Rosetta Achieves Orbit Around Comet 54
schwit1 (797399) writes with an update on the European Space Agency's comet-exploring craft Rosetta: "Rosetta has successfully achieved orbit around Comet 67P/C-G and has transmitted its first close up images. More information here (1) and here (2) about the rendezvous and what science the mission scientists plan to do as they orbit the comet." As pointed out earlier by reader Taco Cowboy, this is the fruit of a 10-year mission. Reuters points out
The mission performs several historical firsts, including the first time a spacecraft orbits a comet rather than just whizzing past to snap some fly-by pictures, and the first time a probe has landed on a comet. ... There is little flexibility in Rosetta's schedule this year. The comet is still hurtling toward the inner Solar System at almost 55,000 km per hour, and the closer it gets to the sun the more active it will become, emitting gases that can make it difficult to predict the trajectory of Rosetta and its probe.
To everyone involved (Score:1)
Thank you!
NavCam animation 6 August (Score:5, Informative)
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Ha, looks like a slightly more sophisticated versioin of Elite. Interesting to think that it's a glimpse into what future asteroid miners will witness as they approach ore bearing bodies.
Doesn't an orbit require gravity? (Score:3, Insightful)
Correct me if I'm wrong, but based on the little I learned from KSP, I don't think anything can reasonably get an orbit around a comet due to its lack of mass.
Re:Doesn't an orbit require gravity? (Score:5, Informative)
You are correct. The gravity is insufficient. So I looked it up.
They will basically "drive" the probe around the comet, firing thrusters as needed. After a bit, they will "drive" it onto the surface, then:
"As Philae touches down on the comet, two harpoons will anchor it to the surface; the self-adjusting landing gear will ensure that it stays upright, even on a slope, and then the lander's feet will drill into the ground to secure it to the comet’s surface in the low gravity environment. Philae carries 9 scientific instruments, including a drill to sample subsurface material." ( http://www.esa.int/Our_Activit... [esa.int] )
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Obligatory XKCD (Score:1)
Sometimes obligatory explainXKCD (Score:3)
Harpoons in space [explainxkcd.com] ...for those as confused as I about what the hell the "Apollo 12 Rum Incident" was all about. Hint: probably some kind of "noodle incident"
Obligatory Futurama (Score:3)
We carry a harpoon
But there ain't no whales
So we tell tall tales
And sing our whaling tune.
Re:Doesn't an orbit require gravity? (Score:5, Insightful)
you are wrong. This comet has 3 trillion kg of mass and escape velocity of 0.5 meter / sec.
Re:Doesn't an orbit require gravity? (Score:5, Informative)
Wrong, the comet has sufficient gravity to hold Rosetta in an orbit, thrusters will be used for corrections. Look it up facts of the matter on proper scientific website rather than the usual watered-down drivel people such as yourself normally read.
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Escape velocity of 1.5 ft/s is close to 1mph, I think.
That's a pretty delicate orbit.
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On the other hand, it is space. The list external forces to worry about are: gravity.
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Solar wind. Solar radiation pressure. Bits of gas and dust blowing off the comet and hitting you. EM. Lots of other stuff going on is space. "had the effects of the sun's radiation pressure on the spacecraft of the Viking program been ignored, the spacecraft would have missed Mars orbit by about 15,000 kilometers" [wikipedia.org]
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Yeah, okay. You're right. I thought about including solar wind, and tabulating how many mph it'd make per year, but decided it was too complicated and standing on false simplicity would make for a better point.
Being objectively wrong like that wasn't a "better point".
Sloppy reporting. (Score:5, Informative)
It is not yet in orbit. (or rather - at the moment, propulsive manouvers are dominant - you can technically say you're in orbit if you jump off the ground, and not be wrong)
Protip - orbits aren't triangular.
https://www.youtube.com/watch?... [youtube.com] - is a two minute animation from ESA explaining the manoevers.
10th sep - it begins its first orbit at 30km - and about 14 day period. After about half an orbit, on the 17th of Sep or so it is tilted 80 degrees and still remains in a 30km orbit.
After a complete orbit, it then moves into 20km orbit, and around Oct 10, 10km.
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Erm, it is an ESA probe not NASA...
Black and White....yes the photos are black and white, they're after quality of image not colour - that can be added later as the cameras sample different wavelengths....NB: this might be a good time to actually look up the specs of the imaging systems on Rosetta...
And did you really have to invoke Godwin's Law...even if really, really badly done.
Rosetta and its probe (Score:2, Funny)
"Rosetta and its probe"
What in the world is that supposed to mean? Sounds dirty.
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Rosetta carries within it a lander called Philae, which is intended to land on the comet in November. DLR is already picking out candidate landing sites - this video [youtube.com] shows the current set (each green dot is an error ellipse, which get stretched out over sloping terrain).
Philae will harpon itself onto the surface, and one task will be to serve as one end of a low frequency radar system with Rosetta - it should be possible to image more-or-less the complete inside of the comet with this system.
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It should be possible to image more-or-less the complete inside of the comet with this system.
What is meant by "inside" in this context?
In Orbit? (Score:1)
How close would the spacecraft have to be to the comet in order to achieve orbit? At just 2.5 miles long, I don't suspect its gravitational pull would be very significant.
