[X] My Assistant

[dilo]

According to the above report, it looks that it is almost impossible to remain fixed at a distance from Itokawa. This report shows a graphic about the position of Hayabusa in almost every day which varies. The Hayabusa teams is trying to control it by learning the controls adjustments. So, it is nothing easy. Lucks for Hayabusa's team

Rodolfo

The plot of Z-distance suggests a free-fall gravitational decay with periodic small chemical thrusts in order maintain an average distance close to 20Km. It's something like ISS periodic re-boosts, with the important difference that here we do not have an orbital movement with a decay induced by atmospheric drag but there is a simple free-fall acceleration. So let's do some newtonian calculation based on this idea! (if you don't like math, jump to final conclusions).
I concentrated on the 15-19 September time window, where a complete re-boost+parabolic free-fall seems to happens.
In fact, a parabolic curve fits very well with data ponts... (I'm making assumption that asteroid dimension is negligible and distance variation is small relative to absolute distance, both true in this case):

Based on curve fitting, the re-boost done around Sept 15.2 produced a vertical positive velocity of 1.6 cm/sec and was followed by a uniformly decelerated free-fall with a=-1.34E-7 m/s2 (slightly above 1 cm/s each day).
Based on the average distance of 16.5Km, we can calculate the resulting Itokawa mass:
M = - a R2 / G = 2,73E+11 Kg
This mass is larger than expected... in fact, based on radar model of Itokawa (an ellissoid with 550x305x275 meters), it give a mean density of about 22.5 g/cm3, an order of magnitude too high!
Based on Hayabusa imagery, I have impression that actual asteroid dimensions are even smaller, so density would be even higher!!!

In conclusion, my simple "newtonian" approach cannot explain spacecraft motion and, probably, we have some other unexplained effects. Are there some ideas about this?

[edstrick]

Orbital mechanics over a few days in solar orbit can result in non-intuitive behaviour, same as 10 minutes of trying to station keep with a sat in orbit around Earth.. If you're not right on the orbit, you find yourself drifting relative to the target.

[djellison]

Orbital mechanics over a few days in solar orbit can result in non-intuitive behaviour, same as 10 minutes of trying to station keep with a sat in orbit around Earth.. If you're not right on the orbit, you find yourself drifting relative to the target.

There spoke the voice of a man who's tried the Gemini scenarios in Orbiter

Or at least - it sounds like you have

Doug

[edstrick]

I haven't played with orbital sims much, but I've been watching rendezvous "antics" since Gemini (audiotaped 2/3 of Gemini 3 and parts of missions starting with the Gemini 6a pad shutdown) and knows some of the hassles they got into when they tried to force rendezvous by just using more propellant and finding the target slipping off to one side.

[Bob Shaw]

I haven't played with orbital sims much, but I've been watching rendezvous "antics" since Gemini  (audiotaped 2/3 of Gemini 3 and parts of missions starting with the Gemini 6a pad shutdown) and knows some of the hassles they got into when they tried to force rendezvous by just using more propellant and finding the target slipping off to one side.

Yup, until Buzz Aldrin came along (with, obviously, other folk involved) the fighter jocks thought you could just point the capsule, burp the thrusters and arrive - hence the terrible problems the early Gemini crews had with station-keeping even with the Titan II second stage. By the time the Agena was ready they'd learned the lesson!

[abalone]

Based on curve fitting, the re-boost done around Sept 15.2 produced a vertical positive velocity of 1.6 cm/sec and was followed by a uniformly decelerated free-fall with a=-1.34E-7 m/s2  (slightly above 1 cm/s each day).
Based on the average distance of 16.5Km, we can calculate the resulting Itokawa mass:................


...........In conclusion, my simple "newtonian" approach cannot explain spacecraft motion and, probably, we have some other unexplained effects. Are there some ideas about this?

Would be interested to know what the magnitude of solar pressure is on the craft, it has of course got big solar panels and is situated directly upsun from the asteroid

[dilo]

Would be interested to know what the magnitude of solar pressure is on the craft, it has of course got big solar panels and is situated directly upsun from the asteroid

I don't know if you are referring to solar wind or light pressure... Anyway, I considered light pressure explaination.
I do not know exactly Hayabusa mass and solar panels extention, but even assuming a very optimistic (and surely wrong) 500Kg mass and specular panels, we need at least 6.5 square-meters panels in order to justify residual acceleration (1.2E-7 m/s2). This is based on a distance of 1 au from the Sun and a perfect alignment between Itokawa, Hayabusa and Sun (both assunptions should be roughly respected).
My impression is that, probably, pressure radiation can explain a significant part of observed acceleration (probably more than gravitational effect); other residuals could arise from solar wind pressure or even orbits small mismatch mentioned by others...
Do someone can make more realistic/precise figures?

