Hayabusa 2 on its way back to earth

[KG3]

On ‎8‎/‎25‎/‎2018 at 4:42 AM, YNM said:

 

However, humans make 3 billion tonnes of cement each year.

Chilling to know that we're building an asteroid every 2 months !

Ryugu seems like such a human scale object.  We actually could build one if we really wanted! 

Also the Hayabusa2 team says "Ryugu is a retrograde rotator. The images that have been posted so far have had south up; the mission is now disciplining itself to post images with north up. The big funky boulder is at Ryugu's south pole."

I wasn't aware of this right hand rule for celestial bodies.

Edited August 26, 2018 by KG3
forgot to mention...

[MinimumSky5]

4 hours ago, KG3 said:

I wasn't aware of this right hand rule for celestial bodies.

Its a debate in astronomy, as to whether the north pole is the pole that points to the northern celestial hemisphere (relative to Earth) or if it's the pole that rotates anti-clockwise when viewed from above.

[YNM]

4 hours ago, MinimumSky5 said:

Its a debate in astronomy, as to whether the north pole is the pole that points to the northern celestial hemisphere (relative to Earth) or if it's the pole that rotates anti-clockwise when viewed from above.

It's the direction where the angular momentum points to. Whichever that is.

I think I already explained it...

9 hours ago, KG3 said:

 Ryugu seems like such a human scale object.  We actually could build one if we really wanted! 

Given the amount of stuff available on Earth it's not too surprising. Many mines can hold Ryugu inside their pits.

[Scotius]

Seems conterintuitive to me. Wouldn't something simple be better as a common frame of reference? Like designating Greenwich observatory as meridian 0. It's absolutely arbitrary, but it's working so far. I bet in the future North will still be designated as direction of north celestial pole seen from Earth - both because of tradition and convenience. And a good dose of human laziness and intellectual inertia

[Green Baron]

For objects in the solar system that would be the ICRF (International Celestial Reference Frame).

But for specific applications other frames can be chosen, for example the ecliptic of a satellite. If the object's rotation is not known, it is assumed to be normal to the ecliptic of the object's orbit around its parent body.

Hope i got that right.

 

Inside this there is a link to a paper describing the dirty details. I just found it via my search fu.

Coordinate transformation is a main chapter in books on astronomy :-)

Edited August 27, 2018 by Green Baron

[YNM]

2 hours ago, Scotius said:

Wouldn't something simple be better as a common frame of reference?

There is - Ecliptic Plane.

That is why our planet's orbit has no inclination I think !

Edited August 27, 2018 by YNM

[Green Baron]

11 minutes ago, YNM said:

That is why our planet's orbit has no inclination I think !

Well, it has no inclination relative to its own ecliptic. I am not sure, but i think the icrf ecliptic has been fixed is relative to the solar system barycenter, to which the earth's orbit changes. Though the changes are minimal (<<1 arcsecond/year), they must be observed for high precision astrometry. So, yeah, the earth's ecliptic has an inclination relative to the icrf ...

Again i hope i got that right :-)

Edited August 27, 2018 by Green Baron
eclitpic nitpick

[KG3]

According to the Planetary Society article previously posted the right hand rule applies to small bodies.  They also say: 

"The images [of Ryugu] that have been posted so far have had south up; the mission is now disciplining itself to post images with north up."

[YNM]

9 minutes ago, Green Baron said:

I am not sure, but i think the icrf ecliptic is relative to the solar system barycenter, to which the earth's orbit changes.

IERS defines the ICRF with Right Ascencion and Declination - this means they're defining the celestial equator and not intrinsically the ecliptic. The ecliptic is allowed to move.

So it's still "the ecliptic", although the probability is that they've documented enough of the drift so that it can be said with certainty the inclination of other orbits wrt the ecliptic. And for why fixing the celestial equator instead of the ecliptic, or planetary average ecliptic, or the Sun's equator, or even the galactic plane, it might comes from the fact that the celestial equator was better defined and tracked, and there's not much reason to use one plane over the other, so they just went and use the one they know well.

[Green Baron]

@KG3: The Ryugu guys execute their right to choose the reference frame :-)

But i think the question was "Isn't there a universal fixed reference frame ?" and the answer is no. If at all, it "quasi fixed" but only for a limited time. Changes must always be taken care of, which results in transformations and feeding formulas with elements who describe the changes and the time for which they are valid.

Edited August 27, 2018 by Green Baron

[DAL59]

10 hours ago, Green Baron said:

But i think the question was "Isn't there a universal fixed reference frame ?"

Use the orientation of the pole relative to the CMB.

[Green Baron]

Err, that's a joke, right ?

[KG3]

So this right hand rule.  If the images and spacecraft around Ryugu are flipped around so that the asteroid appears to be rotating in a prograde direction does that now mean that Ryugu is traveling around the sun in a retrograde direction (along with everything else in the solar system)?  I can see how this could be a source of conflict between inter stellar civilizations.  "Your world spins in the wrong direction!" "Well, YOUR world is traveling around your sun in the wrong direction!" 

