Nuclear Powered Space Missions - Past and Future by Regina Hagen 11/8/98
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4. NASA Plans

4.1 "Upcoming Plutonium Launches" (pre-1997)

4.2 "Potential Future Nasa Space Missions With RTGs" (1997)

4.3 "NASA FACTS: Future NASA Spacecraft" (1998)

4.4 Future RTG Development

4.5 "Advanced Solar Arrays And Solar Reflectors"


Russian information policy with respect to space missions differs considerably from U.S. policy. Therefore, this Chapter deals exclusively with NASA plans to launch nuclear powered space missions. It should, however, be pointed out that the Russian government and space organization also continue to push development of nuclear power systems for space missions. In February 1998, an ITAR-TASS press release announced that the "Russian government approved the concept of space nuclear power development in Russia"24. According to this press information, the Russian government considers nuclear power in space as a key aspect for space and military technology.

4.1 "Upcoming Plutonium Launches" (pre-1997)

During the Cassini protest campaign, one of the web pages from the Florida Coalition for Peace and Justice provoked particular discussions. It is titled "Upcoming Plutonium Launches" and lists twelve planned NASA missions with a total of 132.5 kg plutonium [FCPJ]. NASA as well as DoE kept repeating that the list was wrong and that they did not know how the Florida Coalition came to post this information. However, the data given by the Florida Coalition are a perfect match of a spreadsheet with no indication as to its origin or creation date which is titled "Plutonium-238 Requirements (kg)" [USDOE/c]25. The spreadsheet lists twelve mission names with additional information about the launch year, the number and watts of RTGs, the plutonium requirements for the years 1992 to 2001, and the total plutonium-238 requirements in kg. The following table repeats the information from the spreadsheet with the exception of the individual plutonium amounts for the years 1992-2001.

Table 2: Plutonium-238 Requirements (kg)

Year of LaunchLevel RTG (WE)

Number of RTG's

Total

Outer Solar System

Cassini1997300324.0
Comet Nucleus Mission2002300325.5
Pluto Flyby2003300325.5
Mars
MESUR19991543.5
 20011542.5
 20031584.5
Mars SR20073024.0
20093022.5
Moon  

Site Rover

1998

100

3

13.5

Telescope

1999

400

4

18.0

Network (2) [sic?]

2001

100

2

4.5

Network (2)

2002

100

2

4.5

Total

   

132.5

4.2 "Potential Future Nasa Space Missions With RTGs" (1997)

Although NASA insisted on the invalidity of the "Upcoming Plutonium Launches" list, the organization was rather reluctant to provide up-to-date information about nuclear power usage for their planned deep space missions. In August 1997, a few weeks before the Cassini start, NASA headquarters issued a Fact Sheet with the title "Information on potential future NASA space science missions which may be powered with Radioisotope Power Sources (RTGs or RHUs)". As the Fact Sheet lists the conditions for RTG and RHU use as well as potential missions, it is quoted in full length here:

"Information on potential future NASA space science missions which may be powered with Radioisotope Power Sources (RTGs or RHUs).

Radioisotope Power Sources (RPSs) and/or RHUs are generally considered for potential use on missions constrained by one or more of the following conditions:

  • the mission occurs too far from the sun to make feasible use of solar power,
  • the mission occurs in a space radiation environment too harsh to allow sustained use of solar cells,
  • the mission occurs near a planet's poles where solar illumination is insufficient for solar arrays,
  • the mission occurs on a dust- or cloud-enshrouded world, or in a subsurface application, where the use of solar power is impractical or impossible,
  • the mission must operate in night environments with time frames beyond practical battery capacity,
and/or
  • the mission occurs where the solar intensity is so high as to be damaging (i.e., very near sun environment).

Examples of potential space science missions that are under study [and] fall into one or more of these categories include:

  • Europa (Jupiter's Moon) Ocean Explorer (Orbiter) - this mission would occur far from the Sun (5 times the Earth-Sun distance), in an intense radiation environment, and with the possibility of lengthy eclipses.
  • Europa Lander - this mission would not only occur far from the Sun in an intense radiation environment, with frequent day-night cycles and Jupiter solar eclipses, but might also involve submersible exploration of a Europa ocean.
  • Pluto Express (Flyby) - this mission would occur very far from the Sun (30 times the Earth-Sun distance).
  • Titan (Saturn's Moon) Biologic Explorer - this mission would involve the use of an aerobot within Titan's atmosphere and a data relay orbiter; both would be far from the Sun (9 times the Earth-Sun distance), and the aerobot would be operating in Titan's cloud enshrouded atmosphere.
  • Interstellar Probe - this mission would occur at 150 to 200 times the Earth-Sun distance.
  • Mars landers, rovers, or penetrators involving extended operations (2 to 19 years) in very dusty conditions, at extremely low temperatures, or in subsurface applications (generally, RHUs are required).
  • Venus lander - would involve operation beneath dense clouds; carbon dioxide and sulfuric acid atmosphere.

