NASA Reconsiders a Mission to Pluto

By Keith Cowing
December 21, 2000
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Pluto/Charon - Artist's Concept

NASA has decided to give Pluto another chance to receive a visitor from Earth.
NASA Associate Administrator for Space Science Ed Weiler sat down with reporters this week to discuss a new round of proposals NASA is soliciting for the exploration of Pluto.
Earlier this year Weiler put a stop work order in place on the Pluto-Kuiper Express mission when the projected costs for both the Pluto-Kuiper Express and Europa Orbiter missions got out of hand.

According to Weiler, cost numbers provided by the Outer Planets Project Office at NASA JPL showed that the combined runout cost for Pluto-Kuiper Express and Europa Orbiter had jumped from $650 million to $1.5 billion. Attempts to reduced costs involved slipping the launch of Europa Orbiter from 2003 to 2007. But even these measures would not have been enough for Weiler to be able to fund both missions. A look at the 5 year budget projection produced by the Office of Management and Budget (OMB) shows a total of $1.2 billion over the Fiscal Year 2001 to 2006 period. Weiler said this was insufficient for him to do both missions without some cancellations in other parts of NASA’s Space Science program.

Weiler had to decid between one mission or the other. Since none of the established science committees that advise NASA on mission priorities had expressed a preference for Europa Orbiter or Pluto-Kuiper Express, Weiler said he decided to stop work on Pluto-Kuiper Express. His rationale being that the Europa Orbiter project had a strong astrobiology component – something Weiler said “went along with the agency’s mandate to look for life in the universe.” Weiler made his decision when he did Pluto-Kuiper Express was about to start expending funds at a rate of $1 million per week.

Weiler’s decision to stop work on the Pluto-Kuiper Express mission was not at all popular with the space science community. Reacting to Weiler’s actions, the Division of Planetary Science of the American Astronomical Society said “Pluto is the only planet not yet explored by spacecraft and is therefore of great interest and importance to the planetary science community. It is also moving rapidly outward from the Sun from its perihelion passage in the early 1990s, and if this mission is delayed beyond the 2004 launch, the opportunity to study the tenuous Pluto atmosphere may be lost for centuries.”

Since Weiler was announcing a new call for proposals for potential Pluto missions, he posed the obvious question “What has changed since then? ” “A lot” he answered. One factor affecting NASA’s views were a number of concepts that Weiler had been presented for less expensive ways to accomplish a mission to Pluto. Weiler said that this had given his staff confidence that there were possible ways to do a mission to Pluto at less expense than the Pluto-Kuiper Express concept.

The other reason given was the advice given to him by the Solar Systems Exploration Committee – the committee charged by law with providing NASA with advice on its space science programs. In a letter reflecting a meeting of the SSEC held in October, the SSEC said that it could accept a small slip in the launch date for the Europa Orbiter mission to 2008 if this would allow NASA to get a mission to Pluto before its atmosphere collapses.

The Solar System Exploration Subcommittee (SSEC) said “We must go there. The information needed is mostly unobtainable from telescopic observations, even allowing for future improvements. Though small, Pluto has an atmosphere and is expected to have internal dynamic processes. The Pluto-Charon binary planet system has no other solar system analog in its tidal evolution except possibly Earth and our companion the Moon. The images from the Pluto encounter, collected over an extended period, are likely to fascinate the public. From our perspective close in to the Sun, this is a mission to the frontier of the solar system, an appealing aspect to both scientists and the public.”

As is the case with all planetary missions how long it takes a spacecraft to reach its destination is based on its speed. With “traditional” modes of propulsion (i.e. rockets), this speed is based on the amount of kick the spacecraft’s rocket engines can provide. The more fuel the bigger the kick.. The more fuel the greater the weight of the spacecraft and the larger launch vehicle required. As you increase the capability of a launch vehicle costs go up very fast.

One way to get around this problem is to do gravity assist flybys of planets – in so doing, a little of the planet’s momentum is stolen by the spacecraft accelerating it in the process. Cassini is about to use Jupiter to get a gravity assist to speed up its journey to Saturn. The basic idea behind the Pluto-Kuiper Express mission – and any other mission that would use a gravity assist from Jupiter is to take advantage of a chance alignment between Earth, Jupiter, and Pluto. Jupiter will be optimally placed such that a launch from Earth in 2004 can make maximal use of the gravity assist. There is a less favorable opportunity in 2006. After than it will be another 12 years before Jupiter moves around the sun back into position. By then, any mission launched to Pluto via Jupiter would arrive after Pluto’s atmosphere had condensed back on the planet’s surface. It is Pluto’s atmosphere that is one of the key scinece goals of the Pluto-Kuiper Express mission.

