Rosetta probe on it's way to a comet and will attempt to insert into orbit around it.

I just started reading up on the Rosetta mission and I got to thinking about the challenge of orbital insertion.

here's the official mission website

Here is an overview on the mission

I was curious how the team of scientists would go about calculating the mass of the target object. This is very critical if you want to catch up to and orbit this object in any stable way. It's very small and obscured by the usual veil of gasses and dust that are characteristic of comets. So it's really hard to calculate volume and mass. Have to do it though

So they came up with 102kg/m3 for the density? Really? That's like the density of very dry, loosely compacted snow. A block of ice has a density of about 1000kg/m3. Graphite is 2000kg/m3. I'm not saying it's wrong of course, but if they are close to correct, it' really is a dusty snowball with only a minimal amount of dust at that.

So after you arrive at a value for the mass, you then have to model all the stuff spraying around and rotating, wobbling etc. It occurred to me that this orbital insertion has a tremendous amount of uncertainty. You only get one chance. If it works, it is a big win for the conventional model of comets. I'm concerned that we are undershooting the mass which would mean the probe would come in hot and miss. I believe there is a window of trajectories that would succeed, but if the estimate is way off, it will be a huge miss for conventional comet science.

Anyone have any thoughts or read anything interesting about calculating the mass of this comet?

Paper showing how they did it.

reply to post by InverseLookingGlass


www.abovetopsecret.com...

Older thread

Different questions

InverseLookingGlass
So they came up with 102kg/m3 for the density? Really? That's like the density of very dry, loosely compacted snow. A block of ice has a density of about 1000kg/m3. Graphite is 2000kg/m3. I'm not saying it's wrong of course, but if they are close to correct, it' really is a dusty snowball with only a minimal amount of dust at that.

They did not have to guess. We have had a good idea of the type of material comets are made from since shortly after the link between comets and meteor showers was made back in 1871 after the great Leonid meteor storms of 1833, and 1866.

Photographic studies of meteors eventually told us that cometary meteors are composed of light fluffy material that quickly "burns up" in the atmosphere, and that asteroids are usually composed of harder/denser material that occasionally makes it down to the ground in the form of meteorites. Meteorites (and especially meteorites who's entry into our atmosphere was also photographed) eventually gave us the final clue since we then had samples of material of known density/composition/structure to act as a baseline that could then be compared to photographs of known cometary meteors.

This paper covers at least the earlier aspects I mentioned, but you should be able to find any more info you are after at the same site if you search it:
The Scientific Study of Meteors in the 19th Century

reply to post by FireballStorm


You made me read a paper summarizing meteor science from the mid 1800's. That's when they were still debating that meteors were from extraterrestrial origin. pffft.

I don't know how you ended up with those totally unrelated comments from that. Try reading the paper I linked (and the one you linked) if you want to understand my OP.

InverseLookingGlass
I was curious how the team of scientists would go about calculating the mass of the target object. This is very critical if you want to catch up to and orbit this object in any stable way.

Catching up to the comet is no problem; its location at any one time is determined solely by the celestial mechanics, which have been figured out hundreds of years ago. Robotic spacecraft have visited comets before. As for orbiting the comet itself, it's trickier, but I'm sure the mission control have it figured out. The comet's gravity is extremely small, so it's not like Rosetta will crash into it if there's not enough velocity. Since we know the position of the comet, we can make a careful approach using a spacecraft. Both the comet and the spacecraft will be orbiting the Sun together, with Rosetta's orbit being only slightly adjusted so it slowly orbit the comet.

InverseLookingGlass
reply to post by FireballStorm

 

You made me read a paper summarizing meteor science from the mid 1800's. That's when they were still debating that meteors were from extraterrestrial origin. pffft.

The point of me posting that paper was to show you where it all started - and the science that led up to us knowing about the densities of cometary material. Sorry if that was not entirely clear, but I was just trying to help you answer your questions.

InverseLookingGlass
reply to post by FireballStorm

 

I don't know how you ended up with those totally unrelated comments from that. Try reading the paper I linked (and the one you linked) if you want to understand my OP.

The "unrelated comments" I posted were in fact a brief outline of how we came to know about the density of cometary material - I did not say it was related to that particular paper. I did say if you want further detail that you should search the same site for relevant papers, like these:

Abstract
Using a physical theory of meteors and on the basis of the results of double-station photographic observations of meteors in Dushanbe (Tajikistan), Kiev, and Odessa (Ukraine), the mean mineralogical and bulk densities of meteoroids belonging to nine meteoroid streams and sporadic background are determined. The mean mineralogical densities δm of meteoroids range from 2.2 g cm-3 (Perseids) to 3.4 g cm-3 (Quadrantids, δ-Aquarids, and α-Capricornids). The meteoroid bulk densities δ, which were determined according to the theory of quasi-continuous fragmentation of meteoroids in the Earth's atmosphere, vary from 0.4 g cm-3 (Leonids) to 2.9 g cm-3 (Geminids). Using the relation between bulk density and mineralogical density the porosity of meteoroids was estimated. The Geminid meteoroids are found to have the lowest porosity, while the Leonid and Draconid meteoroids have the most porous structure (83%). These results confirm the porous-structure nature of meteoroids' parent bodies i.e. comets and asteroids.

