Dawn at Ceres Thread

[Whirligig Girl]

Isn't it a weight issue? After all it's running an ion engine.

Ion engines are super-efficient, and the length of burns is already essentially meaningless when you're dealing with Ion-burners. A few more seconds either way. I suspect it was the launch vehicle's mass fraction that created the issue. Might just have been a placement thing, not enough room for a bigger camera.

It is my firm belief that EVERY NASA space mission needs to have good cameras. I will be heartbroken if the Europa mission doesn't have an epic camera.

[CptRichardson]

Actually, I believe it's not the actual imaging camera taking these photos. It's the navigation camera. Good stuff will happen when it gets into orbit.

[Streetwind]

Why would they need a bigger camera if they're going into orbit anyway? These (Ceres + Vesta) are virtually airless bodies, they can come as close as they wish

[Frida Space]

They can come as close as they wish

As long as they don't forget about mascons

[astropapi1]

As long as they don't forget about mascons

This.

I doubt JPL would ignore something like this, but you never know. There's a million things that could go terribly wrong with this kind of missions...

[Newt]

Actually, I believe it's not the actual imaging camera taking these photos. It's the navigation camera. Good stuff will happen when it gets into orbit.

It also is still very far away. If you look at the full images, Ceres is still a small white bright zone in a large black region. Things will get better.

[-Velocity-]

Why would they need a bigger camera if they're going into orbit anyway? These (Ceres + Vesta) are virtually airless bodies, they can come as close as they wish

That's like asking why do you need a microscope if you can bring something right up to your eye. It's always better to see more detail, especially since this may be the ONLY space mission to visit Ceres in the next 50 years or more! It's our ONE look at Ceres in most people's lifetimes, and they send something with a camera that's barely better than the one on my cellphone.

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Actually, I believe it's not the actual imaging camera taking these photos. It's the navigation camera. Good stuff will happen when it gets into orbit.

It is the imaging camera... Dawn does not carry separate navigation and high res cameras. The imaging camera (which is also the navigation camera) is called the "Framing Camera". Dawn has two of them, and they are identical, with a 20 mm aperture. Dawn also carries a spectrometer, and it apparently can take images, but they are not very high res images either. Maybe a bit higher than the Framing Cameras, you be the judge. The image below is a composite image of the surface of Vesta from one of the 20 mm Framing Cameras and the spectrometer; the smaller strips were taken with the spectrometer.

Anyway, I'm not criticizing, though some of you may think that I am. I am not that dumb to think that I know better than them. I just can't figure out why they would send such a small-aperture camera! You don't hire a photographer who takes pictures with his cell phone to take your wedding photos!

Edited February 17, 2015 by |Velocity|

[Frida Space]

That's like asking why do you need a microscope if you can bring something right up to your eye. It's always better to see more detail, especially since this may be the ONLY space mission to visit Ceres in the next 50 years or more! It's our ONE look at Ceres in most people's lifetimes, and they send something with a camera that's barely better than the one on my cellphone.

First, the camera is vital to Dawn's science, so the probe has a redundant one just in case the main one fails. A bigger camera would probably be more massive, which might not be a problem, but you need two of them, so the overall mass of the two camers and their separate systems would probably exceed the limits. Also, the framing camera's resolution is actually not that bad: 415 m/pix during Survey orbit, 138 m/pix during High Altitude Mapping Orbit, and 35 m/pix at Low Altitude Mapping Orbit. Plus, I'm sure there are other reasons I haven't thought or heard of.

[-Velocity-]

Just to contrast, they put 208 mm and 60 mm telescopes aboard New Horizons. Yes, I know, New Horizons flies by Pluto at long range, so large telescopes are absolutely required to get the kind of resolution we'll get with Dawn in low orbit.

But look at some example Mars orbiter missions. According to Wikipedia, Dawn's lowest planned orbit over Ceres will be 375 km. According to Wikipedia, the MRO orbits Mars even lower, at an altitude of 250-316 km, and it packs a 500 mm telescope.

Another example, the THEMIS camera on Mars Odyssey has an aperture of 120 mm.

So yea, compared to all the example spacecraft I looked at so far, Dawn's 20 mm aperture seems pitifully small. It's baffling.

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First, the camera is vital to Dawn's science, so the probe has a redundant one just in case the main one fails. A bigger camera would probably be more massive, which might not be a problem, but you need two of them, so the overall mass of the two camers and their separate systems would probably exceed the limits. Also, the framing camera's resolution is actually not that bad: 415 m/pix during Survey orbit, 138 m/pix during High Altitude Mapping Orbit, and 35 m/pix at Low Altitude Mapping Orbit. Plus, I'm sure there are other reasons I haven't thought or heard of.

I don't think it's mass. The 208 mm telescope on New Horizons only masses 8.6 kg according to Wikipedia, and that sounds pretty reasonable. Compare that to Dawn's starting mass of 1240 kg, and the delta-V penalty of a larger camera doesn't seem prohibitive.

