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Europe's Rosetta probe goes into orbit around comet 67P (bbc.co.uk)
234 points by NickPollard on Aug 6, 2014 | hide | past | web | favorite | 73 comments

You can watch the live stream here http://rosetta.esa.int/

The detailed explanations and behind the scenes looks are really interesting, meeting such tiny piece of rock 400M km away is just crazy.

Sometimes it's also funny (one of the scientists was showing a slow mo video with seemingly nothing happening "well, you got time, right? WE had to wait for 10 years")

EDIT:it's over

> The detailed explanations and behind the scenes looks are really interesting, meeting such tiny piece of rock 400M km away is just crazy.

What makes this even more crazy is that the space craft cruised 31 months powered down, from Mars to beyond the orbit of Jupiter and then back. The precision required to accomplish this is tiny!

The mission controllers must have had a pretty good confidence before giving the ok to power down.

Huge accomplishment and magnificent engineering. Hats off to the guys in ESOC and other ESA facilities.

Is the precision tiny or huge? :>

I think "great" is the word. Tiny margins/tolerances, but not precision. Not that it really matters.

Even if it doesn't matter: "exquisite".

Physics: It just works (TM)

I watched ESA's livestream this morning and they explained that Rosetta is actually just co-orbiting the Sun with 67P as of now. They're making it slowly drift to a few different directions and measuring the acceleration caused by the comet's gravity, so they can build an accurate model of the mass and density distribution of such an irregularly shaped object.

The hardest part is not so much modeling the comet's gravity, but modeling the aerodynamic effects since at this distance from the Sun outgassing is already taking place at a substantial rate.

It's a big blow to the flight dynamics team that the copyright arrangement for the pictures from some of the onboard scientific cameras makes it impossible for the FD team to use them. Sponsors of the instruments keep denying the permission to guarantee that no competing science and publications can be made before they publish.

So much for putting advancing scientific knowledge first, as opposed to making sure you're the first to publish something.

A very bad side effect of scientists only caring about published results, not acquiring the raw data needed for those published results.

All the incentives in the current scientific system are pushing scientist that way. It is sad, but I think it is a bit unfair to say that the "scientist only care about published results". That is a little like saying a musician only care about money because they copyright their songs and publish on iTunes. I think both scientists and musicians care a lot about music and science, despite how the current system works.

With the masses of the comet and the spacecraft being relatively close, it's hard to tell whether Rosetta is co-orbiting the sun or orbiting the comet itself.

> With the masses of the comet and the spacecraft being relatively close

No, they're not close at all. The comet is three by five kilometers across, and the mass is around 3.14 * 10^12 kg. The orbiter at launch weighs in at 2900 kg and the lander is 100 kg.

http://en.wikipedia.org/wiki/Rosetta_(spacecraft) http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimen...

I think jeroen94704 is saying "With the masses of the comet, and the spacecraft being relatively close"

[comma added]

I disagree. At the very least it would need to be "mass" not "masses". The closest statement should be more like "With the mass of the comet, and the spacecraft being relatively distant."

It's orbiting the sun, however its orbit is such that it will take it around the comet in a roughly triangular shape - but this isn't the same thing as orbiting.

This is my favourite part of the article:

The mission gets even more ambitious in November when, after moving Rosetta closer to 67P, mission controllers will attempt to put the Philae lander on the surface.

The lander will use harpoons to anchor itself and will carry out a series of experiments, including drilling into the material that makes up the comet.

The mission aims to add to knowledge of comets and their role in ferrying the building blocks of life around the early Solar System.

Harpoons. Very, very cool. I can't wait to find out more about this comet!

And the obligatory XKCD: http://xkcd.com/1402/

harpoons ;) yeah - http://www.xkcd.com/1402/

I watched the news on BBC. They are right now in an orbit 100 km away from the comet and getting pictures of 2.5 mt resolution. In the next few weeks they will reach upto 30 km orbit and take more high resolution pics.

The head of the mission mentioned about boulder like structures on the comet, origins of which is not knows. The comet also has vast plain areas, suitable for landing a probe. It will be by November when we get detailed results of the chemical composition of the comet.