The amount of plutonium-238 dioxide potentially required for each of these missions generally ranges between roughly 3 grams (one RHU) to 2 kg (for a 150 watt-electric class RPS).

Such missions are currently being studied, but are not yet approved. Other missions that potentially could require RPSs or RHUs are in the conceptual phase - the foregoing being examples that establish power requirements for technology planning purposes.

August 1997" [NASA/e]

4.3 "NASA FACTS: Future NASA Spacecraft" (1998)

In response to a Freedom of Information Act inquiry of journalistic professor Karl Grossman, NASA's JPL finally issued another Fact Sheet in April 1998: "NASA FACTS: Future Spacecraft: Solar Arrays, Batteries, and Radioisotope Power and Heating Systems." When he returned from the international annual meeting of the Global Network Against Weapons and Nuclear Power in Space which took place in the first week of April 1998 in Colorado Springs/Colorado, he found the long-asked-for information in his mail. At the same time, NASA made the fact sheet available in the Internet [JPL/f]. Shortly afterwards, the same text was re-posted on the Internet by NASA under a different web address. The title has been changed to "NASA Facts: Future Missions", the layout has been improved, and all pictures have been included [JPL/e]. As this Fact Sheet states the current NASA policy with respect to nuclear powered space missions and future plans, it is re-printed in full length in this Working Paper.

In summary, the 1998 Fact Sheets says that no nuclear powered missions are planned for the next five years (RHU usage is mentioned in the Fact Sheet but not further considered in this section of the article). Three missions are "under advanced study", i.e. "NASA generally accepts the concept, however, detailed spacecraft and mission design (and sometimes specific funding approval) are needed before development can begin". Another five missions are "conceptual studies" which "means the mission is an idea that might be proposed by or to NASA but has not been selected for advanced study" [all quotes JPL/e].

In the 1998 Fact Sheet, the conditions given for RTG usage are the same as in the 1997 Fact Sheet. This is not quite the case for the planned missions. As an attempt is made to give more details about the planned missions below, the mission list according to the 1998 Fact Sheet is quoted here:

"Examples of future missions, which may require the use of radioisotope power systems are:

  • Pluto/Kuiper Express (Adv. Study): Map the surface and characterize the atmosphere of Pluto and its moon Charon.
  • Europa Orbiter26 (Adv. Study): Study Europa (a moon of Jupiter) in search of possible liquid water oceans beneath the surface ice.
  • Solar Probe (Adv. Study): Study the origin of the solar wind.
  • Interstellar Probe (Conceptual Study): Characterize interstellar dust and gas at 900 million miles from the sun and beyond.
  • Europa Lander (Conceptual Study): Study the seismology and possibly penetrate the ice crust to reach a liquid water ocean.
  • Io Volcanic Observer (Conceptual Study): Extensive study of Io's (a moon of Jupiter) surface and volcanic activity.
  • Titan Organic Explorer27 (Conceptual Study): Use landers or aerobots to investigate the surface and chemistry or Titan's (a moon of Saturn) atmosphere.
  • Neptune Orbiter (Conceptual Study): Extensive study of Neptune's system." [JPL/e]

4.4 Future RTG Development

In addition to the conditions and future missions, the 1998 NASA Fact Sheet gives some basic information about RTGs and RHUs. In this context, NASA basically repeats the information from the previous Fact Sheet: "NASA is working with the Department of Energy to identify power requirements of future spacecraft, and to design smaller and more efficient power systems. These power systems may only need to carry about 2-3 kg (about 4-7 lb) of nuclear material for power generation." [JPL/b]

The need to develop new RTGs is mentioned by several U.S. government organizations. DoE says: "NASA has identified a number of potential missions that can best or only be undertaken using radioisotope power and/or heat sources. These future missions depend upon two important conditions.

First, there must be a reliable and continuing supply of Pu-238 fuel from the U.S. Department of Energy. U.S. facilities that could supply Pu-238 are being considered, as are foreign sources such as Russia, England, and France.

Second, smaller and more efficient power systems will have to be developed consistent with NASA's needs." [USDOE/d, pages 26/27]

In another document, DoE becomes more explicit about the infrastructure required to build RTGs – and about the importance of RTG development for 'national security': "In all, the Department has provided a total of 41 power sources for 24 missions since 1961. DoE continues to maintain the capability to provide power and heater systems to NASA for further missions.

The space and defense radioisotope thermoelectric generator program provides support for radioisotope power source development, demonstration, testing, and delivery. Radioisotope power sources are the enabling technology for space and terrestrial applications requiring proven, reliable and maintenance-free power supplies capable of producing up to several kilowatts of power and operating under severe environmental conditions for many years.