Pluto takes 249 years to complete one orbit of the sun, and, with the exception of several decades when it’s elliptical orbit takes it closer to the Sun than Neptune, is the farthest planet from the sun. Since Pluto and Neptune have orbits in a 3:2 resonance they will never collide with one another. Pluto’s stature as a “planet” has been contested recently given that many astronomers now believe that Pluto is just the largest (known) representative of a large number of objects in the outer solar system. Collectively, these icy bodies form the Kuiper belt.

Pluto has a moon named Charon. Pluto has a diameter of 1,413 miles while Charon’s is 728 miles. No other planet and its moon are so close in size to one another. Only Earth and its moon come close. Unlike the Earth/Moon system, Pluto and Charon’s rotation are both exactly the same (6.387 days) meaning that they always show the same face to each other.

Pluto’s atmosphere is exceptionally thin and is thought to be composed of nitrogen with smaller amounts of carbon monoxide and methane. Its is hypothesized that this atmosphere is seasonal and that it only exists when Pluto is at its warmest. Pluto reached perihelion – or its closest distance to the sun in 1989 and has been moving away ever since. Eventually temperatures will drop and the atmosphere will freeze out or “collapse” back onto the surface of Pluto until warmed again several centuries from now. The goal of the Pluto-Kuiper Express mission was to get a spacecraft to Pluto-Charon system while the atmosphere was still in existence.

According to Weiler, the SSEC recommended that NASA put out an Announcement of Opportunity (AO) calling for new proposals. Weiler said that a draft version of this AO will be online at NASA’s Space Science website on 26 December 2000 and that a final version will be available on 19 January 2001. Proposals will be due on 19 March 2001 and will be reviewed between March and April 2001. Following proposal review, NASA will select two or more of the best proposals for more detailed study to the tune of approximately $450,000 each. At the end of this additional study work, a final proposal will be selected in August 2001.

An announcement in Commerce Business Daily was posted on 20 December 2000. According to Weiler, the AO will call for mission concepts that have arrival dates at Pluto no later than 2015. A launch date will not be set – instead, Weiler said that this will be left up to the proposers – and the mission concepts they propose.

An earlier AO was released by NASA for the Pluto-Kuiper Express (or “old Pluto mission” as Weiler called it) for scientific instrument development. When asked if these proposals would be considered as submitted Weiler said that these proposers would have to resubmit their proposals – and that they would have to do so as part of an overall mission team.

This is in contrast to what the SSEC said in their 27 November 2000 letter which stated that “the recommended boundary conditions are that NASA announce selections of Pluto-Kuiper Express and Europa Orbiter science payloads by January 2001 from those that have been proposed in response to the September 10, 1999 AO (AO-99-OSS-04), that AO solicitation be issued by NASA on or before 1/15/01, requesting proposals that include the totality of the OP program, a la Discovery, and accommodate the NASA-selected science payloads. Responses should be requested by 4/15, with intended NASA selection by 6/1/01.”

Weiler repeatedly cautioned that NASA is not making a commitment to accept and or fund any of the proposals that may be submitted. If NASA is not satisfied that the proposals are feasible or that the costs are low enough (and that cost estimates are reliable) NASA may pass on the opportunity to go to Pluto altogether. As part of the process of getting the science community prepared for their proposals an Outer Planetary workshop will be held in early February 2001.

One of the reasons Weiler said he was not going to specific a launch date was the possibility that propulsion schemes might be proposed that are different from the traditional use of rockets. He cited one possible alternative propulsion approach as being solar electric propulsion – a concept that has been successfully tested on the ground, and more recently on during a 9 month period on the DS-1 mission sponsored under NASA’s New Millennium program.

Despite having put a stop work order on the Pluto-Kuiper Express program some work did continue. A new power system is being developed using RTG technology – one which would provide the same amount of power as the current RTG units – but using less Plutonium to do so. Weiler added that a Pluto mission can be done without the new design using one of the three spare RTGs left over from the Cassini program.

Given that cost was the prime issue driving his earlier decisions, Weiler said that the new solicitation, review, and selection process would be in line with the Discovery and Explorer programs. According to Weiler cost problems were minimal on these programs whereas some of the larger, more complex efforts such as the “strategic plan missions” Europa Orbiter and Pluto-Kuiper Express projects were more prone to cost inflation.