> On 6 June 2014 water vapor was detected being released from Churyumov-Gerasimenko at a rate of roughly 1 litre per second


I find that pretty amazing! That's a lot of water coming off one comet just being sprayed everywhere.

The comet is pretty big. I don’t know if you know London, but this image posted on the BBC earlier put it into perspective: http://ichef.bbci.co.uk/news/625/media/images/76758000/jpg/_...

The probe weighed in at 3,000kg at liftoff back in 2004, with over half of that made up of propellant

Wow I am starting to understand why they are so interested in that propellant-free drive.

Does the comet actually have enough gravity to make an orbit happen or is the orbit completely artificial?

My guess: The probe won't orbit the comet, but rather orbit the sun in sync with the comet. That too would be artificial because the comet's mass and the probe's mass will keep changing (presumably at different rates). So the probe would need to catch up every once in a while.

> Does the comet actually have enough gravity to make an orbit happen or is the orbit completely artificial?

Yes it does.

Rosetta is now at 100 km distance while they are trying to figure out the mass and the mass distribution of the comet (unknown at this time!), and will eventually settle in a 20 km orbit, finally 10 km, around the comet.

This isn't going to be easy, see elsewhere ITT for some detailed discussion.

Bit of both. Rosetta will enter a hyperbolic orbit around P67 at 100km of altitude but it will have to use its thrusters on a regular basis to adjust its trajectory.

Rosetta will eventually settle in a 20 km orbit around the comet tail.

It will still have to use thrusters because orbiting an small irregular blob of melting ice and rock isn't going to work without precise maneuvering.

That's a great article. The author said "invested" rather than "spent" a billion euros, I loved the visualization [0] of how big the asteroid was, and they kept the article not too technical as to bore the audience but technical enough to interest the more science-minded readers.

[0] http://news.bbcimg.co.uk/media/images/76758000/jpg/_76758586...

This is the first step in developing the techniques required to sustain life in space without having to haul everything up Earth's gravity well. If we can get good at this, then it changes the entire economics of space.

What techniques are you referring to specifically?

I assume he's referring to harvesting of resources in space - e.g. comets contain large amounts of (frozen) water, so one way of getting water into space could be to send small spaceships up to harvest it from comets, rather than big spaceships full of water that is then costly to power up out of earth's gravity well.

Of course, how you get that water to the target is another matter - getting it to a space station could be hard, but maybe you could pull chunks off the comet and send them crashing into, say, Mars. This might be a way of 'terraforming', to get water into the atmosphere, maybe even allow some of it to collect into small lakes, then you send some probes over with plants to start growing, bootstrap your way up.

I am guessing OP is referring to the ability to reliably orbit and land on comets (and asteroids). We can use them for manned way stations or construction platforms. Not to mention mining them for whatever useful stuff may be in there.

If we could show that comets have available surface water ice in the right form, then their gravity is low enough that you could plan to drill it off and melt it to be fed into water-ion thrusters. That would let you at the very least launch a mission which could study one comet, refuel, then slingshot away to a different target.

Hunting, fetching and carrying from low mass objects. If we can get a stream of cheap things doing this, then we get rivers of stuff.

I feel like I'm in a new era where more people care about space again, but the reality is probably more that the "right" people care about space, so now more is happening.

When did this era start? This probe was launched 10 years go, and the planning must have started at least a few years before that.

after the 90s.

Hmm so when were was there a lull? Serious question. I grew up in the 80's and although I don't check space news every day it doesn't seem like activity in space decreased in that time. All I'm really aware of is the loss in interest in, say, manned moon missions after Apollo 11. Are you talking about robotic probes?

Maybe it's the fact that I can actually watch all the launches now, and things are making the news. I'd never hear about the launch of anything in the 90s. It was all too mundane or routine to even get a note in the paper apparently.

I wish I could upload my brain to something like Rosetta & blast into the space, travelling around space bodies, planets & glaxies, gather the information & transmit back to space for the other Rosettas.