The program will develop new, state-of-the-art power supplies required to support both the National Aeronautics and Space Administration (NASA) space missions as well as the national security applications. The outyear planning for these missions reflects arrangements with the national security users, NASA, and the U.S. Department of Energy (DoE) to ensure the capabilities of the facility infrastructure to produce RTGs. This infrastructure represents the sole national capability to produce radioisotope power systems. Without these systems, critical national security activities and NASA missions to explore deep space and the surfaces of neighboring planets would not occur." [USDOE/a]

DoE also describes technological solutions for nuclear spacecraft power supply other than the traditional RTGs: "One option is the dynamic isotope power systems (DIPS), which are much more efficient in converting heat into electricity than the RTGs used on recent missions. ... The range of technologies under investigation is wide. For instance, a process called Alkaline Metal Thermal to Electric Conversion (AMTEC) converts infrared radiation into electricity using liquid metal ions, which are charged atoms. By contrast, the thermo-photovoltaic (TPV) converter changes infrared radiation emitted by a hot surface into electricity. Design goals for AMTEC and TPV technology call for even more efficient conversion of heat into electricity of about 20-30%, or a three-fold increase over RTGs. The higher efficiencies of these new technologies mean that future spacecraft may require less Pu-238 than RTGs typically use. ...

Because of its many advantages, it seems likely that nuclear energy will continue to provide power on space missions into the next century, whether in RTGs, other advanced generators, or nuclear reactors." [USDOE/d, page 27]

In May 1998, the U.S. General Accounting Office (GAO) published a report to U.S. Senator Barbara Boxer, "Space Exploration. Power Sources for Deep Space Probes". The report has its focus on the Cassini mission and gives an outlook to future NASA plans. It is one of two official government documents known to the author of this article which deals with financial details of RTG development:28

"During the past 30 years, NASA , DoE, and DoD have invested over $180 million in solar array technology, the primary non-nuclear power source. In fiscal year 1998, NASA and DoD will invest $10 million to improve solar array systems, and NASA will invest $10 million to improve nuclear-fueled systems.29 ... There are no currently practical alternatives to using nuclear fueled power generation systems for most missions beyond the orbit of Mars. ...

NASA is studying eight future deep space missions between 2000 and 2015 that will likely require nuclear-fueled power systems to generate electricity for the spacecraft. None of these missions have been approved or funded, but typically about one-half of such missions are eventually funded and launched." [USGOA, page 3, emphasis added]

"NASA and DoE are working on new nuclear-fueled generators for use on future space missions. NASA and DoE's Advanced Radioisotope Power Source Program is intended to replace RTGs with an advanced nuclear-fueled generator that will more efficiently convert heat into electricity and require less plutonium dioxide fuel than existing RTGs. NASA and DoE plan to flight test a key component of the new generator on a space shuttle mission. The test system will use electrical power to provide heat during the test. If development of this new generator is successful, it will be used on future missions." [USGOA, page 13, emphasis added]

In a statement explaining the DoE budget 1999, Director Terry R. Lash summarizes the requested budget as follows:

"Budget Authority ($ in Millions)
  
Program Element Request FY 1999
Nuclear Energy R&D $116.9
Advanced Radioisotope Power Systems40.5 
Test Reactor Area Landlord7.4 
University Nuclear Science and Reactor Support10.0 
Nuclear Energy Research Initiative24.0 
Nuclear Energy Plant Optimization10.0 
Nuclear Technology R&D25.0 
Program Direction 23.6

Facilities

 

96.2*

International Nuclear Safety

 

35.0

Uranium Programs

 

66.7

Isotope Support

 

22.4

   

TOTAL NUCLEAR ENERGY, SCIENCE AND TECHNOLOGY REQUEST

 

$360.8

*Includes $31.2 million in activities transferred from the Environmental Management budget associated with the Fast Flux Test Facility [USDOE/e].

Terry Lash continues to give details about the individual items, among them Radioisotope Power Systems.

ADVANCED RADIOISOTOPE POWER SYSTEMS

The Department of Energy and its predecessor agencies have provided radioisotope power systems for use in space and terrestrial applications for over 35 years. These systems are safe, proven, reliable, maintenance free, and capable of producing either heat or electricity for many years under the conditions required for deep space and unattended terrestrial missions. The unique characteristics of these systems make them especially suited to applications where large arrays of solar cells or batteries are not practical, e.g., at large distances from the sun where there is little sunlight or in harsh environments. To date, the Department has provided over 40 radioisotope power systems for use on a total of 25 spacecraft; in addition, two spacecraft were launched with radioisotope heaters on board. In FY 1998, NASA launched the Cassini spacecraft to Saturn. Cassini is entirely electrically powered by three radioisotope thermoelectric generators provided by the Department. Many isotope power systems have also been provided for terrestrial applications. Critical national security activities and NASA missions to explore deep space and the surfaces of planets would not occur without these systems.

In FY 1999, the program will continue developing new power supplies required to support both future NASA space exploration such as a mission to Pluto and national security applications. The national security users will require upgraded versions of existing terrestrial power systems. Future missions by NASA will require both new radioisotope power systems as well as the continued use of radioisotope heater units. The emphasis will be on lighter weight, lower power systems. The R&D will include more efficient energy conversion technology and new materials. There will be an emphasis on developing a relatively standardized family of systems that could meet a range of power requirements based on mission needs at reasonable cost.