[cubinator]

13 minutes ago, KG3 said:

So this right hand rule.  If the images and spacecraft around Ryugu are flipped around so that the asteroid appears to be rotating in a prograde direction does that now mean that Ryugu is traveling around the sun in a retrograde direction (along with everything else in the solar system)?  I can see how this could be a source of conflict between inter stellar civilizations.  "Your world spins in the wrong direction!" "Well, YOUR world is traveling around your sun in the wrong direction!" 

Luckily, we can resolve this issue by making everybody stop arguing about it to think about ants instead. 

Honestly, I'm fine with Venus and Ryugu, etc. being considered "upside down" in reference to rotation. Something's got to be backwards in this case, so it might as well be the orientation.

Edited August 28, 2018 by cubinator

[Green Baron]

2 hours ago, KG3 said:

If the images and spacecraft around Ryugu are flipped around so that the asteroid appears to be rotating in a prograde direction does that now mean that Ryugu is traveling around the sun in a retrograde direction (along with everything else in the solar system)?  I can see how this could be a source of conflict between inter stellar civilizations.  "Your world spins in the wrong direction!" "Well, YOUR world is traveling around your sun in the wrong direction!" 

Ryugu orbits the sun like the planets and nearly all other Asteroids, at least all Apollos. That is prograde in respect to the sun. Ryugu's proper rotation is retrograde in respect to its orbit. That's it. Edit: imagine the cloud the solar system formed from. It was rotating in a direction and the impulse of that cloud is preserved in the bodies that formed from it. When this is not the case (like with Ryugu), something must have happened in between, the simplest being collisions. When it orbits its main body in a retrograde sense like some moons of the outer gas planets it must have been captured from outside.

If that means dig out the war ax with interstellar civs because of violation of traffic regulations then so be it. But i think we can stay relaxed here. We just answer "then flip the pictures and you're good !". That's what Jaxa will do to have object rotate from left to right.

 

Edited August 28, 2018 by Green Baron

[KG3]

52 minutes ago, Green Baron said:

 But i think we can stay relaxed here. We just answer "then flip the pictures and you're good !". That's what Jaxa will do to have object rotate from left to right.

 

Ok, I'm relaxed.  Sorry.

I do really like this particular mission.  It looks like they are using the same sample method they used on the first Hayabusa mission.  Firing a pellet into the soil and hoping some material gets blasted up into the collection container.  It seemed like they got less material with this method than they had hoped for.  Did they make any changes to their collection method for Hayabusa 2? 

[Green Baron]

No worries :-)

Yep, according to the Jaxa page there will be improvements, but in principle it follows the same method.

Maybe it can answer why Ryugu refuses to rotate with the mainstream ...

 

Edited August 28, 2018 by Green Baron

[YNM]

15 hours ago, KG3 said:

I can see how this could be a source of conflict between interstellar civilizations.

We'd agree on yet another plane, for sure...

And to add to the confusion, all axial tilt (the thing that makes is it rotating prograde or retrograde) is measured against the body's orbital plane (wikipedia refers to USNO almanac). So, for example, Mars is inclined by 1.85° to our ecliptic, and is further tilted by 25.19° to it's own orbital plane - so at times it's tilted 27.04° to our ecliptic !

We also have what is called as the invariable plane - basically the plane that's perpendicular to where the sum of all angular momentum from all bodies in the Solar System points to.

 

The world isn't perfect...

[Green Baron]

6 minutes ago, YNM said:

The world isn't perfect...

I find this perfect.

If only it was modeled in KSP, i'd soon start new missions ...

[YNM]

1 hour ago, Green Baron said:

If only it was modeled in KSP, i'd soon start new missions ...

Yeah that'd be a real challenge XD

[KG3]

2 hours ago, Green Baron said:

I find this perfect.

If only it was modeled in KSP, i'd soon start new missions …

How different is KSP? 

[Superfluous J]

5 minutes ago, KG3 said:

How different is KSP? 

In KSP every planet and moon (and the Sun) rotate such that their North Pole faces exactly the same direction. I guess you could make them rotate backwards by using negative numbers for their rotation periods (though I find it odd I've never seen this in a planet pack so it makes me think either it doesn't work or I've just not happened to see it) but then the South Pole would point that exact way.

There is no way to tilt a planet like how Earth is tilted, or more dramatically how Uranus is tilted on its side.

[Green Baron]

[snip]

Too slow ... :-)

Edited August 29, 2018 by Green Baron

[KG3]

2 hours ago, YNM said:

 

We also have what is called as the invariable plane - basically the plane that's perpendicular to where the sum of all angular momentum from all bodies in the Solar System points to.

The world isn't perfect...

Messy, I see.  Invariable plane is new to me!  It looks like it should come with some sort of standard error... thing.  If there were a single super Jupiter orbiting in a polar orbit the invariable plane might not be close to any of the objects in the system.  Right?  

[adsii1970]

43 minutes ago, 5thHorseman said:

In KSP every planet and moon (and the Sun) rotate such that their North Pole faces exactly the same direction. I guess you could make them rotate backwards by using negative numbers for their rotation periods (though I find it odd I've never seen this in a planet pack so it makes me think either it doesn't work or I've just not happened to see it) but then the South Pole would point that exact way.

There is no way to tilt a planet like how Earth is tilted, or more dramatically how Uranus is tilted on its side.

Bit it is possible to set a retrograde orbit...