What more it'll take to create an artificial intelligent life which can sustain herself almost everywhere?


For those who don't get the reference, it's a fantastic book by Charles Stross that you can get for free from his website:


Thanks for that link, did not know that was freely available! It's a blast to read.

This is incredible.

Wait.. I thought it said the live airing was happening at 13:00 CEST today?! did I miss it?! NOooo!!

Rosetta project is incredibly inspiring.

If you are ever stuck trying to name a variable, fire up your browser, look at Rosetta's trajectory and let the fact that it's a flawlessly-executed ten-year billion-kilometer plan to catch up with a fleck of rock in space sink in. Then go back to your coding :)

If anyone feels like planning a trajectory like Rosetta's or, more precisely, writing a computer program to plan a trajectory, ESA's Global Trajectory Optimization [0] contest is for you. It's dubbed the "America’s cup of rocket science".

The past contests and their entries are quite interesting, ranging from mapping the whole surface of the Galilean moons of Jupiter (Io, Ganymede, Callisto, Europa) or doing an asteroid hopping mission in the main asteroid belt.

I might participate in the next one.

[0] http://sophia.estec.esa.int/gtoc_portal/

>'Then go back to your coding...'

...where you imagine that you actually are working on trajectories - at Gattaca Aerospace [1].

(Great movie, one of my favorites.)

1: http://en.wikipedia.org/wiki/Gattaca

My friends at work proposed to have one of us each day doing nothing just helping others when they need a name for something.

It's the hardest thing in programming, and the fact that we do it in foreign language doesn't help.

> Then go back to your coding :)

How could I possibly?

I assume that it isn't actually orbiting the comet, but instead is just adjusting to follow the same orbit as the comet, right?

It's being described as a "triangular orbit" :-)

"Rosetta will, follow, at least for now, a three-legged triangular orbit that requires a small thruster burn at each apex. The legs are about 100 km long and it will take Rosetta between three and four days to complete each one."


Here is a YouTube video from ESA demonstrating the orbit. It will be performing these small thruster (in the order of centimeters per second velocity change) burns to stay ahead of the Comet while doing fine measurements of the acceleration caused by the gravity of the comet.

When the mass and mass distribution has been established, the spacecraft will eventually settle in an orbit about 20 kilometers from the comet and finally down to 10 kilometers.


It looks like it's only in a triangular orbit while approaching but it will eventually be in an elliptic orbit.

> It looks like it's only in a triangular orbit while approaching but it will eventually be in a elliptic orbit.

Yes, exactly. But since the comet is highly irregular in shape and mass distribution (both are still unknown at this point!), the orbit won't be a perfect ellipse, not even close.

Does the shape and mass distribution even matter? It's going to have a center of mass, so once that's located, shouldn't the orbit be close to a perfect ellipse?

Yes, mass distribution and shape have a huge effect in the orbits.

For example, the perturbations from Earth's equatorial bulge causes the perigee of an orbit to shift eastwards (see: Molniya orbit). The Moon's gravity field is very irregular and orbiting the Moon at low altitude is just black magic.

But it will be a stable orbit, right?

The issues is that, technically, orbits aren't stable in the sense of being described perfectly as ellipse with a point mass at one focus.

That approximation can describe an orbit for a while, but after time the position will differ from the predicted orbit, until at some point you have very little idea of where it will be.

The time it takes for this to happen is called the Lyapunov time. For example, the Solar System is chaotic that way in the ~100 MY time scale. See http://en.wikipedia.org/wiki/Stability_of_the_Solar_System .

A Rosetta goal now is to figure out the comet's mass distribution, so that the orbital predictions can be better. The orbit won't be "stable" but it will be more "predictable."

Even then, we expect Rosetta to release gas, dust, and water vapor which affects the orbit. I expect there will be orbital corrections over the next few years.

tl;dr: no, it won't be a very stable orbit for very long.

One definition for the "stability" of an orbit is that if you give the orbiter a small nudge, it will not radically change its orbit. Lagrangian points L4 and L5 are not stable (with this definition) while L1, L2 and L3 are (only in theory, though, in practice perturbations matter a lot).