The outyear planning for future space missions reflects arrangements with the national security users, NASA, and the Department to ensure maintenance of the facility infrastructure to produce radioisotope power systems. This infrastructure represents the sole national capability to produce radioisotope systems. The Department of Energy recognized the need to keep these facilities operational, and maintenance level operations will continue at each facility with limited amounts of hardware being fabricated. Maintenance of this capability will allow for a quick transition into a production mode without having to requalify facilities and personnel as new missions become finalized. In accordance with arrangements with our customer agencies, NASA, or other users, will provide funds to the Department to pay for mission specific costs including development, hardware fabrication, and other support costs.

A key factor in the ability to provide radioisotope systems for future missions is to have an adequate supply of plutonium-238 (Pu-238) that is used in all of these systems. It is very important to note that Pu-238 is not weapons-grade material and is not useable as the explosive in nuclear weapons. The current inventory of this isotope, with the exception of approximately nine kilograms that were purchased from Russia, was produced in Savannah Rivers K-reactor and processing facilities that have been, or are in the process of being, shut down. In the near term, the inventory will be augmented by purchasing additional Pu-238 from Russia, while development of a domestic production source is investigated further. The Department is discussing with NASA a new funding approach that would have NASA provide the Department with funding prior to making purchases of Pu-238. This new approach could be implemented beginning in FY 1999.

The Advanced Radioisotope Power Systems Program is an important part of the R&D efforts of the Department. In conjunction with the user agencies, the Department will maintain the capability to supply these systems for future missions that are important to the exploration of space and vital to U.S. security interests." [USDOE/e]

With respect to the development of a new RTG type, Pluto Express seems to play a key role: "NASA has asked DoE to sponsor design studies on a lower-power RTG that could be used on the proposed Pluto Express mission, which is under very restrictive mass and cost constraints. In order to reduce both mass and the amount of plutonium-238 a number of advanced thermal-to-electric conversion options are being considered, including small Stirling engines, thermophotovoltaics (essentially solar cells tuned to the infrared radiation of the radioisotope heat source), and alkali metal thermal-to-electric conversion (AMTEC). Maintenance of these technology options is essential to meet the power requirements of the new, smaller, cheaper space missions such as the Pluto Express mission." [AIAA]

4.5 "Advanced Solar Arrays And Solar Reflectors"

NASA information about the future mission Io Volcanic Observer (see Section 4.6.5, Io Volcanic Observer) contains a link to another page titled "Advanced Solar Arrays and Solar Reflectors"30. This page shows two pictures subtitled "Linear Concentrator Array" and "Inflatable Antenna Experiment". The text of this Internet page describes development work for deep space solar technology (spelling changed by author of this article):

"Improved solar array technology will enable solar-electric propulsion and inexpensive missions to the Jupiter System.

  • 50-100 W required at Jupiter
  • Up to 15 kW at 1 AU for SEP31
  • Efficiency exceeding 100 W/kg
  • Radiation and thermal tolerance

Current solar array technology at 40 W/kg is too heavy for many future missions

  • New millenium ‚Scarlet'
  • Inflatable demo completed May 1996

Solar concentrators can focus sunlight on collection surface/converter

Inflatables or rigid surfaces

Inflatable technology also required to deploy large panels of advances solar arrays." [JPL/a]

4.6 Details About Future Missions

The missions mentioned by the Florida Coalition [FCPJ] and by NASA [NASA/d and JPL/e] amount to a total of 20 launches for which RTG usage has been considered in recent years. The following sections provide some basic information about the objectives of the individual missions. Where available, additional information about the missions' power supply or other features is also given. As will be shown in the quotations below, official NASA documents point to the feasibility of solar power alternatives for several of the RTG missions listed in the 1998 NASA Fact Sheet!

4.6.1 Comet Nucleus

"NASA has selected the 5th and 6th missions to be conducted on behalf of its DISCOVERY program for low-cost interplanetary probes. The US$216-million GENESIS spacecraft will be launched in January 2001 to collect solar wind particles and return them to Earth in August 2003. The US$154-million COMET NUCLEUS TOUR (CONTOUR) mission will be launched in July 2002 to flyby comets P/Encke in November 2003, P/Schwassmann-Wachmann-3 in June 2006 and P/d'Arrest in August 2008." [ORBITAL]

Science Objectives
CONTOUR's goals are to dramatically improve our knowledge of key characteristics of comet nuclei and to assess their diversity. The targets span the range from a very evolved comet (Encke) to a future 'new' comet such as Hale-Bopp. CONTOUR builds on the exploratory results from the Halley flybys, and will extend the applicability of data obtained by NASA's Stardust and ESA's Rosetta to broaden our understanding of comets. Key measurements include:

  • Imaging nuclei at resolutions of 4 m (25 times better than Giotto).
  • Spectral mapping of nuclei at resolutions of 100­200 m.
  • Detailed compositional data on both gas and dust in the near-nucleus environment at precisions comparable to those of Giotto or better. ...

