Emily Lakdawalla • May 06, 2006
OPAG, Day 1: Status of radioisotope power and communications support for future missions
Following the mission- and science-focused presentations of the morning, there came two rather alarming presentations: one on the status of the "RPS Program" (RPS stands for Radioisotope Power Supply) and one on the status of the Deep Space Network. Both of these programs are run by NASA independently of the funding of any particular mission, although of course both respond to the demands of present and future missions -- everything is linked.
The presenter was Ajay Misra, from NASA Headquarters. "The GPHS RTG used on Cassini is no longer in production." That stands for General Purpose Heat Source Radioisotope Thermoelectric Generator. "But we have a lot of spare parts, so we can maybe produce two more of these units; if we are lucky we can get three. So we decided a few months back we would invest some money. Two or three could be available for flight 2012 and beyond." The limiting ingredient in the production line apparently is a thermocouple that is no longer in production, and whose production would be difficult and expensive to restart: "It would be 40 to 50 million dollars to restart the line, and there is no other customer but NASA. But we can do better than this.
"You heard this morning about future uses for the Multi-Mission Thermoelectric Generator (MMRTG) Development. It is designed for use on Mars and subsequent RPS-powered missions. The product is to be available in 2009 for Mars Science Laboratory." The basic specs: nominal power: 125 Watts at beginning of mission; 100 Watts after 14 years.
Misra showed a slide listing the numbers of radioisotope power supplies needed for the missions that the science community (if not NASA) has been discussing doing over the next couple of decades; the list was far too long for me to write it all down. Misra summed it up as "Lots. We need a lot of these things. But there is nothing in the budget for RPS for any of these missions. Delay and uncertainty of future science missions have resulted in a significant reduction of the RPS budget. 'You don't have anything in the future, so you don't need the money.' So they cut us by about 30 percent."
His presentation moved on to some explanations of the kinds of RPS that will follow after the MMRTG has been developed, including one with a Stirling design. The MMRTG will not achieve significant increases in efficiency over the current RTG design; the Stirling will enable them to get more power out of each kilo of plutonium. And that, as it turns out, is a goal we need to be aiming at, for reasons Misra explained next. (I have to say I also found this presentation interesting for its open discussion of plutonium supply; as a kid of the Cold War, it seems strange for us to be chasing Europeans out of the room one moment and talking openly about our plutonium supply the next. There were no ITAR restrictions on Misra's talk!)
There is a paucity of plutonium-238. "5 kilograms has been delivered from Russia; 5 kilograms more is on order." These orders are to supply the plutonium necessary for the Mars Science Laboratory mission. Each future MMRTG, which will produce roughly 100 Watts of power apiece, requires I think 2.9 kilograms. "Russia has only 15 kilograms of inventory beyond MSL. The Department of Energy can buy 5 kilograms more under its current contract. There is an option in the contract for it to be extended to purchase the remaining 10 kilograms.
"The DOE is proposing to produce plutonium-238 at Idaho National Laboratory. The US has 300 kilograms of neptunium 237, the feed material for producing plutonium 238. Production is proposed to start in 2013. There would be 5 kg/year of production, of which 2.5 kg/year for NASA use. But there is no budget yet for starting production at INL -- it has always been proposed as over the guideline, and request has been stopped at OMB." They could not show actual missions that need to use this material, so the OMB has balked at paying to start the production. "So right now nobody has put the money to the Department of Energy to start producing."
Someone in the audience asked whether Russia is capable of producing more. Misra: "Russia can produce more if they are paid to. Their production is shut down right now but they can start it pretty easy; easier than us." Someone asked Misra how much neptunium-237 the Russians have, and he didn't know; that particular question, again, tickled my Cold War sensibilities; how preposterous it would have been to ask that question 20 years ago!
Curt Niebur commented, "Russia has also realized last year that NASA really wants plutonium. Prices have gone up from $2.4 million to $2.7; they will probably go up above $3 million per kilogram." Just one more thing to raise the cost of missions.
Misra finished with a request that OPAG support a more robust investment by NASA in the development of future radioisotope power supplies and the plutonium-238 necessary to power them. I have to comment that I'm not unsympathetic with the OMB not funding plutonium production when there aren't missions that need it being planned. That is what needs to be fixed -- NASA has got to say that they do plan future missions. Then there will be no reason not to fund plutonium production. The technology investment is another matter; and that's a theme that was also raised by VEXAG on Monday and Tuesday. NASA has more to do than plan missions; it's also got to investigate in new technology developments to enable those missions.
Which brings me to the next talk, which was given by Bob Preston, on the status of the Deep Space Network, which is called the DSN by pretty much everybody in the space business. Preston came out swinging. "Over the next 30 years, deep space communications will have to accommodate orders of magnitude increases in data transmitted to and from spacecraft, and at least a doubling of the number of supported spacecraft. The present architecture is not extensible. So NASA must develop a new strategy so that the DSN is no longer a restriction on mission capability, but instead an enabler."
