Asteroid 2005 YU55 - something I just don't get!

[lukebl]

Perhaps someone can explain something to me regarding Asteroid 2005 YU55, which is passing by on Tuesday.

About 18 months ago it passed fairly close to the earth and the Aricebo radio telescope took this radar image of it, which has been published all over the web, showing a roughly spherical object about 400m across

Very nice and interesting, but my question: if this is a radar image of a roughly spherical object, why does the object appear to be illuminated and with a distinct phase, as in a visual image? If it was a radar image, surely you would see the entire object, not just the illuminated part. The 'dark' part should be just as visible as the 'light' part.

Or am I missing something here?

[rightguard]

i kinda see what you mean but then again i dont.

the moon has a dark part when it has a crescent. that means that the light either a) isnt hitting that part at all, or the light is being reflected but not towards us on the earth.

same thing happening with the radiowaves i guess, as in the ones on the dark side are either not reaching that part at all or just arent ebing refelcted back towards the detector.

edit* remember that if we here on earth shone IR light at the moon we would see the dark side but only because we can send the signal and receive it. thats not the same thing as being able to see it from the emr emitted from the sun.

[bamus]

same thing happening with the radiowaves i guess, as in the ones on the dark side are either not reaching that part at all or just arent ebing refelcted back towards the detector.

.

this would be true, if radar was in space in certain position.. as radar was Earth based, we should be able to see whole shape of asteroid.

[lukebl]

Sorry, but that can't be right. If you held a bright light and shined it at a spherical object, you would see the entire sphere. Just like at full moon.

The same is true for radar images. You are viewing the object from the source of the 'light' (the radio waves) so you should see the whole thing.

[rightguard]

Sorry, but that can't be right. If you held a bright light and shined it at a spherical object, you would see the entire sphere. Just like at full moon.

The same is true for radar images. You are viewing the object from the source of the 'light' (the radio waves) so you should see the whole thing.

depends what angle too. what would a cylinder look liek head on vs sideways?

[rightguard]

You are viewing the object from the source of the 'light' (the radio waves) so you should see the whole thing.

im sorry, but that just cant be right.

if i held a torch and shone it at somehting with 100% asborbance, I wouldnt see a thing. just because you are at the source of emr does not mean you will be in the right place to detect any reflections. what if all the emr from the torch was reflected obliquely? then it owuld never reach my eyes and i would never see it.

you used false logic op.

[Adrian]

I thought that radio telescopes just view in radio wavelengths. They don't transmit because to get a signal back from most objects viewed by radio telescopes (like stars & galaxies) you would need a LOT of power and have to wait a long time

So the image will be the reflected radio "light" from the sun and so can be a crescent

(it's an image in radio frequencies of the electromagnetic spectrum, not a radar image)

[rightguard]

I thought that radio telescopes just view in radio wavelengths. They don't transmit because to get a signal back from most objects viewed by radio telescopes (like stars & galaxies) you would need a LOT of power and have to wait a long time

So the image will be the reflected radio "light" from the sun and so can be a crescent (it's an image in radio frequencies of the electromagnetic spectrum, not a radar image)

thats got it

the clue was in the name! telescope! it just detects the radiowaves being reflected from objects, the same way our telescopes detect the visible waves of light

[Shibby]

I've just done a little reading.

The gradient in brightness is caused by the doppler shift imaging technique used and the rotation of the asteroid causing a doppler shift. The top of the asteroid is moving quickly towards us and therefore appears brighter when interpreted as an image.

[rightguard]

(it's an image in radio frequencies of the electromagnetic spectrum, not a radar image)

'radar' was the one thing that got me here. had me thinking of earth bound radars that ships have etc. didnt even occur to me that the source and detector could be in different places

not sure if radar is so called because it uses radiowaves or because of the type of device it is. i wouldve thought a typical 'radar' on a ship should work equally well with IR or microwaves

[Adrian]

I've just done a little reading.

The gradient in brightness is caused by the doppler shift imaging technique used and the rotation of the asteroid causing a doppler shift. The top of the asteroid is moving quickly towards us and therefore appears brighter when interpreted as an image.

I would have thought that if this were the case then other wavelengths would be doppler shifted to the correct wavelength and it would lokk uniformly illuminated... unless I'm misunderstanding how the doppler shift is affecting this.

[Shibby]

As the asteroid spins, the top of the asteroid moves relatively towards us more quickly than the centre does, this means that the radio waves are more compressed by the doppler effect when returning from the top and appear brighter.

[lukebl]

Thanks for the replies gents. So, it is capturing the reflection off the asteroid of radio waves emitted from the sun, which is why it appears as a crescent then? The 'light' source is the sun, not a transmitted from earth. I somehow assumed that it was bouncing back radio waves sent to it from the direction of the telescope, like 'traditional' radar. Apologies for the dumb question!

[Stu]

http://www.nasa.gov/vision/universe/solarsystem/asteroid-20061219.html

Luke, you didn't ask a dumb question. This link says that the images are produced by beaming microwave radar at the asteroids and detecting the reflections. Not sure why the partially illuminated image as per your original question, it does look like the source is coming from a different location which is not right. Shibby's doppler/rotating theory does make sense though as the brightest part is the top edge which would be moving fastest towards us presumably

Stu

[lukebl]

Thanks, Stu. So I was correct in my assumption that the image is produced from the reflection of microwaves emitted from the telescope location. Still don't quite get why it should show a crescent, though. In theory, it should appear 'full', like the moon does when you are viewing it from the same direction as the light source.

