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Out To The Horizon Of Sol

A temperature map of Pluto

Part two continued,...
The possible near-future disappearance of this intriguing atmosphere has always been one of the main reasons for the need to rush a Pluto probe in the near-future - as the planet moves slowly away from the Sun, its air may freeze out onto the surface within just the next two decades. Nobody is sure how fast this will happen, or even if it will happen at all - one theory suggests that the light-colored frosts accumulating on Pluto may cool it down even more and produce a sudden runaway freeze-out in the next few years, while another theory states that a change in the thermal properties of nitrogen ice as it gets colder may prevent Pluto's mostly-nitrogen air from freezing out at all. We simply don't know - indeed, this is one of the reasons to explore the place.

This is especially true when one also considers that Pluto - whose spin axis is keeled over fully 122 degrees - has also been at the point in its orbit where it is side-on to the Sun, but is now, as it revolves around the Sun, starting to point one of its poles more and more away from the Sun, shrouding an increasing fraction of its surface in permanent unphotographable darkness.

However, it would be harder to advocate this mission if it didn't have other scientific targets besides Pluto and Charon, although they comprise optional parts of the mission. Both POSSE and New Horizons would operate for at least 4 years after the Pluto flyby - giving the opportunity to arrange for them to also make close flybys of one or more of the smaller Kuiper Belt objects, which represent leftover remnants of the original solar nebula that are likely to be less altered by other factors than are the surfaces of Pluto and Charon.

New Horizons' ability to carry out further midcourse maneuvers to divert itself to a flyby of such an object, of course, has limits - and up to now, no such object has been located within range of its post-Pluto trajectory. But, oddly, this doesn't demoralize researchers, for the Kuiper Belt - completely unknown 10 years ago, save for Pluto itself - is turning out to be one of the most important portions of the Solar System, utterly dwarfing the Asteroid Belt in its size and number of objects. It is thought to contain something like 100,000 objects 100 km or more in diameter - only a tiny fraction of which have been discovered, with dozens more now being discovered each year by ever more powerful telescopes - and hundreds of billions of other objects down to the size of individual comet nuclei. And so the odds are thought to be overwhelming that at least one Kuiper object more than 100 km wide will be discovered within range of New Horizons by the time it actually flies past Pluto in 2016 or 2017. (A funded ground-based search for such targets is, in fact, part of the mission.)

Indeed, it's considered likely that it can visit more than one. The current rough estimate is that, on the average, New Horizons would encounter one 100-km Kuiper object within range of its remaining maneuvering fuel supply every 18 months - so the odds are excellent that it may fly by two or three of them during that 4-year period, and perhaps one or two more if it survives longer.

Photography and surface-composition analysis of these little worlds is scientifically invaluable; we need to know just how big they are, how the count of craters on their surfaces indicate the dynamic history of the belt, and how their composition reflects (or has been changed from) the original solar nebula. After all, the giant planets were also formed out of similar icy planetesimals - after which their great gravitational fields diverted the remaining ones all over the Solar System, flinging some inwards to crash into the Sun or the inner planets (which may have gotten most of their water supply from them), hurling others completely away from the Sun or into the tremendously distant "Oort Cloud" of comets and larger icy objects trillions of kilometers from the Sun, and reorganizing the form of the remaining Kuiper Belt itself. Even if a Pluto probe is delayed to the point that its atmosphere freezes, a spacecraft study of the Kuiper Belt and its "Solar System fossil remnants" at some point will be important.

The main argument against a Pluto probe - which was, in fact, used by the Office of Management and Budget officials who spoke against it at the Decadal Survey meeting - is that Pluto, by itself, is a miserable little world which is simply not worth the expense of a near-term half-billion dollar mission. And indeed it is true that the outer planets will be arranged for the next two decades so that such a probe could not fly by any other planets except (perhaps) Jupiter. But such a mission, properly designed, can still find other scientifically useful targets - not only in the Kuiper Belt, but perhaps even before it reaches Pluto itself, despite the much smaller flexibility in its acceptable flight path during that period.

So far, only one tiny main-belt asteroid has been located that might be within range of a flyby by New Horizons - but there are two other intriguing possibilities. First, the number of tiny captured "irregular" satellites of Jupiter --orbiting millions of kilometers from the planet, and thought to be the pieces of two asteroids that were captured by the planet in its earliest days and fragmented at that point - has suddenly shot up over the past few years from 8 to 22 as improved telescopes see smaller and smaller objects. None has ever been seen as more than a point of light - except for an extremely fuzzy long-range view of the moon Himalia captured by Cassini, which reveals little more than its overall shape - and researchers will keep a genuinely serious eye out for any new ones that might be within observational range of New Horizons (or any other future Jupiter probe).

Second, there are the "Centaurs" - large Kuiper Belt objects that, like multitudes of the smaller comets, have been diverted by Neptune's gravitational tuggings into the inner reaches of the Solar System. About two dozen Centaurs have been discovered orbiting at various distances between Saturn and Neptune - and, once again, the count of newly found smaller ones is growing steadily, and a serious attempt will be made to discover one positioned acceptably for a flyby by New Horizons before it reaches Pluto. (In fact, Dr. Esposito informs me that a similar search by "POSSE's" team found two or three possibly acceptable candidates among the Centaurs already known.)

At any rate, there still seems to be a very strong scientific argument for NASA funding this 2006 Pluto mission; and a proper reshuffling of agency funds currently devoted to the expensive and delayable Europa Orbiter, the development of ion-drive technology which can also be acceptably delayed, and perhaps even money from the currently unpredictable final resolution of the Space Station mess would clearly allow it to fly. Despite denials by some officials, the imminent report by the Decadal Survey Committee could be a key factor in persuading either the White House or Congress to firmly order this mission next year.

But whether it becomes the first mission in NASA's new Medium-class planetary exploration program or not, a growing number of other Medium-class mission concepts for the outer Solar System are also starting to look plausible, and could be crucial in making the exploration of that vast realm economically practical. In the next part of my Decadal Survey report, I'll examine some of them.

  • Back to Part One of this Report

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    Out To The Horizon Of Sol
    Part two continued,...
    There is considerable flexibility in the plan for closest approach. The current Pluto flyby distance is 9600 km; but if continuing Earth-based scientific observations of Pluto justify more detailed but smaller-area mapping, it could be reset for distances as close as 3000 km. Similarly, New Horizons' closest approach to Charon is currently set at 27,000 km - but it could be set as close as 10,000 km.







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