The CONTOUR Comets
Encke: A unique object. Comet Encke has been observed at more apparitions than any other comet including Halley. It is one of the most evolved comets that still remains active. In its present orbit, Encke returns to perihelion (dist. ~ 0.34 AU) every 3.3 years. Because Encke has been in this orbit for thousands of years, its continued high level of activity is rather puzzling.

SW3: First discovered in 1930, the activity pattern of SW3 is usually very predictable. However, in late 1995, this comet displayed dramatic variability, and split into at least three pieces. When CONTOUR arrives in 2006, it is likely that relatively unmantled materials will be visible in the cleaved areas, and that evidence of internal structures will remain exposed.

d'Arrest: Since this comet's discovery in 1851, the repeatability of its visual light curve from apparition to apparition suggests that the rotation state is stable, and that its surface outgassing vents change very little with time. ...

CONTOUR Spacecraft

  • ... Body-mounted solar array
  • ... Designed for 0.75 to 1.5 AU solar distance" [JHUAPL]

Comet Nucleus is part of NASA's Discovery program. Discovery missions are not permitted to use RTGs. Therefore, Comet Nucleus will not use RTGs32 to produce electricity but solar arrays. This mission is no longer listed in the 1998 NASA Fact Sheet.

4.6.2 Europa Lander (Europa Lander Network)

"Europa stands out among outer solar system objects in that it may possess subsurface liquid water in global shells, regional zones, or in isolated pockets. As such the top science objective for such a mission is to detect and characterize these zones." [NASA/i]

Not much information about this mission is provided in the NASA web. However, the information available clearly points to the feasibility of solar power for the mission.

"Science Objectives:

  • Measure ice thickness
  • Tomography of layers
  • Chemical analysis of surface

Mission Description:

  • Minimum of 3 landers though precursor mission could use just 1 for seismicity measurements
  • Semi-hard landing with caging
  • Some penetration of ice surface (for rad protection and seismic improvement)
  • Precursor mission

Tech[nology]:

  • ... Efficient, lightweight solar power generation at Jupiter distance ..." [JPL/g]

This mission is listed as one of eight nuclear powered space missions in the 1998 NASA Fact Sheet.

4.6.3 Europa Orbiter (Europa Ocean Observer)

Europa Orbiter, which is also named Europa Ocean Explorer, is planned to be launched in 2002 or 2004. This mission is part of NASA's Outer Planets Program33. NASA's Solar System Exploration Subcommittee identified several criteria which should be addressed before deciding about the Europa Orbiter mission (radar sounder development, radiation tolerance of electronics, propulsion technology, and interpretation of additional Galileo science data, [NASA/i]) – development of non-nuclear power systems for the Europa mission is not one of them.

As for the Europa Lander mission, little information is available about the Europa Orbiter/Europa Ocean Observer mission. But the little information mentions feasibility of solar power for this mission:

"Science Objectives:

  • Verify presence of liquid layer
  • Measure ice thickness and interior properties
  • Image surface features

Tech[nology]:

... Efficient, lightweight solar power generation at Jupiter distance." [JPL/h]

This mission is listed as one of eight nuclear powered space missions in the 1998 NASA Fact Sheet.

4.6.4 Interstellar Probe

"In our present view of the large scale structure of the heliosphere, the solar wind flows radially outward to a 'termination shock' surrounded at somewhat greater distance by a contact surface called the heliopause, which is the boundary between solar wind and interstellar plasma. A bubble of solar wind therefore shields the inner heliosphere from the plasma, energetic particles, and fields of the interstellar medium; to observe these directly, one must get outside the heliopause. Although the size of the heliosphere is not certain, several recent estimates place the distance to the termination shock at ~80 to 90 AU34, with the heliopause somewhat further beyond.

The Interstellar Probe Mission would be designed to cross the solar wind termination shock and heliopause and make a significant penetration into nearby interstellar space. The principal scientific objectives of this mission would be to (1) explore the structure of the heliosphere and its interaction with the interstellar medium; (2) explore the nature of the interstellar medium, and its implications for the origin and evolution of matter in the galaxy, and (3) investigate fundamental astrophysical processes occurring in the heliosphere and interstellar medium. ...

To accomplish its objectives an Interstellar Probe should acquire data out to a heliocentric distance of ~200 AU [which] requires spacecraft velocities of ~10 AU/year to achieve this within ~25 years or less." [NASA/g]

"Technology Requirements: Advanced propulsion and non-solar power source (if not RTGs) ...

Mission Description: ... Potential methods: close Jupiter/Sun flybys; nuclear or RTG electric propulsion."35 [NASA/f]

This mission would operate at a distance from the sun where currently only RTGs can provide the required electricity. It is listed as one of eight nuclear powered space missions in the 1998 NASA Fact Sheet.

4.6.5 Io Volcanic Observer

Information provided about the Io Volcanic Observer mission explicitly points at the feasibility of solar power.

"Io's extraordinary rates of volcanism and heat flow make it a prime target for the study of planetary evolution. Understanding how Io's volcanism is generated and sustained is key to understanding how planets generate and loose heat. The Earth, while contrasting the styles of volcanism on Io with those of Moon, Mars and Venus, provides a window through which we can view mantle composition and differentiation on these different planets. Galileo's recent results have shown that high temperature volcanism is abundant on Io, and that the active volcanic centers are more numerous than previously thought. ...