First, Preston explained the current situation with the DSN, and why it's not "extensible." "The DSN has three major sites around the globe, with 16 large antennas. It currently services about 35 spacecraft for both NASA and foreign partners. A large number are planetary, but a significant number are heliophysics, astrophysics, and technology demonstration missions. DSN is of course the spigot for all of science data and is part of all radio science experiments. At a cost of $2 billion, it is a good deal. But many of the current DSN assets are obsolete or well beyond the end of their design lifetimes. For example, we have just celebrated the 40th anniversary of the 70-meter antennas." And the 70-meter antennas, he said, cannot be upgraded to support the higher-data-rate Ka-band transmissions that are being deployed with newer spacecraft.
This aging infrastructure is being asked to support a vastly expanding fleet of spacecraft. "Future US missions will require a factor of 10 more bits returned per decade, and a factor of 10 more bits sent per decade. Spacecraft will require more precision navigation for entry, descent, and landing, as well as outer planet encounters. And improvements are also needed to support human missions.
"NASA has neglected investment in the DSN over the decades. Compared to 15 years ago, the number of DSN tracked spacecraft has grown 450%, but the number of antennas has grown by only 30%. Drivers on demands on the DSN are increased complexity and performance of missions. We've gone from brief flybys to detailed orbital remote sensing. Three orbiters at Mars are constricted largely by communication time." In other words, we are not getting back all the data that we could from our spacecraft at Mars, because the DSN is a bottleneck. "We're going from short-lived missions to long-lived missions." Also, the demands are increasing from non-planetary missions, because they are orbiting father and farther from Earth. "We're going from low-Earth-orbit missions to much more distant orbits; Spitzer, which trails Earth in its orbit, now needs the 70-meter dishes. And we're going from single spacecraft to arrays." The latter point hasn't happened yet; but there are missions on the drawing board that include whole clusters of satellites being launched as part of a single mission.
"Where we'd like to be in terms of data rate is around 5 Megabits per second; but Cassini, for example, is only capable of just shy of 80 kilobits per second." At this point Preston was interrupted with the question: "Can the needs of human exploration be leveraged to improve the DSN assets?" Preston's reply: "Human exploration is interested in Ka-band communications. However, at the Moon, you don't need big antennas. You do need high-data-rate systems, so there is a possibility of leveraging there.
"By 2030, we need a thousand times the downlink performance increase, and there'll be two times the number of spacecraft. What process is NASA using to plan for the future of the DSN? NASA and JPL have developed a DSN roadmap. This is being integrated into an agency-wide program plan by the Space Communications Architecture Working Group, or SCAWG," which Preston pronounced "scay-wig." "Mike Griffin has declared that NASA has neglected the DSN and communications infrastructure, and requested a plan to deliver to Congress by 2007."
The new and improved DSN will not look the same as the old DSN with its 35- and 70-meter dishes. "Radio communication with large arrays of small antennas will be the backbone. This would serve all missions -- large, small, new, old. Costs will be recovered over time through much lower operation and maintenance costs. Now we have antennas from 26 to 70 meters. For the arrays we are talking about antennas ranging from 6 to 18 meters, hundreds at each site, to meet our requirements. This isn't ridiculous because radio astronomers already do it."
Someone in the audience asked how the DSN will save money by increasing the number of antennas, replacing single antennas with arrays. He answered, "Right now, every antenna is different. We want to go to a system where the antennas are nearly identical, where some fraction will always be down for regularly scheduled maintenance, with a centralized, automated maintenance strategy. Right now there's a lot of manual work, with an operator at each antenna. Large radio telescope arrays have a single operator, who often does maintenance as well."
Preston went on. "In addition to the ground-based assets, we know that for assets like probes, landers, and rovers, that it's much more efficient to get data back not direct to Earth but to go up to an orbiter around that body and then come back to Earth. Almost all data from the Mars Exploration Rover mission is coming back that way. So orbital data relays at Moon and Mars are in the plan. Eventually, optical communication would be brought in, with trunklines from Mars to the Moon." In other words, spacecraft may always carry radio antennas; but one orbiting telecom satellite at Mars could receive the communications from all Mars orbiters and landers and relay it via a much higher-data-rate optical transmitter to Earth or the Moon.
"Small antenna arrays have more resiliency and redundancy, more graceful degradation of performance in case of antenna or receiver failures, and they are easily scalable when growth is required -- build it in parts, build it as you need it. You can track many spacecraft at once instead of having one big antenna on one spacecraft. We would just pile up the right number of antennas instead of wasting surplus aperture from one large antenna." The DSN has experience with this kind of arrayed communications already, Preston said; it was used by Voyager for its operations at Uranus and Neptune; by Galileo; and even Cassini uses arrayed 34-meter antennas.
Finally, Preston explained the DSN's plan for increasing data rates the factor of 1,000 that is needed for the future. The new antennas give you a factor of 10 increase. Switching over to higher-frequency Ka-band communications from the current X-band gives another factor of 4. Another factor of 5 are will come out of advanced coding and compression of the data itself. And another factor of 10 will be achieved with higher-power spacecraft transmitters.
That's it for the first day of the meeting (phew!). Now on to the second day.
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