Edited February 17, 2015 by |Velocity|

[Frida Space]

I'm not sure what to say, but I'm pretty certain there must be something that we are overlooking. However, just to make things clear, imaging is not Dawn's main objective, not even in LAMO. As PI Marc Rayman wrote in a blog post, "the priority will be on three other sets of measurements which probe even beneath the surface."

Also, images taken from LAMO will be well over 800 times better than the best we have now from Hubble Space Telescope, so again not that bad.

Plus, I'm assuming that taking very high-res images would increase the download time, and, quoting once again Rayman, "Even if the two remaining reaction wheels operate, hydrazine will be running very low, so time will be short. [...] And when the last puffs of hydrazine are expelled, it will no longer be able to aim its instruments at the surface, any of its ion engines in the direction required to maneuver, its antenna at Earth, or its solar arrays at the sun. The battery will be depleted in a matter of hours." I guess the main imaging objective is to map the entire surface, not to map in very high res just a few bits here and there. I could be wrong though, this is just an assumption I made, I have no sources for it.

[-Velocity-]

I'm not sure what to say, but I'm pretty certain there must be something that we are overlooking.

Me too. If even someone who is not a spacecraft expert can recognize that Dawn's camera is abnormally small, the most logical assumption is that there is a good reason for it.

Plus, I'm assuming that taking very high-res images would increase the download time, and, quoting once again Rayman, "Even if the two remaining reaction wheels operate, hydrazine will be running very low, so time will be short. [...] And when the last puffs of hydrazine are expelled, it will no longer be able to aim its instruments at the surface, any of its ion engines in the direction required to maneuver, its antenna at Earth, or its solar arrays at the sun. The battery will be depleted in a matter of hours." I guess the main imaging objective is to map the entire surface, not to map in very high res just a few bits here and there. I could be wrong though, this is just an assumption I made, I have no sources for it.

Thanks! That sounds like it could be a reasonable explanation.

Edited February 17, 2015 by |Velocity|

[Kerbin Dallas Multipass]

WP states that the entire project was cancelled numerous times, So maybe the tech on board is just a bit old. Also consider that 1 MPixel doesn't even sound so bad regarding that Hubble had just 2.56 MP.

This is what the cameras look like btw:

(notice all the space tape)

Edited February 18, 2015 by Kerbin Dallas Multipass

[rkman]

Speculating about the cameras: Ceres is relatively close to the Sun so lighting is not a big problem. No reason not to save on payload mass.

[Frida Space]

Speculating about the cameras: Ceres is relatively close to the Sun so lighting is not a big problem. No reason not to save on payload mass.

I don't think the distance to the Sun has a lot to do with lighting, I think (but I could be wrong) that it has more to do with the albedo, and Ceres has a really low albedo (less than a third that of Earth)

[rkman]

Intensity of sunlight decreases with the square of the distance. All the way out at Pluto the intensity is about 1600 times less than at Earth, at Ceres it in the range of 6 to 9 times less than at Earth.

I think that goes some way toward explaining the difference in size of the telescopes on Dawn and New Horizon.

[Frida Space]

Intensity of sunlight decreases with the square of the distance. All the way out at Pluto the intensity is about 1600 times less than at Earth, at Ceres it in the range of 6 to 9 times less than at Earth.

I think that goes some way toward explaining the difference in size of the telescopes on Dawn and New Horizon.

Sure, that's pretty straightforward. What I was wondering is which more important, the albedo or the distance from the Sun.

[-Velocity-]

Intensity of sunlight decreases with the square of the distance. All the way out at Pluto the intensity is about 1600 times less than at Earth, at Ceres it in the range of 6 to 9 times less than at Earth.

I think that goes some way toward explaining the difference in size of the telescopes on Dawn and New Horizon.

I doubt it. I think the main reason they go for aperture is resolution; the closest New Horizons gets to Pluto is 12,500 km according to Wikipedia; at that range, you'll need a big telescope to see small details on the surface.

Additionally, aperture and light gathering power does not work the way most people think it does. Surface brightness- the amount of light per unit of angular area of an image of an extended object- does not increase with aperture. If you've matched your focal plane array pixel size to some some small multiplier (like 1.0) of the size of a point source on the image plane- which you do if you want to squeeze the maximum amount of resolving power out of your system- then surface brightness of extended objects does not increase. In other words, as long as you match the pixel size to the some fixed constant of the size of a point source, then when you look at an extended object- like a planet, a nebula, a galaxy, etc.- then the number of photons falling on each pixel is the same, regardless of whether you're hooked up to 20 mm telescope or a 2000 mm telescope.

However, if you're willing to use pixels that are not fixed in size with regards to the angular resolution of the telescope, then you CAN increase the number of photons hitting each pixel by going with a short (small) f-ratio. This is what is meant by "fast" telescopes- they have short focal lengths and have low power (for their aperture size). "Fast" originated from the world of photography; you could take images of faint objects faster with "fast", small f-ratio telescopes, because they concentrated more light per unit area on the film. HOWEVER, their resolution suffers because of this.