To understand this very active planet [sic! Io is a moon of Jupiter], a mission is needed which can unequivocally determine the total heat flow and the mechanism which sustains it, the degree of differentiation of the mantle and the composition of the lavas which rise from it, and the mechanism which feeds clouds surrounding Io. ...

Tours which orbit Jupiter require a delta V of about 1.2 km/sec, which is achievable within Discovery resources. A flyby requires significantly less delta V, permitting a larger spacecraft (or larger solar panels). Solar Electric Propulsion (SEP) is practical and is a good match with the large solar arrays needed at 5.2 AU. Inflatable and concentrator arrays both appear useable at Jupiter, though the radiation effects can be serious for missions of long duration." [JPL/o]

This evaluation is repeated in another NASA document: "Tech[nology]: ... Efficient, radiation-tolerant solar arrays." [JPL/i] (See also Section 4.5, Advanced Solar Arrays And Solar Reflectors)

This mission is listed as one of eight nuclear powered space missions in the 1998 NASA Fact Sheet.

4.6.6 Mars Missions (5 Launches)

The Florida Coalition web page [FCPJ] lists a total of five upcoming Mars missions: 3 MESUR launches in 1999, 2001, and 2003, and two Mars SR launches in 2007 and 2009.

Mars landers, rovers, or penetrators are listed as RTG missions in the 1997 but not in the 1998 NASA Fact Sheet. Quite on the contrary: a picture subtitle in the 1998 Fact Sheet explicitly explains: "The Mars Surveyor Program would embark on a mission to bring back soil samples from Mars. These samples would help us understand whether life ever existed on Mars. The Lander and Rover can use solar arrays and batteries for power, but may need RHUs to keep electrical components warm enough to survive the cold Martian nights." [JPL/e]

In all, NASA plans a total of ten Mars launches within the next years:

  • Mars Surveyor '98 consisting of Mars Climate Orbiter (planned launch Dec. 10, 1998) and Mars Polar Lander (planned launch Jan. 3, 1999)
  • Mars Surveyor 2001 consisting of the 2001 Orbiter (planned launch Jan. 27, 2001), the 2001 Rover (planned launch on April 3, 2001) and 2001 Lander (planned launch April 3, 2001)
  • Mars Surveyor 2003 with an Orbiter, a Lander, and a Rover (planned launch May/June 2003)
  • Mars Surveyor 2005 with an Orbiter and a Lander for sample acquisition and return of the samples to Earth (planned launch July/August 2005).36

4.6.7 Moon Missions (4 Launches)

The Florida Coalition web page [FCPJ] lists four Moon missions as Upcoming Plutonium Launches: Site Rover (1998), Telescope (1999), and two Network launches (2001 and 2002).

One Moon mission, the Lunar Prospector, was launched by NASA on Jan. 6, 1998. The NASA web pages contain no information which points to any future planned Moon missions.

4.6.8 Neptune Orbiter

"The results from the highly successful Voyager Neptune encounter pose many profound questions that only follow-on missions will be able to answer. Recent investigations of other star systems have resulted in fundamental questions that may be approached through probing our solar system's gas giants as astrophysical analogs and solar system laboratories. The Neptune Orbiter mission is a high priority part ... for the future NASA Solar System Exploration program. The potential science returned from a Neptune Orbiter mission is in the break through category and enabled by advanced technologies." [JPL/p]

"Science Objectives

  • Atmospheric structure and circulation at Neptune and Triton
  • Ring particle physical properties, dynamics, and distribution
  • Magnetosphere structure and dynamics
  • Map the gravity field (Neptune)
  • Composition, structure, and activity of Triton surface

Mission Description

  • Delta-class launch vehicle
  • Flight time: 6-7 years using advanced SEP
  • Autonomous operation and navigation
  • Aerocapture for orbit insertion
  • Daily flybys of Triton possible" [JPL/b]

In addition to investigating Neptune, the mission is also planned to explore Triton, Neptune's largest moon. Although NASA plans to use high-power solar electric propulsion for this mission, the use of solar panels is not feasible at Neptune. Neptune is too far away from the Sun, consequently there is not sufficient light available.

This mission is listed as one of eight nuclear powered space missions in the 1998 NASA Fact Sheet.

4.6.9 Kuiper-Express (2 Launches)

"Pluto is the largest of a class of primordial bodies at the edge of our Solar System which have comet-like properties and remain relatively unmodified by warming from the Sun. Pluto is thought to be compositionally similar to Triton, the largest moon of Neptune, which was reconnoitered by Voyager 2. These two bodies may also be similar to Charon at 10 to 20 AU37 and the recently discovered Kuiper belt objects out at 40 AU and beyond. All of these objects probably hold important clues to the origin of comets and the evolution of the solar system. Pluto has a large moon, Charon, which has properties very different from Pluto, and this bizarre double body system may have resulted from a catastrophic planetary collision.

