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Final Pluto Showdown Looming

Los Angeles - Jul 22, 2002
The interminable on-again off-again saga of the proposed U.S. Pluto probe -- and whether it will be launched early enough to take advantage of a Jupiter gravity-assist flyby that would boost it at redoubled speed into the outer Solar System -- is, at long last, approaching its end. And the ending is likely to be bizarre, as noisily melodramatic as the climax of any grand opera, and entirely unpredictable.

Up to now, NASA and the Bush Administration have been extremely reluctant to launch such a probe by January 2006, which is the last possible window allowing a Jupiter flyby until 2014. Widespread pressure from the scientific community and the general public forced NASA last year to request competitive bids for the design of such a probe, resulting in the selection of the "New Horizons" craft proposed by Johns Hopkins University's Applied Physics Lab and based mostly on the design of the CONTOUR probe that was successfully launched July 3 to begin its long journey to at least two comets. Indeed, Congress provided the project with $30 million in preliminary funding

However -- at NASA's own request -- the Bush Administration's Fiscal Year 2003 budget for NASA contained no actual money for this launch. Instead, the agency proposes to launch a Pluto probe (perhaps New Horizons) several years later, missing the Jupiter opportunity, and instead using an attached module containing several ion engines to ram the probe into the outer Solar System at high speed without a Jupiter gravity assist.

Indeed, earlier this year new NASA Administrator Sean O'Keefe told Congress that the best way to do so would be to use a revolutionary "Nuclear-Electric Propulsion" module in which the ion engines were actually powered by a small nuclear reactor, which would allow them continue firing steadily at full thrust even when the craft was far from the Sun and thus from any supply of solar-generated electricity. He said that the additional speed thus acquired would be sufficient to get the probe to Pluto as soon as it would arrive via Jupiter flyby.

And an arrival as soon as possible is uniquely important for Pluto, unlike any other Solar System target. The planet is both moving away from the Sun in its elliptical orbit and (since its spin axis "lies on its side") is now steadily tilting more and more of its south polar region continuously away from the Sun.

These events have two serious effects on its study. First, of course the latter effect is steadily increasing the amount of Pluto (and its similarly tilted moon Charon) that is shrouded in permanent night and thus cannot be photographed again until Pluto reaches the opposite side of its vast solar orbit 124 years from now. Second, most scientists think that Pluto's extremely tenuous but scientifically important atmosphere -- thanks to its movement away from the Sun -- is on the verge of completely freezing out onto its surface, starting at the night-shrouded south pole. If so, it won't reappear until Pluto next approaches its perihelion, a full 248 years from now. The best estimate of the deadline before this happens is about 2020, and so NASA has set itself the goal of reaching the planet before then.

However, as virtually every astronautical engineer pointed out at the time, O'Keefe (who has no actual training in astronautics) was absurdly wrong in saying that a nuclear-electric propulsion system was the best answer -- for the simple reason that it will take at least a decade, even with high funding, to develop such a miniature space-going nuclear reactor. Even traveling away from the Sun at high speed, a NEP-propelled probe could not reach Pluto until 2020 at the earliest -- whereas, if New Horizons is launched in 2006 and flies by Jupiter, it can reach Pluto as early as 2015.

The final crunch came last week with the release of the Solar System Decadal Survey Report, commissioned by NASA itself from the National Academy of Sciences to recommend the proper goals and mission launches for Solar System exploration up to the year 2013. This report said flatly that a spacecraft that would fly by Pluto and Charon, and then proceed onwards to fly by as many smaller objects in the scientifically vital Kuiper Belt as possible, was the highest priority Solar System mission in this period. It also reemphasized the importance of arriving at Pluto as soon as possible:

"... About 200,000 square km of terrain will be lost to imaging and spectroscopic mapping on Pluto alone for each year of arrival delay between 2015 and 2025. Beyond the proportional damage this does to the [central] global geology and composition mapping objectives... this loss of [observable] terrains will also severely affect the ability to answer key questions about the extent and nature of the polar volatile reservoirs on Pluto, the origin of the polar cap dichotomy on Pluto [that is, the difference in sizes between the two caps], and the possibility that volatiles capable of [sometimes] generating an atmosphere on Charon are sequestered in [its] polar regions.

