[X] My Assistant

[JRehling]

Probably some perfect "grand tour" type alignment, where each spacecraft-planet encounter is as close-in to the planet as possible for a departure trajectory that's approximately tangent to the planet's orbit or as close to tangent as possible.

Eris is also well off of the ecliptic at present (and for a long time coming). I doubt that keeping things in the ecliptic for three flybys then counting on Neptune to provide all of the work to acquire a high inclination is feasible. Maybe a Jupiter-Saturn combo could do it, assuming the rings weren't a problem.

That would actually be a scenario that would unfold fairly often.

Uranus is actually in a pretty good position right now for an assist to Eris, but it'll soon move out of that good position and not come back for 8 decades. Neptune, however, is moving into position, but again, Neptune can't bend the path down in very good proportion to Jupiter's bending it out.

In only 230 years or so, Eris will come within 40 AU of the Sun. Let's plan on an Eris Orbiter/Lander then. Start the buzz now.

[Alan Stern]

You probably already know about The Great Planet Debate meeting coming this week near DC, if not, see:
gpd.jhuapl.edu.

To register for Great Planet Debate conference web participation, click: http://tinyurl.com/6xcqec
Watch the talks and debate on line!

-Alan

[Juramike]

Objects smaller than the Sun occupy a multidimensional continuum of values. Level of differentiation, atmosphere, size, distance from parent star, crustal materials, past/current geologic processes, etc. Choose the right dimensions, and there will be a unique place for everything: Io, Pluto, Eris - everything could get a coordinate.

All of the objects could be plotted in multidimensional space and subjected to cladistic analysis to draw up clusters of objects. But when you define up the clusters, you may end up drawing up borders that may not make much sense in the big scheme of things. And some of these might end up being arbitrary.

For instance, my own personal level of "interesting objects" [crustally differentiated objects with a solid surface]: Mercury, Venus, Earth, Moon, Mars, Ceres, Io, Europa, Ganymede, Titan, Triton, Pluto.

My list of "preferred interesting objects" [crustally differentiated objects which may have had "recent" geological activity] includes: Venus, Earth, Mars, Io, Europa, Ganymede, Triton, Pluto

And my arbitrary list of "most preferred interesting objects" [crustally differentiated objects which may have had recent geological activity and have an atmosphere and that we've taken pictures of the surface]: Venus, Earth, Mars, Titan

All this demonstrates that the groupings and definitions we give objects are arbitrary in the eye of the definer. (The data itself is not, but how the data is grouped is.)



Pluto will still be Pluto whether we call it a "planet" or not.

Whether Pluto is a "planet" or not, for me, is an uninteresting discussion.

A much more intereseting discussion would try to answer:
How is Pluto similar/different from Earth?
How is Pluto similar/different from Titan?

Those are the questions I get excited about.

-Mike

[Alan Stern]

Objects smaller than the Sun occupy a multidimensional continuum of values. Level of differentiation, atmosphere, size, distance from parent star, crustal materials, past/current geologic processes, etc. Choose the right dimensions, and there will be a unique place for everything: Io, Pluto, Eris - everything could get a coordinate.

All of the objects could be plotted in multidimensional space and subjected to cladistic analysis to draw up clusters of objects. But when you define up the clusters, you may end up drawing up borders that may not make much sense in the big scheme of things. And some of these might end up being arbitrary.

For instance, my own personal level of "interesting objects" [crustally differentiated objects with a solid surface]: Mercury, Venus, Earth, Moon, Mars, Ceres, Io, Europa, Ganymede, Titan, Triton, Pluto.

My list of "preferred interesting objects" [crustally differentiated objects which may have had "recent" geological activity] includes: Venus, Earth, Mars, Io, Europa, Ganymede, Triton, Pluto

And my arbitrary list of "most preferred interesting objects" [crustally differentiated objects which may have had recent geological activity and have an atmosphere and that we've taken pictures of the surface]: Venus, Earth, Mars, Titan

All this demonstrates that the groupings and definitions we give objects are arbitrary in the eye of the definer. (The data itself is not, but how the data is grouped is.)



Pluto will still be Pluto whether we call it a "planet" or not.

Whether Pluto is a "planet" or not, for me, is an uninteresting discussion.

A much more intereseting discussion would try to answer:
How is Pluto similar/different from Earth?
How is Pluto similar/different from Titan?

Those are the questions I get excited about.

-Mike

Mike-

I agree with the high "interest factor" in your two questions just above, start a thread! But regarding planet definition, I hope the the topic (for everyone, not just this forum) needs to move from a "contest" over Pluto (let it fall where it may) to a rational one about planet categorization in general. Putting Pluto in the middle of it clouds the arguments, with people claiming there are issues of sentimentality, American pride, etc.; it distracts attention from the important issue of getting a workable definition and categorization of planets. Sykes and others (not sure about Tyson) have accepted this point and will be echoing it at GPD later this week.

Are you going to be at GDP?

-Alan

[Greg Hullender]

But regarding planet definition, I hope the the topic (for everyone, not just this forum) needs to move from a "contest" over Pluto (let it fall where it may) to a rational one about planet categorization in general. Putting Pluto in the middle of it . . . distracts attention from the important issue of getting a workable definition and categorization of planets.

It does seem to me that Planetary Scientists are the only ones in a position to do such a thing. If there were a consensus around a really solid definition, I'd expect the other IAU members would defer to it.

Maybe it would help to give a special name to the Magic Eight; call them "Classical Planets" for example. Round, isolated, in circular orbits in the ecliptic -- they certainly SEEM special enough to merit special attention.

