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I asked this in other boards but no one could give me answers.

So, can someone tell me:

  1. Why can New Horizons (which is older than Dawn) take photos in a way higher resolution than Dawn can (NH: 70m/pixel at an altitude of 17.000km; Dawn: 140m/pixel at an altitude of 1400km)?

  2. Why does a probe which should observe objects optical not take the latest camera technique along?

  3. Why isn't there a minimum standard equipment on each probe that makes results and images of missions comparable to each other?

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    $\begingroup$ Fun fact: New Horizons had a \$720 million budget, while Dawn had $450 million. Having a few extra million to burn (literally) changes things too. $\endgroup$ – corsiKa Dec 9 '15 at 6:55
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  1. New Horizons was designed to do a fly-by of Pluto in a relatively large distance. It was only in range of Pluto for a couple of hours. Dawn is designed to orbit Ceres at much closer range. To accomplish its mission, New Horizons had to have such a high resolution camera to get the desired results.

  2. Instruments are designed specifically for a certain mission. Imaging alone is only one of several mission objectives that is accomplished by a space probe. A probe is severely limited by the amount of weight and available space it has for all its instruments. So sometimes to make room for other experiments, some things have to be replaced, made smaller or dropped altogether.

    A camera like with New Horizons wasn't needed for Dawn because Dawn can get much much closer to Ceres than New Horizons ever got to Pluto.

    And last but not least, the project budget must be met. A higher resolution camera that fits the probe costs a lot more than a low resolution camera.

  3. Because there is a lot of planning time involved for each mission and each mission is designed specifically for the technology safely available and affordable at that time.

  4. Don't forget that the costs for New Horizons were already much higher than for Dawn. The project cost for New Horizons was about double that of Dawn, so it's easier to squeeze in another million when the project budget is already rather large and you only get one shot at this in 20 years than on a small budget that could possibly be repeated in half that time. ;)

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  • $\begingroup$ b) but if I estimate correct even at the lowest altitude the resolution of Dawn's images will be far lower than the ones of New Horizons. I quite agree with you that the weight of a probe has to be as low as possible. But even the first Hubble images showed up the mysterious spots for example and let scientists speculated on their origin. So wouldn't it be advisable to get the best result of what makes you curious? Why wait 10 years to get only low resolution images? $\endgroup$ – Hellmachine Dec 8 '15 at 14:44
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    $\begingroup$ See the edit. You can just get so much out of a mission budget. Dawn has two redundant framing cameras that do not only chart Ceres but should also make a multispectral analysis of the surface. It has an IR spectrometer and a gamma ray/neutron spectrometer to analyse the surface structure of Ceres. Btw. the next batch of surveys at 375 km altitude will increase Dawn's resolution to 35 m / pixel. $\endgroup$ – Adwaenyth Dec 8 '15 at 15:05
  • $\begingroup$ c) ok, but what about planning from mission to mission? Or benefit from the efforts of similar missions? To get reliable information of e.g. a dwarf planet, a protoplanet, an asteroid [...]? Put simply: Is it possible to come to a conclusion when you know that Dwarf Planet A has a nitrogen-rich atmosphere but you don't know how it looks but therefore you know that Dwarf Planet B has a wonderful grey/orange surface in HD but youe don't know the composition of a possibly existing atmosphere (I know that the instruments are not this limited - it's just to reinforce the thoughts of my question). $\endgroup$ – Hellmachine Dec 8 '15 at 15:10
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    $\begingroup$ You would not want similar missions. Atm it is still too costly to just confirm what you already know, so when you plan a mission to another celestial body, you take into account all you know about the body, take into account all that you learned from other bodies and then try to adjust your experiments to cover what ever and where ever you expect to find something new. Also "what you know" changes during the planning and construction phase. You mustn't forget that i takes at least one or two decades to plan and prepare something like that. Also HD images are the least of the worries thy have. $\endgroup$ – Adwaenyth Dec 8 '15 at 15:17
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    $\begingroup$ @Hellmachine That's a lot of new questions! Please avoid long discussions in chat and/or extending your question. That can render existing answers incomplete and discourage their authors in providing new ones, if they constantly have to rewrite old ones. Instead, rather ask a new question and link in them to this one if it helps provide context. We're a Q&A, not a discussion forum. Comments are there to help address issues with posts or ask for clarification, not for extended discussion. See About and Help center. Thanks and welcome to the site! $\endgroup$ – TildalWave Dec 8 '15 at 16:22
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SWaP and SWaP-C - Size, Weight, and Power (and Cost) can almost always explain decisions beyond the mission objectives. Reducing any one of these for one instrument means they are available resources for another instrument, either an existing one, or an additional instrument. Once you achieve the minimum specifications for each mission objective, you balance the rest according to the desired outcome.

Some missions do prioritize visual imagers, but the hard reality is that they provide limited scientific value. The vast majority of money that goes into visual imagers is usually justified by PR needed to keep space activities funded. A lot of science is more readily accomplished by instruments that aren't visual in nature, and giving them more space, power, weight, or funds may exponentially increase the scientific value.

Once you get to a certain point in visual imaging, increasing beyond it significantly doesn't significantly increase the science gained.

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Relative to your 3rd question, a common suite of instruments for every probe doesn't work for several reasons. First, the targets of the missions or their environments may vary considerably. For example, although both New Horizons and Dawn were designed to study "dwarf planets", the illumination differs greatly. The asteroid belt is about 2.7 ± 0.5 AU (astronomical units = Earth-Sun distance) from the Sun, while Pluto is currently just under 40 AU away. So the solar illumination at Pluto is more than 200 times less than that at the asteroid belt. Second, the purpose of missions vary, so the instruments necessary to serve that purpose vary. The suite of instruments on a given mission are chosen to meet the requirements of the scientists that will use the data and the particular characteristics of the target(s).

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