Re:In Orbit? (Score:4, Informative)
It will use thrusters to maintain orbit, because the gravity is indeed insufficient:
"Rosetta will have to continue to fire its thrusters every few days to maintain a hyberbolic orbit at 100km above the rotating rock. "
http://www.bbc.co.uk/news/scie... [bbc.co.uk]
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Hyperbolic orbit? I'm afraid the BBC got that wrong. The plan is to eventually get to as close to a stable orbit as you can about an irregularly shaped lump that's moving on its own, and that orbit won't look anything like a hyperbola. Yes, it will still have to correct often, but eventually the plan is to maintain an elliptical orbit for as long as possible. Try looking at http://www.esa.int/spaceinvideos/Videos/2014/01/Rosetta_s_orbit_around_the_comet for the pretty pictures.
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Eventually, but not now.
Right now the are spending a few 100 gm of propellant per day or so to maintain these triangular orbits, and each leg is hyperbolic (well, probably actually parabolic, but you get the idea).
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This is something they are choosing to do. My understanding is this basically a safety thing. If the spacecraft went into a safe mode, in these slow flyby orbits it would keep moving slowly away from 67P, at maybe a km / hour or so, until the problem could be fixed. I heard that they didn't want to risk being in an unstable orbit, and maybe hitting the comet if they went into safe mode.
Re:In Orbit? (Score:5, Informative)
Today the comet is 186,444,271 km from the Sun Where is Rosetta? [esa.int]
Using F=GMm/R^2 [wikipedia.org], the Sun's gravity on Rosetta is equal to 67P's gravity on Rosetta at 700m from the center of Rosetta on 6 August 2014, which means that Rosetta will never really be completely within 67P's field. (At Perihelion on 13 Aug 2015, 67P's gravity field will be as strong as the Sun's only 250m from the centre) However, now that Rosetta is in the same orbit as 67P we can mostly disregard the Sun's gravity and the elliptical path that Rosetta and 67P now share as of today. (Earth's pull on Rosetta is at least a million times weaker than the Sun's pull - so forget any influence from the Earth's mass.)
The "orbits" at 100km are called hyperbolic [wikipedia.org] because Rosetta is not trapped in 67P's gravity well since the gravity is so weak and because Rosetta is still moving FAST at 1 m/s. But this hyperbola is so weak it is effectively a straight line.
Rosetta will turn 60 degrees after every 100 km of a hyperbolic path to make a triangular "orbit". This triangular path cannot be called an orbit because it is not a conic section [wikipedia.org], nor is the comet at a focal point of the conic section Kepler's First Law [wikipedia.org].
These "straight"/"hyperbolic" paths of 100km and 50km are deliberately done for two reasons:
-to calculate exactly the gravity field of the comet, because it is clearly not a uniform sphere. They will likely use radar&cameras to continuously measure the precise distance to the comet
-to keep in front of the comet to avoid its coma and tail.
After these maneuvers, Rosetta will go into a 30 km "orbit", so that the task of mapping 80% of the surface all happens from the same distance. This orbit is not natural and will be powered because a natural 30km orbit of 67P takes 26 days.
Here's how to calculate the natural circular orbits for 67P (it won't be circular, because of the crazy shape, but close enough). Kepler's 3 Law [wikipedia.org] gives us
T^2=4pi^2/GM*r^3. 4pi^2/GM=0.19 for this comet. G=6.67×1011 N(m/kg)2 [wikipedia.org]
if r=30km=3e4m, the natural orbit would have a period of T=2.3e6 seconds=26.11 days
If r=2.5km, the natural orbit would have a period of T=15 hours
If r= 5km, the natural orbit would have a period of T=1.77 days
If r= 100km, the natural orbit would have a period of 159 days So I could imagine that when Rosetta gets within 5km it is mostly using the natural orbit and hence saving fuel.
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Glad to see that you jumped in on this: good description.
Because the comet is so small, the gravity changes a lot with "altitude" from the surface. For a 2-km diameter sphere, say, then the difference in gravity between an altitude of 2-km and 6-km (i.e. between 4 and 8-km from the centre of the sphere) is a factor of 4. On the Earth, it barely changes at all between altitudes of 2 and 4-km, because this is a tiny change relative to the 6400-km radius of the Earth.
So, yes, at 100-km and 50-km, we'll b
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A 30 km orbit would take 8 - 25 days and have an orbital velocity of 0.1 - 0.3 m/sec, and would probably be stable.
Re:Why Black and White? (Score:5, Interesting)
Eleven years (Score:5, Informative)
Rosetta was originally supposed to launch in January 2003, to comet 46P/Wirtanen, and was all ready to go when problems with the Arianne 5 forced a delay its launch. That meant finding a new target and a total redesign of the mission, leading to a launch in March, 2003.
So, for the Rosetta team it has been the perils of Pauline since before the launch, and an 11 year mission to get to their comet.
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67P (Score:3)
They should rename come 67P to Rosetta Stone.
Ain't she a beautiful sight? (Score:4, Funny)
Half a billion kliks from the sun
I'd left Earth 'bout ten years ago
And was ready to have some fun
I'd buzzed past Mars and a coupla asteroids
And saw this comet goin' roun' and roun'
He says "Tin Can, this here's Rubber Duck
And I'm about to put the hammer down"
'Cause we got a little ol' convoy rockin' thru the night
Yeah, we got a little ol' convoy, ain't she a beautiful sight?
Come on and join our convoy, ain't nothin' gonna get in our way
We gonna roll this truckin' convoy 'cross the invariable plane
Convoyyyyy....
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