[abalone]

I don't know if you are referring to solar wind or light pressure... Anyway, I considered light pressure explaination.
I do not know exactly Hayabusa mass and solar panels extention, but even assuming a very optimistic (and surely wrong) 500Kg mass and specular panels, we need at least 6.5 square-meters panels in order to justify residual acceleration (1.2E-7 m/s2). This is based on a distance of 1 au from the Sun and a perfect alignment between Itokawa, Hayabusa and Sun (both assunptions should be roughly respected).
My impression is that, probably, pressure radiation can explain a significant part of observed acceleration (probably more than gravitational effect); other residuals could arise from solar wind pressure or even orbits small mismatch mentioned by others...
Do someone can make more realistic/precise figures?

With solar pressure I was assuming the combined effects of both solar wind and light pressure and yes the solar panel would be large to power the ion thruster and yes the allignment would be quite favorable, so this might explain it

[dilo]

New interesting update on the site, with other technical nav infos and no images.
Now they explain this policy: "High resolution images themselves can be readily used for improper and inaccurate interpretation. And Hayabusa project intends to add adequate calibration and speculation to those data and will make them public later but in timely manner. To this end, Hayabusa project for the time being will not release every detailed information. We in advance would appreciate the understanding of the Hayabusa supporters in the world."
http://www.jaxa.jp/missions/projects/sat/e..._c/index_e.html

[edstrick]

Bob Shaw: "....the fighter jocks thought you could just point the capsule, burp the thrusters and arrive ..."

That, of course, works just fine, if you have rocket propellents made of unobtanium with a specific impulse of 2000 or 20,000 instead of 200, and can burn to make delta-V like crazy and go from here to there in a straight line and just stop and do it in a few minutes. ]

THAT is the fundamental problem of Astronautics. Chemical bonds just don't have enough bounce-per-ounce to make Buck Rodgers/Flash Gordon/Tom Corbett rocketships, where the loaded fuel tanks are say 15 or 30% of the mass of the vehicle. There was a general assumption that there'd be a way to do it with "Atomics", but with the real nuclear physics we have to deal with, instead of Pulp Scientifiction nuclear physics... we sure don't have a clue how to do it.

Ghods, I want TorchShips!

[odave]

Now they explain this policy: "High resolution images themselves can be readily used for improper and inaccurate interpretation."

But that's the fun part!

Seriously, I'm little disappointed in that attitude. I'm all for having the scientists on the team having the first crack at the data, but I don't think any harm has been done to the MER or Cassini missions by having the raw images published on the web.

[ljk4-1]

But that's the fun part! 

Seriously, I'm little disappointed in that attitude.  I'm all for having the scientists on the team having the first crack at the data,  but I don't think any harm has been done to the MER or Cassini missions by having the raw images published on the web.

Is it a cultural thing as well? I wonder if Japanese scientists and space administrators are used to the level of public participation as Americans
conduct with their space missions?

[RNeuhaus]

Based on curve fitting, the re-boost done around Sept 15.2 produced a vertical positive velocity of 1.6 cm/sec and was followed by a uniformly decelerated free-fall with a=-1.34E-7 m/s2  (slightly above 1 cm/s each day).
Based on the average distance of 16.5Km, we can calculate the resulting Itokawa mass:
M = - a R2 / G = 2,73E+11 Kg
This mass is larger than expected... in fact, based on radar model of Itokawa (an ellissoid with 550x305x275 meters), it give a mean density of about 22.5 g/cm3, an order of magnitude too high!
Based on Hayabusa imagery, I have impression that actual asteroid dimensions are even smaller, so density would be even higher!!!

In conclusion, my simple "newtonian" approach cannot explain spacecraft motion and, probably, we have some other unexplained effects. Are there some ideas about this?

Thanks Dilo for your comments and Maths,

Yes I agree that the density of Itokawa (22,500 kg/m3) is exaggerated. I tought that their density would be around 2,000 kg/m3.

Rodolfo

[The Singing Badg...]

Now they explain this policy: "High resolution images themselves can be readily used for improper and inaccurate interpretation."

But that's the fun part!  smile.gif

Seriously, I'm little disappointed in that attitude.  I'm all for having the scientists on the team having the first crack at the data,  but I don't think any harm has been done to the MER or Cassini missions by having the raw images published on the web.

To be fair, the person who writes the Hayabusa website does not have the best English skills in the world. He/she may not mean exactly what they seem to be saying here.

[helvick]

Yes I agree that the density of Itokawa (22,500 kg/m3) is exaggerated. I tought that their density would be around 2,000 kg/m3.

22.5g/cm3 would make it just slightly less dense than solid iridium or osmium which would be a great scientific discovery but unfortunately _very_ unlikely. Clearly it's an order of magnitude or two out. Asteroid densities vary from the low end (for very porous rubble piles) around 0.2g/cm3 to 3g/cm3 for solid chondrites.