The science criteria for the exploration of Pluto has already been set by a Science Definition team.. This group defined a series of categorized and prioritized science goals. These criteria were included in the Pluto-Kuiper Express AO issued a year ago. The main groups of science objectives were identified: global geology and morphology, surface composition, and atmospheric studies. As proposals are reviewed and selections are pondered, Weiler said that he would be looking for over-bidding and over-performance rather than under-bidding and under-performance – that is, he’d rather see proposers be conservative in proposing missions such that any deviation would be to everyone’s benefit. Weiler felt that his new approach was preferable and since “science is best chosen by the scientists”.

When asked if he had briefed Congress on this issue, Weiler said that some inquiries had been received from Congress and that NASA had replied. When asked if he expected Congressional hearings, Weiler chuckled and said that there hadn’t been any when the stop work order had been issued so he didn’t expect any in the future- especially since NASA was now trying to find a way to do a Pluto mission after all.

One reporter asked Weiler to comment on other outer planet missions such as follow on missions to Europa – and perhaps a mission spawned from discoveries that might be made by the Cassini mission to Saturn and Titan. Weiler replied that he though that “people are too focused on only Europa and Pluto” and that the approach he was taking would ensure more flexibility in overall program planning such that new opportunities might be more easily taken advantage of.

Weiler made certain to state that none of the missions that have been circulating such as a Europa Ocean Probe had been approved – or even formally proposed by NASA. He added that significant challenges still remained before the designers of the Europa Orbiter and Pluto missions with the Europa Orbiter project being the more technically challenging of the two. In particular, during its one month operational time in orbit around Europa, the Europa Orbiter spacecraft would received a radiation dosage equivalent to 40 complete Galileo missions. While Weiler is willing to solicit overall mission concepts from a wide variety of bidders, he does not feel that the Europa Orbiter can be done by anyone else on Earth except JPL.”

In addressing the question of what telescopic observations of Pluto and Charon using the Hubble Space Telescope (HST) and the Next Generation Space Telescope were not considered Weiler replied that the increase in resolution to be offered by the NGST is only three fold over that offered by HST. Weiler noted that contrary to what one would expect from a fast flyby mission, the period of time wherein observations were to be made was substantial. He made a point of noting that imagery 100 days out from Pluto and Charon would exceed the resolution of that provided by the HST (or 30 days out in the case of the NGST).

Since one of the main science objective is characterization of the surface histories of Pluto and Charon (including crater counts) these things couldn’t be accomplished using telescopes in the inner solar system. There is also the need for radio occultation experiments where a telemetry signal is monitored as a planet’s atmosphere moves between the spacecraft and Earth. This can only be done with a spacecraft in the vicinity of a planet.

As the session closed Weiler was asked if he thought there was a significant chance that a proposal would be submitted that could be funded. Weiler refused to speculate on the odds that a worthy proposal would arrive and dismissed the suggestion that he was just going through the motions of soliciting proposals to appease the science community.

Related Links

° Pluto-Kuiper Express mission, NASA JPL

° Europa Orbiter mission, NASA JPL

° 21 December 2000: Public Wants NASA To Explore Europa, Pluto, Sky and Telescope Magazine

° 20 December 2000: Pluto Kuiper Mission AO Announcement, Commerce Business Daily

° 20 December 2000: NASA Seeks Proposals for Pluto Mission; Plans to Restructure Outer Planets Program, NASA PAO

° 20 December 2000: On to Pluto? – The Planetary Society Statement

° 27 November 2000: Letter from Michael J. Drake, Chair, Solar System Exploration Subcommittee (SSEC) to Jay Bergstralh, Office of Space Science, NASA HQ regarding the findings and recommendations of the SSEC’s recent meeting.

° 22 September 2000: Planetary Scientists Express Major Concern Over Work Stoppage On Pluto Mission, American Astronomical Society

° 22 September 2000: Earmarks, Rising Costs Threaten NASA Missions, Science, [summary – can be viewed for free once registered. A subscription fee is required for full access.]

° 22 September 2000: Planetary Society Urges Congress Not to Put Pluto on Ice, press release

° 4 August 2000: Dear Colleague Letter from NASA HQ regarding Pluto-Kuiper Express (PKE) mission

° 28 July 2000: Don’t Cancel Mission to Pluto: The Planetary Society Begins Public Campaign

° 30 December 1999: Pluto’s moon Charon is covered with crystalline water and ammonia Ice, SpaceRef

° 10 September 1999: Deep Space Systems Program Including Europa Orbitor Pluto-Kuiper Express And Solar Probe, NASA Announcements of Opportunity Soliciting Proposals for Basic Research, AO: 99-OSS-04 , Issued: September 10, 1999

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