Source: Densities and porosities of meteoroids

Abstract
This paper is a review of the present knowledge on the structure of meteoroids. A summary of the evidence concerning the common occurrence of fragmentation among both photographic and radio meteors is given first. Then, an attempt is made to examine all the present observational, theoretical and laboratory data on the luminous and ionizing efficiencies of meteors, with the aim of establishing a mass scale. This allows the computation of the bulk density of meteoroids, which, on the average, turns out to be about 0.3 g/cm3. The paramount importance of progressive fragmentation, the behavior of abrupt-beginning meteors and the low density of nearly all meteoroids (even of those of relatively large sizes) support a porous and fragile structure for most of these particles. In turn, the crumbly structure and the cometary origin confirm Whipple's theory of comets and meteor production. A critical analysis of recent papers proposing different conclusions shows that the new theories always arrive at results which do not agree with well-established observational data.

Source: Structure and Fragmentation of Meteoroids

Abstract
The phenomenon of meteoroid fragmentation in the Earth's atmosphere was recorded repeatedly by means of different methods and especially using the photographic technique of instantaneous exposure. Among the four principal forms of fragmentation, the quasi-continuous fragmentation, i.e. a gradual release of the smallest fragments from the surface of a parent meteoroid and their subsequent evaporation, is most common. The analysis of photographic observations shows that a substantial fraction of meteoroids is exposed to this type of fragmentation. According to the theory of quasi-continuous fragmentation and on the basis of light curves of meteors photographed in Dushanbe (Tajikistan), the mean bulk densities of meteoroids belonging to six meteoroid streams and the sporadic background have been determined, which vary in the range from 0.4 g cm-3 (Leonids) to 2.9 g cm-3 (Geminids).

Source: Fragmentation and densities of meteoroids


I guess this is why I no longer post on here much any more - try to help someone and it gets thrown back in your face without even a "thank you", and people on here wonder why they are not taken seriously. pffft.

The comet's gravity is extremely small, so it's not like Rosetta will crash into it if there's not enough velocity.

reply to post by wildespace


I was just thinking if there is more gravity than they expect, they would accelerate and end up on an escape vector rather than crashing.

I guess I'm wondering how big the window is that they can adjust to and insert. I'm convinced this might be the definitive test for the "homogenous dusty snowball" model of comets. Failing to insert could mean a lot of things and one of the possibilities is that the mass estimate was way off. It's the closest I'll ever get to comet science.

Fascinating stuff. Looking forward to the observations.

Any physics buffs here could explain in simple terms how the comet's mass and density was found, based on this paper? www.lpi.usra.edu...

As for it possibly being much heavier (such as being made of solid rock), I don't think it would affect Rosetta to any large degree. The comet's gravity will still be extremely small.

reply to post by InverseLookingGlass


They have actually had a lot of practice ("They" being different space agencies around the world). Here is a list of cometary missions.

Each time a space craft flies by a object in space, telemetry from the space craft can show how and if it's path has been affected by any gravity from that object.

Once you know what the gravitational acceleration is from that data, and if you know the size of the object, you can then determine it's density and mass.

Having encountered 7 different comets successfully, the telemetry from the crafts gives us a good idea of what sort of mass and density to expect, based upon a comet's size.

Now, it is possible for a comet to have ratios of material that will make it's density and mass greater or smaller, however because we are dealing with such a small object (small as in compared to a moon or planet) changes in the gravitational acceleration will be small, meaning the craft has time to also make small changes to ensure proper orbital insertion.

The main problem would be if the comet was made from extremely dense material (IE degenerative matter), or some sort of space craft failure to execute commands. The latter is much more likely of course.

a reply to: InverseLookingGlass

Brilliant!

Earth sucks these days but damn the ESA geeks are doing some great work.

I'm intrigued with the calculated low density of this comet, too. If it's mostly frozen water, as expected, it must be something like fresh snow, as noted in the original post. Comets are typically about six times as dense as this. I am wondering how something like snow could form on a small, airless body in space. On Earth, snow forms as large hexagonal crystals within, and influenced by Earth's atmosphere.

originally posted by: Ross 54
I'm intrigued with the calculated low density of this comet, too. If it's mostly frozen water, as expected, it must be something like fresh snow, as noted in the original post. Comets are typically about six times as dense as this. I am wondering how something like snow could form on a small, airless body in space. On Earth, snow forms as large hexagonal crystals within, and influenced by Earth's atmosphere.

I'm pretty sure there's much more to come.

I mean we have like what? 20 or 30 years history of new models of how solar systems evolve, including planets, asteroids, comets and all the stuff in between.

I think Pluto/Charon will also surprise us at a closer look .