That said, it is important to understand that how "fast" an imaging system is has nothing to do with its aperture; it is solely dependent on the f-ratio.

It's a similar situation as to what occurs in visual astronomy, which I've been doing for over 20 years now. The pupil of the human eye cannot open wider than about 7 mm. This means that if the cone of light entering your eye is wider than 7 mm, you are wasting light from your telescope. The diameter of the light cone as it enters your eye is known as the "exit pupil". To calculate it, you just need to divide the telescope's aperture by the magnification.

Say I've got a 200 mm telescope operating at a 7 mm exit pupil. That's 28.6X. Let's compare that to a 400 mm telescope operating at a 7 mm exit pupil- 57X. Let's say we're looking at the Orion nebula. The 400 mm telescope gathers 4X more light, but must be operated at TWICE the magnification. This means that the light that is gathered is spread over 4X more area. So, while the bigger telescope gathers more light, it spreads that light over proportionally more area. The surface brightness- the amount of light per unit area of the image- does not change. The Orion nebula appears larger and more detailed in the bigger telescope, because the larger a low surface brightness object is, the easier it is for a human eye to see it. Sometimes, this is perceived as the object being brighter, because in truth, the total brightness of the magnified object has increased by 4X because it's area is 4X larger. But the brightness per unit area has not changed.

This is actually a "law" in optics- that optics cannot increase surface brightness- and I've read about it elsewhere. I forgot the name of it though. I think there was even a Wikipedia page on it, but I can't find it right now.

Edited February 23, 2015 by |Velocity|

[Frida Space]

Thanks, great explanation! That was what I was looking for.

[moronwrocket]

Another thing to think about is that a high-resolution, narrow angle camera needs really good maps to know where to look. MRO has 3 cameras, one of which is just to provide context of the area around the high resolution camera. If we already had full maps of Ceres, and found something worth looking at high resolution, then it would make sense to send a higher resolution camera.

I lament the loss of a magnetometer and laser altimeter more than a higher resolution camera. Clearly launch mass was a major concern, and if given the extra kilograms I bet Marc Rayman choose a magnetometer over higher camera resolution. A magnetometer would be able to provide definitive evidence of a liquid ocean, as any salts in the ocean would make it react to the solar magnetic field.

[rkman]

Pluto albedo: ~0.5

Ceres albedo: ~0.1

Not a very significant difference compared to the difference in sunlight intensity.

That said, it is important to understand that how "fast" an imaging system is has nothing to do with its aperture; it is solely dependent on the f-ratio.

Yes, but it's technically easier to make a telescope fast when the aperture is large.

If a telescope needs to be faster and/or needs to have larger magnification (NH/Pluto), it makes sense to use a larger aperture than for a telescope that can do with lower magnification and/or slower f-ratio (Dawn/Ceres).

Imo the coolest thing about Ceres is that it's the closest thing to a Mün that we're likely to find.

Edited February 23, 2015 by rkman

[Frida Space]

Pluto albedo: ~0.5

Ceres albedo: ~0.1

Not a very significant difference

Not a lot of difference? That's a HUGE difference! Albedo goes from 0 to 1. Earth's albedo is 0.306.

Imo the coolest thing about Ceres is that it's the closest thing to a Mün that we're likely to find.

Never saw Ceres that way. That's cool

[rkman]

Not a lot of difference? That's a HUGE difference! Albedo goes from 0 to 1. Earth's albedo is 0.306.

About an order of magnitude less difference than the difference in intensity of sunlight due to distance.

[-Velocity-]

Yes, but it's technically easier to make a telescope fast when the aperture is large.

It is? I thought it was the opposite- a big, fast telescope, if the objective is single piece, requires A LOT of glass to be removed, because the higher the f-ratio, the closer your objective can be to having a spherical shape. So a big, fast telescope requires A LOT of polishing to get to a parabolic shape. Then again, I may be trying to extrapolate my experience with reflecting telescopes too far. Maybe this applies less to modern multi-element refracting, catadioptric, and reflecting designs that use multiple shaped mirrors (like Cassegrain or ritchey creitian or however you spell it).

Regardless, I would assume that the price of the optics was pretty negligible compared to the cost of the rest of the mission. A very high quality 8" mirror only runs a few thousand dollars. Less than that, really, but if you're the USG, you'd probably end up paying like $20k to get the same thing us amateurs can get for $1k.

Edited February 24, 2015 by |Velocity|

[Kryten]

Regardless, I would assume that the price of the optics was pretty negligible compared to the cost of the rest of the mission. A very high quality 8" mirror only runs a few thousand dollars.

And if there was a man sat inside the probe telling us what he saw through a scope, that'd be relevant. As it is, there's a lot more in this kind of instrument than just a mirror and a couple of lenses.

[Frida Space]

About an order of magnitude less difference than the difference in intensity of sunlight due to distance.

You can't compare two measurements which have different units. It's like saying, 10 J is an order of magnitude greater than 1 meter. Doesn't really mean anything.