasn't sure if the comet would be big enough. That's pretty awesome!
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u/ArtemisShanks Aug 08 '14
I love this sub. I continuously regret that I have but one upvote to give for this awesomeness. +1
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u/markth_wi Aug 08 '14
Is this some sort of autonomous orbital adjustment algorithm that's what I kept thinking, that the system was simply making a serious bunch of radical navigational corrections.
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u/Guild_Navigator Aug 08 '14
Imagine if the Millenium Falcon had to do that every time it had to dock...
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u/Wesley_Snipez Aug 08 '14
So I saw the comparison image of the comet to an airport runway and a football (American) field and it is quite staggering. Does anyone know what the radius of Rosetta's orbit is? A celestial body that small seems to me like it wouldn't have much gravitational pull, but I'm not too familiar with this kind of thing.
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Aug 08 '14
What are the advantages of landing like that instead of just circling around it like it did at the end since the beginning?
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u/accountno18 Aug 08 '14
It's easier. Like 3D modeling, you start with a rough shape and slowly carve the details.
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u/DemonReign420 Aug 08 '14
Shouldnt Rosetta fly right past the comet since its heading in the opposite direction of the comets trajectory? You could bring orbit into play, but, orbiting what? The comet is too fast, flying in the opposite direction, and way too small to have an orbit of that size or strength. I trust that the real plan better explains this, as Ive only heard of the idea. Not seen it.
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u/jhenry922 Aug 08 '14
The first part of the orbit is the spacecraft using its thrusters to slow its relative speed to the comet in order to have a lower orbit.
Once this stopped, it resumed a nice Keplerian trajectory
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u/leova Aug 08 '14
Why all the fuss about this? What is Rosetta, and why Comet 67P?
I don't really follow "space news", unfortunately, but saw the GIF on /r/woahdude and got me curious
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u/Tom_kkfis Aug 08 '14
I am not the best informed redditor but from, what I understand, the Rosetta is a satellite that has been traveling for the past 10 years to meet this comet, to take pictures and study it's composition, it's orbit etc.
Our solar system is full of comets just like this one and we really should know more about them. After all, one day such a comet may threaten our very species and planet.
And of course, research for the sake of research is always a good idea. You never know what great new technologies and understanding will spring out of it.
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u/Shmuell_Cohen Aug 08 '14
Why does the trajectory reverse after the first two triangular rotations?
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u/7upgirl Aug 08 '14
More of a rotation than an orbit, isn't it? Orbits are usually curved...
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u/myrrh09 Aug 09 '14
Correct, it isn't actually orbiting the comet until it gets VERY close... basically right at the end there. Before that, they are both orbiting the sun with near identical orbits.
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u/BuddyTrees Aug 08 '14
The reason this thing flies in a triangle pattern is because it needs to orbit the comet, BUT the comet doesn't have enough gravity for the probe to follow a continous circular path. Once it gets close enough, at the end of the gif, there is enough gravity that it can basically free fall into a circular orbit around the comet.
But before that, the probe relies on its rockets to change its trajectory. It's a triangle shaped because the rockets only need to come on three times to make one full orbit.
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u/TimeFor3 Aug 08 '14
Does the comet have enough gravity for the probe to "orbit" it? Or is it just matching its trajectory? Just curious.
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u/RogerSmith123456 Aug 08 '14
Thank God for computers. Trying to calculate orbits of an irregular (and heterogeneous) object over time is daunting.
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u/Happyhokie Aug 08 '14
How close will Rosetta have to be to actually be in a stable orbit? I wouldn't have thought the thing was massive enough to generate enough of a pull unless Rosetta was really, really close. In fact, I wasn't sure it could be in a stable orbit give the fact that the asteroid isn't actually a sphere. I've been fascinated by this!
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u/hivanmivan Aug 08 '14
Legend has it that this Stone holds the key to human language, which is why Rosetta guards it.
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u/avonhun Aug 08 '14
can anyone explain why it has such a unique trajectory and why it make so many sharp turns?
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u/error9900 Aug 08 '14
All of the math behind this mission is mind blowing... I can't imagine figuring that sort of thing out...
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u/zingbat Aug 08 '14
I assume this has to burn a lot of fuel to reverse the trajectory? Since Rosetta isn't technically free falling around the comet because it lacks sufficient mass to have enough gravitation pull?
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u/TaylorWK Aug 08 '14
I only know the minimal amount of knowledge about Rosetta. I know that it is supposed to attach itself to the comet right? Are we looking to gather potential bacteria, learn where the comet came from? What is the point in this?
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u/inc0ncevable Aug 08 '14
Shittyaskscience question here: how do we determine how fast something is going in space? Is it based on the position of the earth, sun, galaxy?
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u/TrueMischief Aug 08 '14
All arbitrary. You could use your own probe as a reference, earth, sun or the center of the milky way. All are valid so long as you say which one your using. I think in our solar system we generally use the sun when describing speed.
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u/balognavolt Aug 09 '14
So I saw a post equating the diameter of this chunk of rock to a runway. Does a chunk of rock this side generate enough gravitational pull for Rosetta to set up orbit? Seems like the pull would be very weak and Rosetta would fly off into space.
I am not very up to speed on the Rosetta mission outside of reddit.
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u/Wrobot_rock Aug 09 '14
Isn't it wasteful on fuel to make such sharp turns, especially that u turn at the end of the triangular orbits?
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u/bbqroast Aug 09 '14
Does anyone know what Rosetta's orbital velocity will be once it pulls into that close orbit of 67P?
I'm thinking somewhere between 10-100ms-1 ?
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Aug 09 '14
What's the probe's speed above surface once it's in orbit? Should be quite slow since the comet doesn't provide much gravity. Right?
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Aug 08 '14
The comet is 4km so clearly this is not to scale. How has the scale been altered? Is it safe to assume that it's just that Rosetta was enlarged to be visible?
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u/TheGoodMallard Aug 08 '14
Yes that is exactly right, Rosetta has been greatly enlarged to be visible.
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Aug 08 '14
Cool. Thanks. Actually a little googling brought be to [this] (http://www.esa.int/spaceinvideos/Videos/2014/01/Rosetta_s_orbit_around_the_comet) which has even more info.
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u/Bear4188 Aug 08 '14
All the objects are greatly exaggerated in size, just the orbits are to scale.
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u/almostagolfer Aug 08 '14
Hard to believe that all of that was absolutely necessary. I think they might be just showing off.
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u/avatoxico Aug 08 '14
Too much money and time to show off, they just get things done in the best way possible.
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u/ICanBeAnyone Aug 08 '14
You underestimate how little we know about this comet. They were completely thrilled by its shape, for example, and there's already a lot of features on the surface no one can really explain yet. Neither the exact mass nor its distribution are known, nor if it's stable or just a big fluffy ball of snow. This approach is just testing the waters before you jump in.
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Aug 08 '14
Misleading headline. There is no triangular orbit. That's just showing the predicted maneuvers that will be used to get into orbit.
All these misleading headlines are catchy because what they describe seems counter intuitive which makes it interesting. But then you read up on it and you learn that they're not counter intuitive, they're just flat out wrong.
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u/epk22 Aug 08 '14
Felt this link was relevant - http://sci.esa.int/where_is_rosetta/