Things like the moon or Ganymede could be "Secondary Planets". Again, from a Planetary Science point of view, I don't see how any rational definition can leave them out.

Planets that are part of a belt (like Ceres or Pluto) could stay "Dwarf Planets" but those would now be "planets," not minor planets.

Stuff above the meteorite level could be "subplanets" and we'd just retire the term "minor planet".

Probable planets (e.g. fairly bright KBOs) could be called "planetoids" until we had enough data to class them as dwarf planets or subplanets.

I'm still not ready to consider comets to be any kind of subcategory of planet, but I suppose you could "define" comet as an icy subplanet in a highly elliptical orbit etc.

Kids in school would learn the classical planets. Enterprising ones would learn the secondary and dwarf planets on their own.

--Greg

[JRehling]

Maybe it would help to give a special name to the Magic Eight; call them "Classical Planets" for example. Round, isolated, in circular orbits in the ecliptic -- they certainly SEEM special enough to merit special attention.
[...]
Kids in school would learn the classical planets. Enterprising ones would learn the secondary and dwarf planets on their own.

I don't understand what the motive is to create a term (even a reasonable one) for the purpose of singling out a group. If the purpose is to create a cutoff for educators, perhaps. But scientifically, Mars is still more similar to Pluto than it is to Saturn. Your comments above, especially the ones I've quoted, imply that this isn't about science but about providing direction to educators.

I'll point out that elementary school math teachers neither seek nor find direction in cutting-edge research mathematics, showing just how different those two sides of the topic are. Serious academics in mathematics interact with "quotients" all the time, but they aren't any better than your typical fourth-grade math teacher (who likely has forgotten any calculus they ever knew) to determine the nomenclature in an elementary school text. In fact, the math wonks are possibly especially poorly-suited to do that.

But this puts a pinpoint on the divide between the science issue and the educational issue. And I think the shame of it is that while educators should look to researchers for the basic facts (eg, does Venus have volcanoes?), which is an objective matter, this labeling and nomenclature is not an objective matter, and handing the reins to the researchers to help decide what the kiddies will be taught is just going to make people jaded about what scientists do.

As an aside, I have bought a number of books about the planets for my son, and the subtle misinformation that's rampant in most of them tells me that any possible lesson in classifying bodies correctly (if there were a "correctly") is way below the signal-noise ratio as it is.

(Sample misinformation: Venus's thick clouds MAKE its atmosphere dense.)

[Greg Hullender]

Your comments above, especially the ones I've quoted, imply that this isn't about science but about providing direction to educators.

Educators are an interested party, since kids can understand this stuff. Contrast math, where higher math is utterly unintelligible to even a talented undergraduate student -- much less an elementary school kid.

It's not completely frivolous to consider educators. Fascination with the planets is a first step into science for many people.

To recap:

Planetary Scientists (not the Planetary Society!) ought to have first call; this is their field of study, so they have the biggest stake.

The IAU itself needs a criterion for assigning names, and that does need to be based on what can be observed -- not what properties might be observed if only we could send a probe.

But educators inspire the next generation -- both the young scientists and the interested non-scientists who are happy to see their tax dollars spent exploring space. Their needs cannot drive the definition, but any definition certainly needs to take them into account.

--Greg

[JRehling]

Educators are an interested party, since kids can understand this stuff. Contrast math, where higher math is utterly unintelligible to even a talented undergraduate student -- much less an elementary school kid.

It's not completely frivolous to consider educators. Fascination with the planets is a first step into science for many people.

To recap:

Planetary Scientists (not the Planetary Society!) ought to have first call; this is their field of study, so they have the biggest stake.

The IAU itself needs a criterion for assigning names, and that does need to be based on what can be observed -- not what properties might be observed if only we could send a probe.

But educators inspire the next generation -- both the young scientists and the interested non-scientists who are happy to see their tax dollars spent exploring space. Their needs cannot drive the definition, but any definition certainly needs to take them into account.

But kids cannot understand the comparative pros and cons of alternative classification systems. Not before about ninth or tenth grade, on average. Younger than that, and a discussion like that will just be noise. And if instead you just give them the IAU definition as fiat, you're passing opinion off as science.

No, it certainly is not frivolous to consider educators. I'd go much further -- this issue is primarily of interest to educators, and has done them the disservice of asking them to accept an arbitrary and divisive definition with the misinformation that it is a scientific truth newly had.

"This" is not Planetary Scientists field of study. As I mentioned before, the class of body that Pluto might be does not inform scientific papers on Pluto, just as papers about Mars do not hinge on Mars being a planet. Research scientists have no stake in this, but have chosen to browbeat educators into accepting a fiat which is not science, shaky classification, and poor education. And this would be equally true if they had (or "will", as they likely will, in time) handed down a definition ruling Pluto to be a planet.

Just to take an arbitrary example of the non-usefulness of this term for scientists, here's a poster/paper about the spectra of Pluto and Charon: http://www.lpi.usra.edu/meetings/acm2008/pdf/8150.pdf

The paper doesn't anywhere use the terms "planet" or "dwarf planet", or any terms for Charon, for that matter. Can we conceive of any way in which such terms could better inform the science, if they were included in this paper? For any proposed definition of "planet"? I sure can't think of one.

In essence, the term has no use for scientists. It has a folk use, like "hill" and "river", and is therefore useful in folk senses, in the classroom. For scientists to call this their field and therefore their term is like the legislature saying that city names are their field and therefore theirs to decide. So it began in Pittsburg and so it ended in Pittsburgh.