At the present time, Pluto has just passed perihelion at 30 AU and is now moving farther away from the Sun on its way out to 50 AU. Stellar occultation observations have shown that Pluto currently has a temporary atmosphere now that it has been warmed by the Sun during this very brief 'summer' in its 248 year orbit. It is anticipated that these gases will freeze out onto the planet's surface sometime over the next 2-3 decades. It is highly desirable to observe this atmosphere with UV and radio occultation experiments before it disappears, and to observe surface features and chemical makeup that may be obscured if and when the atmosphere collapses." [JPL/CIT]

"Planned launch date: 2001
Launch vehicle: Delta or Russian Proton
Planned on-orbit mass: <100 kg
Power System: Radioisotope Thermal Generators (RTGs) of 65 W

Originally designated the Pluto Fast Flyby (PFF), the Pluto Express mission is planned to be a two spacecraft mission designed to make studies of the planet Pluto and its satellite Charon. Its major science objectives are to: (1) characterize the global geology and geomorphology of Pluto and Charon; (2) map the composition of Pluto's surface; and (3) determine the composition and structure of Pluto's atmosphere. Intended to reach Pluto as quickly as possible (before the tenuous Plutonian atmosphere can refreeze onto the surface as the planet recedes from the Sun), the two Pluto Express spacecraft will arrive one year apart after 6-9 years of travel, depending on the ultimate mass of the spacecraft. Studies of the double-planet system will begin 12-18 months prior to closest approach. The overall structure of the spacecraft is an aluminum hexagonal bus with no deployable structures. Power will be provided by radioisotope thermal generators (RTGs) similar in design to those used on earlier missions (e.g. Galileo). ... A potential cooperative effort with Russia may lead to the inclusion of Zone probes, to study the Plutonian atmosphere." [UNKNOWN]

The information about two launches for the Pluto-Kuiper Express mission is repeated in an official document: "The current plan is to have two launches to Pluto, each carrying one flight system and possibly attached probes." [JPL/CIT]38

"In order to reduce the launch costs the Pluto Express sciencecraft will loop around Venus three times, building momentum with each passing before getting a final tug at Jupiter to fling them on through the outer Solar System.

This trajectory path is one of several options39 being considered providing an opportunity to arrive at the distant double-planet system in 2013." [JPL/k]

The optimum trajectory is a major issue for NASA. The direct trajectory to Pluto would be preferred but is considered too expensive as a large rocket with an additional stage would be required for the launch. "Today's funding environment" does not allow for this option. Therefore, a flyby trajectory must be chosen.

"In order to allow for lower cost missions on Delta or Molniya class launchers without the expense of an upper stage, there are other mission design options. Earth/Jupiter gravity assist trajectories can achieve flight times of around ten years, but require the spacecraft to be capable of surviving significantly higher radiation levels40, and require a much larger onboard propulsion system. Another drawback with these trajectories is the amount of effort needed to ensure that the probability of an Earth impact during the Earth flyby is acceptably low. A straight Jupiter Gravity Assist (JGA) trajectory is available for Delta and Molniya class launchers in 2003 and 2004. ... There is an attractive option for a Venus/Venus/Venus/Jupiter Gravity Assist (VVVJGA) trajectory which avoids an Earth flyby and can be launched on a Delta or Molniya class vehicle without an upper stage, with a flight time of about 11.8 years, launching in March 2001. There is a backup Venus/Venus/Jupiter Gravity Assist (VVJGA) trajectory available in July 2002." [JPL/CIT]

Pluto is the last planet in our solar system. Its distance from the sun is so large that virtually no light is available which could be used to produce solar electricity. This mission is listed as one of eight nuclear powered space missions in the 1998 NASA Fact Sheet.

4.6.10 Solar Probe

As for several of the other missions, NASA documents show the feasibility of solar power for the Solar Probe mission:

"The solar corona is one of the last unexplored regions of the solar system and one of the most important to understand in terms of Sun-Earth connection. SOHO and Ulysses results have focused understanding of regions to the point when the in situ measurements are necessary for further progress.

This report describes a robust, scientifically important space mission to explore the source of the solar wind from inside the solar corona at 2 to 110 solar radii from the Sun.

Our primary science objective is to understand the processes that heat the solar corona and produce the solar wind. ...

The mission and spacecraft designs are partly derived from concepts developed for earlier missions but with important differences which result in cost saving and enhanced science return:

A science payload mass of under 16 kilograms, requiring less than 16 watts and a data return of up to 100 kilobits per second meets the focused science objectives." [JPL/l]

"The present design uses non-nuclear power systems, ..." [JPL/n] "As shown above [in a picture], low illumination solar panels will provide power for the spacecraft from 5 AU to 0.7 AU, where the panels will be discarded. In the baseline mission, high temperature arrays will be used from 0.7 to 0.2 AU, where this second set will be jettisoned. Power will be supplied by batteries from 0.2 AU to perihelion plus 14 hours. In a mission option, high temperature arrays which are currently under technological development will be used from 0.7 to 0.1 AU and will then be tucked into the spacecraft umbra inside 0.1 AU. They will be redeployed at 0.1 AU on the outbound leg and the mission will continue until perihelion plus 17 days." [JPL/m]

This mission is listed as one of eight nuclear powered space missions in the 1998 NASA Fact Sheet.