"...Pluto's withdrawal from perihelion is widely anticipated to result in a substantial decline, if not a complete collapse, of its...atmosphere. Searches for an atmosphere around Charon, an extremely desirable mission objective... will also be adversely affected or wholly lost, as will be the opportunity to study atmospheric transfer between Pluto and Charon [due to the tidal pulls between the two close worlds] -- something unique in the solar system... Among the other atmospheric science that will be lost at Pluto if the atmospheric pressure collapses or significantly declines before arrival will be the ability to:

  1. Test for hydrodynamic escape [as a trickle of the atmosphere is actually swept away from the planet by the solar wind] -- (a mandatory objective).
  2. Determine the base pressure and vertical haze/temperature structure of the atmosphere... (another mandatory objective).
  3. Pin down volatile transport rates (an extremely desirable objective).
  4. Sample the atmospheric chemistry and the production of organics and nitriles during its maximum pressure (i.e., perihelion) state (another mandatory objective)."
The Decadal Survey Report went further, virtually endorsing the New Horizons spacecraft (which would achieve all these goals) without ever quite naming it, and flatly contradicting the testimony that O'Keefe had delivered regarding the usefulness of Nuclear-Electric Propulsion for it: "The technical readiness of this mission [Kuiper Belt Object/Pluto Explorer] is judged high due to the ongoing development of a technically equivalent mission concept... This mission is ready now, has no requirements for new technology, and can use one of the few remaining [currently existing] RTGs [for electrical power]... It is premature to consider an orbiter for any of [its target objects]. For this reason, and because of the low relative flyby velocities required and the requirement to reach Pluto at the earliest possible date, an NEP option with the necessary advanced ion engines is not appropriate. There is no confidence that both can be developed in time, nor are they necessary for this mission."

NASA, having officially commissioned this report from the nation's leading planetary scientists, is in no position to disavow it. But the Report did leave open another possibility: skipping the Jupiter flyby window, and instead attaching the probe to a Solar-Electric Propulsion module in which a set of ion engines were powered by big solar arrays -- a greatly enlarged version of the single ion-engine module on the Deep Space 1 probe that flew by comet Borrelly in 2001.

Such a SEP module would have to carry out all its acceleration while the probe was still in the inner solar system, since by the time it was 600 million km from the Sun the sunlight level would be insufficient to power the ion engines' high-voltage circuits. Thus it would have to contain more (or bigger) ion engines than a NEP module, and solar panels big enough to provide them with a very high level of power -- up to 25-30 kilowatts.

Sources tell SpaceDaily that NASA's leadership -- who were described as infuriated by their contradiction by the Academy's report -- is now advocating just such an alternative SEP mission to Pluto, to be launched around 2008. This mission has one advantage over the Jupiter form of New Horizons: it doesn't require $122 million in startup money in Fiscal Year 2003.

But in every other respect it's grossly inferior. The SEP module required will be very large -- it must include fully 5 to 10 copies of Deep Space 1's "NSTAR-1" ion engine, and huge solar arrays capable of providing 20 to 30 kilowatts of peak power for them (as opposed to only 2.5 kW for Deep Space 1's module). Developing such a new system will be very expensive: at least $200 million. Added to the $500 million cost of New Horizons in its current form, this may very well bust the absolute $650 million cost limit placed by NASA on any of the series of "New Frontiers" medium-size planetary missions of which the Pluto flyby is supposed to be the first -- which could very well force complete cancellation of the mission.

It has other major problems. It cannot possibly reach Pluto before 2018 (for the 10-engine version), or 2020 (for the 5-engine one) -- thus greatly increasing the odds of an atmospheric freezeout before it arrives. And even to do this, it must utilize not only its SEP module but a gravity-assist flyby of Venus, which in turn requires that the spacecraft must be designed to endure the high solar temperatures at that planet.

Moreover, it must spend 3 to 4 years in the inner solar system before finally departing -- so that, as with an NEP-propelled Pluto probe, to reach Pluto by 2020 it must then fly through the outer solar system and past the planet at higher speed than New Horizons, which (as the Report says) produces scientific problems of its own. Pluto and Charon are little worlds -- the faster a craft flies by them, the less time it has to make observations, and the more motion-blurred its photos are likely to be. (Also, it will later have more trouble veering off its previous course at a sharp enough angle to intercept more small Kuiper Belt objects -- an increasingly important part of this mission.)

Nor would those solar arrays, big though they are, provide enough power to run the spacecraft itself at Pluto's distance from the Sun -- after the ion engines are shut down and ejected, the spacecraft itself must still carry a plutonium-fueled RTG for power, with its associated cost and launch safety problems.

As mentioned, the one advantage a 2008 SEP Pluto probe has is that it does not require any large expenditure in FY 2003, whereas a 2006 New Horizons launch would require $122 million of funding in that year (during which NASA and the White House want to spend only $15 million for the entire New Frontiers program) . However, since the total cost of the Pluto mission would be cut $200 million by picking New Horizons, NASA spending on the New Frontiers program in all the fiscal years afterwards would actually drop sharply -- which might allow earlier launch of the next solar system mission, or simply save money for other purposes.