Since the mass distribution is unknown, we can't quite predict if the orbit will be stable or not. The mass distribution may be very uneven and there may be spikes in the distribution (due to shape and porosity of the comet) that could cause the orbit to be unstable. The comet also rotates and makes things more difficult.

Now the goal of the crew is to find an orbit that is reasonably stable and predictable given the knowledge of mass distribution they have. Nevertheless, the space craft will probably have to do thrust maneuvers to stay in orbit around the comet.

And once the comet comes close enough to the sun and starts to lose mass to the vapor trail, all bets are off. That almost certainly implies some kind of unstability due to mass changes as well as the drag from the vapor trail. The comet may even break down to several pieces when it comes close to the sun.

Another definition of "stability", perhaps more relevant here is: if you complete an orbit without crashing into the body you're orbiting, does that guarantee you won't crash into it later?

When orbiting a uniformly-dense sphere in an otherwise empty universe, this holds. When orbiting a body that approximates a uniform sphere in a universe where all other large bodies are relatively far away, this mostly holds. But significant deviations from a spherical shape can easily make most orbits unstable.

Little known fact: the Earth's Moon deviates enough from a uniform sphere to make long-term orbits tricky. Wikipedia article:


Nice bit about a small satellite released by Apollo 16:

"Instead, something bizarre happened. The orbit of PFS-2 rapidly changed shape and distance from the Moon. In 2-1/2 weeks the satellite was swooping to within a hair-raising 6 miles (9.7 km) of the lunar surface at closest approach. As the orbit kept changing, PFS-2 backed off again, until it seemed to be a safe 30 miles away. But not for long: inexorably, the subsatellite's orbit carried it back toward the Moon. And on May 29, 1972—only 35 days and 425 orbits after its release"—PFS-2 crashed into the Lunar surface.

And this is all without any of the additional trickery coming from outgassing.

It's not so much the shape of the moon but the mass concentrations. The moon has significantly stronger gravity over impact basins. Based on shape alone (low plains) you'd expect lower gravity there.

If a comet breaks up into several pieces, will those pieces continue to travel next to each other in the same direction, maybe re-combining via gravity back into a single rock? Or will they just drift apart?

Depends on how fast they are ejected from each other. The pieces will (more or less) have an "escape velocity". If they are faster than this velocity they will drift ever further apart. Slower, and they will slowly fall back towards each other.

My understanding is that it's unlikely that they would ever recombine into a solid object since the gravity between them is so weak.

What a fascinating, seeming convoluted (but clearly intentional and purposeful), flight path. Fascinating stuff, and it's amazing it has that much residual thrust available.

AFAIK they're only doing it for a while to measure 67P's mass and density distribution, so they can calculate a "real" orbit for Rosetta later (but it will still require regular adjustments due to the highly irregular shape of the comet).

> and it's amazing it has that much residual thrust available.

We're not talking about propellant residue, that was very well planned ahead.

The spacecraft carried about 1.5 tons of propellant at launch. That gives the spacecraft a delta-v budget that is several kilometers per second.

The next thruster burns (on the triangular "hyperbolic orbits") will be tiny, in the order of centimeters per second.

It's moving under power in a trajectory around it at the moment. I would call that orbiting, even if it's not due to the gravitational pull of the comet.


That's correct. And because the orbit forces it to slowly overtake the comet, it needs to slow down periodically to stay at 100km of it.

> That's correct. And because the orbit forces it to slowly overtake the comet, it needs to slow down periodically to stay at 100km of it.

No, not quite. Remember, we're talking about a three body problem here so any intuition from two body dynamics doesn't really hold.

The spacecraft travels 100km closer to the sun than the comet, but that's a very small fraction of the total distance. When you factor in the small acceleration from the comet body, we can no longer think of the orbit as being a slightly smaller orbit with a shorter period (and thus overtaking the comet).

My best understanding is that the triangular "orbit" is to measure the mass and gravitational properties of the comet in order to orbit as well as scientific goals.

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