4.6.11 Titan Organic Explorer(= Titan Biologic Explorer)

"Titan has an atmosphere with a higher surface pressure than Earth's that is filled with organic compounds produced by the action of sunlight. A Titan organic explorer would determine the composition of organic compounds in Titan's atmosphere and on its surface, and whether these organics display pre-biological characteristics." [OSS]

Although this mission is mentioned in several NASA documents, no further details are provided. Titan is a moon of planet Jupiter. The ESA probe Huygens of the joint ESA/NASA mission Cassini/Huygens (Section 3, Past Missions – a Chronology for more details) is planned to descend to Titan and explore the moon's atmosphere.

Although not explicitly mentioned, it seems that Titan's atmosphere is so dense that not enough sunlight might pass through to power solar panels. This mission is listed as one of eight nuclear powered space missions in the 1998 NASA Fact Sheet.

4.6.12 Venus Lander

The full mission name is Venus Geophysical Network Pathfinder (VGNP) and includes a Venus surface lander. In addition to some mission details, Malin Space Science Systems, Inc. (MSSS) specifies the power supply of the mission as follows: "On top of the lander, beneath the boom, is the cylindrical housing of the radioisotope thermoelectric generator, which generates 6500 thermal watts to supply 260 electrical watts needed to power a three-stage refrigeration system. The system is capable of cooling the electronics, housed in a dewer, to about 80 deg. C. The VGNP is designed to survive in the 460 deg C, 93 bar ambient Venusian environment for at least one Earth year." [MALIN/b]

According to MSSS, Venus Lander is planned to be launched in early June 1999 and due to arrive at Venus appr. 120 days later.

The MSSS document informs about the VGNP power generation in some detail:

"Although the science instruments and their support electronics require only a small amount of power (~12 W, only 5.5 W within the refrigerated dewar), the refrigeration subsystem itself will require a significant amount of uninterruptable electric power for the duration of the mission. Several options were examined in terms of meeting the following criteria:

  1. The power system must operate in the ambient Venusian atmosphere for a period of at least one year.
  2. The power system must utilize technology of a developed and proven nature. Modifications were allowed within the overall paradigm.
  3. The power system must meet cost and schedule constraints consistent with a Discovery-class mission.

There are few power sources available at the surface of Venus. Batteries would not meet the mission duration requirements, and would have difficulty in the ambient environment. Sunlight reaching the Venusian surface is roughly 2% at its cloud tops, mostly long wavelength and very diffuse – that, along with the high operating temperature, precludes the use of solar cells. Although wind energy may be a potential source of power, it is probably unreliable for continuous operations on the timescale of a year. Brayton, high condenser temperature Rankine, and Sterling technologies are not reasonable power systems for reasons noted earlier. After careful consideration, Radioisotope Thermal Generators (RTGs) utilizing silicon-germanium thermo-electric elements were chosen as the most appropriate technology for this mission. Based primarily on a design utilized by Cassini and planned for MESUR41, General Electric/AstroSpace Space Power division have outlined a system which could operate on the Venusian surface." [MALIN/a]

It should be pointed out that MSSS mentions the involvement of General Electric (GE) in RTG development and production. After some explanations about required RTG modifications for the Venusian environment, the MSSS document continues to talk about financial matters:

"Future availability of RTGs is presently42 a topic of considerable discussion within the Federal government. The Department of Energy's (DoE) Special Projects office provides RTGs to NASA more or less at cost. NASA has not, in the past, been required to pay a fee towards maintaining DoE's ability to provide these devices. However, with the decrease in demand for weapons-grade Pu and other issues leading to the shutdown of DoE facilities, there is concern that RTGs may not be available in the future. This proposal assumes that NASA must maintain access to radioisotope power generation, both for large unmanned missions and for initial power systems for large space endeavors. Discussions with GE and DoE indicate their willingness and ability to meet the VGNP requirements, and the cost estimate given assumes a worst case wherein VGNP would be responsible for the entire production cost." [MALIN/a]

Venus Lander is part of the Discovery program, therefore RTG usage is not permitted. Consequently, this mission is no longer listed in the 1998 NASA Fact Sheet.

4.7 Future Missions Summary

Of the 20 missions which have been mentioned as nuclear powered missions during recent years, the 1998 Fact Sheet lists still eight (Pluto/Kuiper Express is listed as one mission although it will involve two launches.)

Of the eight missions listed, four are technically feasible only when RTGs are used (based on today's technology and under the assumption that Titan Organic Explorer can not be powered by solar arrays.)

For four of the eight missions listed in the latest Fact Sheet, however, other NASA information clearly shows that the missions can be done with solar power - and that corresponding planning is under way!


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