One such purpose might be the proposed Europa Orbiter, which both NASA leadership and the Decadal Survey strongly support -- but which cannot possibly be flown within the $650 million New Frontiers cap. NASA's budget as now designed does not allow this mission to fly at all; additional money must be provided from somewhere to make it possible.

However, SpaceDaily's sources nevertheless report that NASA officials -- for puzzling reasons, possibly having to do simply with their reluctance to admit an earlier mistake -- are determined to cram the SEP Pluto probe down the throats of the scientific community, and may even order a new round of competitive mission proposals for it.

The question at this point is whether Congress will go along. It previously ordered an additional $30 million of funding in the FY 2002 budget for a possible Pluto mission (making the selection of the preliminary design for the Pluto spacecraft possible), and the staff of Sen. Barbara Mikulski, chairman of the Senate VA-HUD Subcommittee (which is responsible for NASA spending), tells SpaceDaily that she is still a very enthusiastic advocate of the 2006 New Horizons mission and will continue to push it ardently. Rumors before the release of the Decadal Survey Report indicated that both the House and Senate subcommittees for NASA appropriations are inclined to go along with her in providing New Horizons' needed additional $107 million, given that the total cost of the Pluto mission would indeed drop drastically.

The Senate subcommittee's vote on such changes ("markups") in the NASA budget is set for July 23, and the corresponding House subcommittee will vote shortly afterwards, followed by probable approval by both chambers' main Appropriations Committees, and then by House-Senate negotiations. If they approve a budget including New Horizons, it's extremely unlikely that the White House -- despite the fact that, like NASA, it strongly opposes the mission -- would veto it merely to stop the mission.

There is, however, one final strange complication. Because New Horizons' RTG power supply is fueled with highly radioactive plutonium-238, its launch requires approval from the EPA after an official study -- which would have to be accelerated more than usual for this particular launch to get off the ground by Jan. 2006. And it also requires presidential approval. Thus, even if Congress funds this mission, it's still possible for the Bush White House to block it using environmental regulations.

This, however, is the same White House which has announced its strong support this year for a large increase in funding for new and improved versions of RTGs for future planetary missions (including the 2009 Mars "Smart Lander", which was previously designed to use solar power) -- as well as for the Nuclear Electric Propulsion program. As such, it will look decidedly peculiar if it uses anti-nuclear arguments to block the New Horizons launch. At any rate, the next chapter in the story -- the Senate subcommittee's vote -- will be written next Wednesday. If New Horizons is rejected then, that's the end of it. But if the subcommittee does favor funding it, there is a very real chance that it will actually get off the ground.

Postscript: A Response To The New DPS Press Release

  • The DPS completely fails to mention the fact that an SEP mission to Pluto will cost far more than the 2006 New Horizons mission -- at least $200 million more.

  • In the opinion of several planetologists I've talked to, it also seriously overstates the chances that Pluto's atmosphere will not freeze out by 2020. It's quite possible that Stansberry and Yelle are right in saying so, but they seem to be decidedly in the minority on this subject.

  • The DPS says that, unless Congress adds an additional $107 million to NASA's total FY 2003 Space Science budget, New Horizons cannot be funded this year without "sacrificing other important NASA programs." However, an article by Brian Berger in the June 24 issue of "Space News" says: "An appropriations staffer [unnamed, and not identified as belonging to the House or Senate] said funding for the proposed Nuclear Systems Initiative could be imperiled if Congress decides to go against NASA's wishes and continue funding the Pluto flyby. 'If political pressures are such that we are going to fund the Pluto mission this year, I don't see the rationale for spending a huge amount of money on the Nuclear Systems Initiative', the staffer said."

    NASA had requested $46.5 million for NEP development this year. However, the Decadal Survey report not only denounced the idea of using Nuclear Electric Propulsion for the Pluto flyby, but went to considerable length to say that it was not recommended for any other Solar System mission launched before 2015 or so.

    Also, NASA's FY 2003 budget request included $67 million for the proposed "Starlight" mission (formerly known as Deep Space 3), which has since been canceled. The funds from NEP and Starlight by themselves are more than adequate to fund New Horizons in FY 2003 without any additional money at all being added to NASA's Space Sciences budget.

  • However, the DPS release does add one important argument in favor of funding the 2006 New Horizons that isn't in my original article: "...[T]he inclination of Pluto is rapidly increasing [that is, it is moving farther away from the ecliptic], making it ever more difficult to target other Kuiper Belt objects after the Pluto flyby." And the Decadal Survey Report greatly emphasizes the importance of having the Pluto spacecraft fly by as many additional smaller Kuiper Belt objects as possible afterwards, in order to survey their variety. Related Links
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     Washington - Jul 12, 2002
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