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KBO encounters
john_s
post Aug 23 2012, 04:13 AM
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Right, our camera aperture is small enough that even though we're closer to the KBOs there's no advantage to searching from the spacecraft. Plus our maximum exposure time is 10 seconds compared to the hours we can integrate from the ground, and it takes thruster fuel to hold the spacecraft steady during those 10 second exposures, so we can't take too many of them. Oh, and because of the need to use thrusters to hold the spacecraft steady, our best spatial resolution for those long exposures is about 4 arcseconds, compared to the ~0.6 arcseconds we can get from the Earth (on a good night). The lower spatial resolution makes it difficult to distinguish and KBOs from all the background stars.

New Horizons would have a sensitivity advantage for KBOs that are very small and close to the spacecraft, as algorimancer says, but we don't think there are many of those, and we don't have the onboard smarts to find them autonomously in the images onboard the spacecraft, and we don't have the bandwidth to send enough of them back to Earth for processing even if we could afford the fuel for all those long exposures...

So we'll just have to keep searching with the big telescopes here on Earth.

John
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elakdawalla
post Aug 23 2012, 04:46 AM
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John, I was talking to someone at LPSC (and now I can't remember who it was) who was dismissing the usefulness of obtaining lightcurves at high phase angles. Obviously people do think it's useful because New Horizons and Cassini have been doing lots of high-phase-angle observations of unresolved objects. Could you explain what it is you (by which I mean planetary astronomers in general, not just the New Horizons team) hope to learn with these lightcurve studies?


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machi
post Aug 23 2012, 08:59 AM
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Light curves are useful for many things.
This is citation from abstract Rotation Periods of Irregular Satellites of Saturn, which describes observations of small irregular moons from Cassini.

"Motivation is the determination of basic
properties of these objects like rotation periods, polar
axes orientations, object sizes and shapes, phase
curves, colors, or the search for binaries."

Another field of research is about asteroids and analyzing their light curves for determination of their physical properties.
I saw comparison between two shape models of Lutetia. One was obtained by modeling from light curve and second from Rosetta OSIRIS camera.
Both of them were almost identical (basic shape in low resolution).
High phase observations are useful for Hapke modeling of surface properties as surface roughness etc.
But I think that usefulness of this technique strongly depend on quality and quantity of images.


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paxdan
post Aug 23 2012, 10:26 AM
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Motivation is the determination of basic
properties of these objects like rotation periods, polar
axes orientations, object sizes and shapes, phase
curves, colors, or the search for binaries

reads like amazing blank verse.
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algorimancer
post Aug 23 2012, 01:10 PM
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QUOTE (john_s @ Aug 22 2012, 11:13 PM) *
...need to use thrusters to hold the spacecraft steady....don't have the onboard smarts to find them autonomously in...


Thanks for clearing that up. I had assumed that reaction wheels were used for adjusting orientation, if thrusters are needed this is clearly a non-starter. I was also a bit concerned that communication would require re-orienting to point at Earth, which would add additional complications. Perhaps someday a similar approach might be tried on a subsequent mission.
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john_s
post Aug 23 2012, 07:03 PM
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QUOTE (elakdawalla @ Aug 22 2012, 09:46 PM) *
John, I was talking to someone at LPSC (and now I can't remember who it was) who was dismissing the usefulness of obtaining lightcurves at high phase angles. Obviously people do think it's useful because New Horizons and Cassini have been doing lots of high-phase-angle observations of unresolved objects. Could you explain what it is you (by which I mean planetary astronomers in general, not just the New Horizons team) hope to learn with these lightcurve studies?


Actually we're more interested in the phase curves than lightcurves per se (lightcurves would be interesting too, to get rotation periods and shapes, but can be done from HST). A rotational lightcurve at high phase angles may be hard to interpret in terms of shape, but is fine for getting the rotation period, which I think is the prime motivation for the Cassini lightcurve observations. For Cassini, I don't *think* there's any preference for high phase angle observations- the phase angle is just whatever it happens to be when there's a block of time available for the observations.

Phase curves (which can't be done from Earth for KBOs) are useful for constraining surface textures. However we also have a practical interest on New Horizons, because we will need to acquire our flyby target KBO at higher phase angles (maybe 20 degrees) than can be observed from Earth, so we'd like an estimate of how bright it will be at that phase angle. The dimmer our KBO is, the less time we will have between our first onboard OpNav and the encounter, and the more fuel we'll need to hold in reserve for final targeting.

John
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john_s
post Aug 23 2012, 07:07 PM
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QUOTE (algorimancer @ Aug 23 2012, 06:10 AM) *
Thanks for clearing that up. I had assumed that reaction wheels were used for adjusting orientation, if thrusters are needed this is clearly a non-starter.


Yes, no reaction wheels on New Horizons (I think because they would have required too much mass and power). Thrusters are better than reaction wheels for the rapid slewing we'll need to do at Pluto- they just aren't as good at keeping a spacecraft rock-steady for time exposures, and of course they use fuel.