[sbooder]

Maybe the makeup of the asteroid produces different readings depending where lets say for instance dense Iron is?

[Naz]

I thought that radio telescopes just view in radio wavelengths. They don't transmit because to get a signal back from most objects viewed by radio telescopes (like stars & galaxies) you would need a LOT of power and have to wait a long time

So the image will be the reflected radio "light" from the sun and so can be a crescent

(it's an image in radio frequencies of the electromagnetic spectrum, not a radar image)

The object is close enough for ground (Earth) based radar observations. The stars and galaxies are too far away for radar, never mind the power required, but we would have to wait for centuries for returning signals from nearby stars, and millions of years for galaxies. The first radio broadcasts from here on Earth will have only travelled less than a hundred light years into space, if they reflected of anything, it would take another hundred years to return to Earth, and we would probably miss them (apart from them being very weak )

Radio telescopes "look" at radio frequencies and the astronomers build visual images for us mortals to see. Maybe this image is an interpetitation of data for consumption by the general public

Radar requires the transmission of a signal and the reception of same, the asteroid is well within the range of earth based equipment, Arecibo and other large radio telescopes, have the ability to act as radar, transmitting and receiving signals.

The observations that generated the image, were most likely done to get data, such as velocity, rotation, distance and trajectory of the asteroid.

Maybe we are looking at this radio image and trying to understand it in visual terms. Worse case, is to consider that it has been heavily "photoshopped":eek:

[Adrian]

Curse you Occam's Razor!

The object is close enough for ground (Earth) based radar observations. The stars and galaxies are too far away for radar, never mind the power required, but we would have to wait for centuries for returning signals from nearby stars, and millions of years for galaxies. The first radio broadcasts from here on Earth will have only travelled less than a hundred light years into space, if they reflected of anything, it would take another hundred years to return to Earth, and we would probably miss them (apart from them being very weak )

That was my point

Radar requires the transmission of a signal and the reception of same, the asteroid is well within the range of earth based equipment, Arecibo and other large radio telescopes, have the ability to act as radar, transmitting and receiving signals.

Well, you live & learn. I didn't realise Radar could go that far. I was basing my thinking on broadcast transmissions (Doh!) which would not get much signal back at that distance and would probably be swamped by background noise. Obviously if you direct up the radar signal (say, with a nice big dish) you can get a good return, even at large distances.

The observations that generated the image, were most likely done to get data, such as velocity, rotation, distance and trajectory of the asteroid.

In which case they would use a very narrow band precisely to get a doppler shift (as Shibby said was the cause).

It all makes more sense now. Cheers guys

Maybe we are looking at this radio image and trying to understand it in visual terms. Worse case, is to consider that it has been heavily "photoshopped":eek:

If it were 'shopped, wouldn't it look better?

[LeoLion]

This paper should have some of the answers ( inc Shibby's doppler comment/s back up ) http://echo.jpl.nasa.gov/asteroids/ast3_ostro+.pdf . Quite detailed to get your head around ( and I've only 'skim read' so far . )

[michael.h.f.wilkinson]

The Doppler shift image should explain the crescent up to a point, i.e. if you assume they clipped either the receding or approaching part of the surface. The full image should show both red-shifted and blue shifted return signal. Maybe they did not like the visual appearance of that version, and used this "That's no moon!!" version instead.

[Retro]

Hi all,

Can someone tell me whereabouts to look for YU55 from the ground, which constellation/s it will be passing by.

Cheers.

[Stu]

Hi Luke

You are indeed correct about it being radar sent from earth and picking up the reflections. My attempt at explaining what you see is below.

If you imagine a sphere spinning on a horizontal axis, with the top coming towards you and bottom going away....

Then consider the velocity in the direction towards you. The very top will have the highest velocity towards you, the centre will have no velocity towards you, and the bottom will have the highest away from you. The areas in between will have some form of gradient between the extremes. With this represented as highest velocity equals brightest, it quite easily explains the image, assuming a s Michael says that they disregard any negative data which is why the bottom area appears black.

That hopefully makes some sense, although I am quite prepared for you to say that it seems like the ramblings of a madman :-)

Cheers

Stu

[Stu]

Retro, there is a thread somewhere with a link to a video showing the path through the different constellations. I'll try to find it

[cantab]

Were the rotation speeds of Venus and/or Mercury determined by Earth-based Doppler radar? I vaguely recall something about getting radar returns off one of those planets.

Amateur radio operators, incidentally, get returns from the Moon with about 400-1000 Watts of transmit power. It can be used as a communications method, called "moonbounce" or Earth-Moon-Earth (EME).

[michael.h.f.wilkinson]

Were the rotation speeds of Venus and/or Mercury determined by Earth-based Doppler radar? I vaguely recall something about getting radar returns off one of those planets.

Amateur radio operators, incidentally, get returns from the Moon with about 400-1000 Watts of transmit power. It can be used as a communications method, called "moonbounce" or Earth-Moon-Earth (EME).

Radar reflection off Venus was certainly used to get distance measurements (calibrating the AU, later improved by using passing asteroids). I do not know if earth-bound radar was used for the rotation rate, however.