John
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TheAnt
post Aug 28 2012, 03:57 PM
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Well at least I did not expect any images taken of "VNH0004" since it will be at a distance comparable to Mars at opposition.
It would take one huge telescope to get a good spectra and even less to resolve anything even if it had been located in the inner solar system.

When one consider that it will have to be for one object that receive ~1000 times less sunlight (about 900 times less at Neptune) it become one daunting task indeed. Even to coerce information out of any possible post-Pluto encounter, that might 'only' be some few millions of kilometers will be quite an achievement - we need to find a possible candidate for anything such first though. smile.gif
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Tom Tamlyn
post Sep 6 2012, 06:46 AM
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QUOTE (vjkane @ Aug 5 2008, 12:38 PM) *
The Voyagers and Pioneers had capable particles and fields instruments. Anyone know if NH's SWAP and PEPSSI instruments would be good for exploration of the heliopause? Also, NH left Earth with a smaller than expected payload of plutonium. How far could it go and still (1) operate the instruments and (2) call home?


QUOTE (Alan Stern @ Aug 5 2008, 01:39 PM) *
We could probably run the s/c and 1 instrument to the mid-2020s to explore the heliosphere 50-70 AU, but that's not the mission-- the mission is to maximize the KB science, and that means spending all the fuel to do that.

Voyager's 35th anniversary got me thinking about a possible particles and fields terminal mission for New Horizon, and I was working on some follow-up questions when I discovered that Alan had answered most of them in a recent PI Perspective.

QUOTE
It's possible that NASA could approve a second extended mission, allowing New Horizons to explore the deep heliosphere as the two Voyager spacecraft are doing now. Although we won't get as far as the Voyagers before we run out of power, we expect New Horizons can operate successfully out to about 90 or 100 times as far from the Sun as Earth.

That would allow us to probe this area of our planetary system with our Solar Wind Around Pluto (SWAP) and Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instruments the most sophisticated and sensitive space plasma instruments ever flown to this distant region and to explore the distribution of dust in this region for the first time using the Venetia Burney Student Dust Counter (SDC). Other science involving our LORRI and Ralph imagers, Alice ultraviolet spectrometer, and our radio science experiment (called REX) will also likely be possible.


I'm curious why NH won't last as long as the Voyagers. Of course it has a substantially smaller RTG, but the decay rate should be roughly comparable. Perhaps NH's spacecraft housekeeping takes proportionately more of its power than is the case for the Voyagers?

Edit: I've belatedly realized that "last as long" and "get as far" are different questions, because the Voyagers are traveling faster than New Horizons. But Alan indicates that "New Horizons has the power and technical capabilities fly late into the 2020s or even into the 2030s if its health remains good," and I still wonder why the shorter life-span.

This post has been edited by Tom Tamlyn: Sep 6 2012, 09:17 AM
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Alan Stern
post Sep 6 2012, 02:45 PM
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New Horizons has 1 RTG, not 2 as the Voyagers do.Also, some will recall we got shorted on Pu-238 owing to two closures of LANL during the mission build-- we launched with 25 watts less power (read: 10+ years lifetime) than we originally hoped for. Double that with 2 RTGs and you have the basic answer to your question.
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Tom Tamlyn
post Sep 6 2012, 09:18 PM
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OK, I finally get it (less margin), and I apologize for being dense. Thanks as always for taking the time to monitor this thread.
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Tom Tamlyn
post Sep 7 2012, 06:18 PM
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As I was thinking about Alan’s answer, I remembered that the SwRI site for New Horizons has a comprehensive collection of freely downloadable technical papers written by scientist and engineer participants in the mission, found at http://www.boulder.swri.edu/pkb/, the kind of basic information that you have to scrounge for with most missions.

One paper, Fountain, et al., The New Horizons Spacecraft (02 Feb 2007) contains a four-page discussion of the NH power system. In particular, Figures 10 and 11 at p. 25 were particularly helpful to me in visualizing the relationship between the RTG’s initial power output and the mission’s lifespan.

Attached File  NH_power_chart.pdf ( 89.74K ) Number of downloads: 336



It’s immediately obvious how an extra 25 watts of initial power would have translated into 10 more years of fully operational lifespan.

TTT (when I figure out how to convert the 90k attachment into an inline image or thumbnail, I'll do it)
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stevesliva
post Sep 7 2012, 08:09 PM
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I suppose it's worth asking whether the instruments on NH measure anything that the Voyagers and IBEX don't. Certainly another in-situ measurement is good, but I wonder, in terms of measuring empty space, what's new?
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imipak
post Sep 7 2012, 08:15 PM
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The NH power charts Tom posted, inline:
Attached Image

o`


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Tom Tamlyn
post Sep 7 2012, 09:22 PM
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Thanks imipak! Looks great, just what I wanted.

How did you do that? I tried copying the attachment url into the "insert image" tool, but I got an error message about dynamic links. I know zippo about html ....
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