Attempting a landing there: the case for a Europa lander
by Jeff Foust
|With an estimated mass of about six metric tons, Europa Clipper will be the heaviest mission built by JPL to dat. With its solar panels unfurled, it will be 27 meters from tip to tip.|
Advocates for such a mission are ignoring that warning (some point out that Clarke himself gave NASA “permission” to land on Europa at a meeting years ago, when discussions about Europa missions were just getting started.) With development of a flyby mission, Europa Clipper, picking up steam, NASA is looking at designs for a Europa lander. The challenge for such a mission, though, will come not from monoliths but from a federal government whose views on such a mission are far from monolithic.
Before a lander mission, though, will be Europa Clipper. The mission, which formally took on that name earlier this year after using it informally for years, seeks to place a spacecraft into orbit around Jupiter that will make dozens of flybys of Europa.
Work on Europa Clipper is going well, even though the mission is still in its early phases of development. Barry Goldstein, project manager for Europa Clipper at JPL, discussed the progress being made on the design if the spacecraft at a March 29 meeting of the Committee on Astrobiology and Planetary Science of the National Academies, including progress being made on systems ranging from its solar panels to its avionics.
He cited as one example development of an avionics testbed, something that he said is usually done much later in a spacecraft design. “It’s really a testament to trying to make as much out of the advanced funding that we were blessed with on this mission,” he said. That’s a reference to largesse Europa Clipper received from Congress, which in recent years funded it well above any request offered by the administration. That’s thanks largely to Rep. John Culberson (R-TX), chairman of the House appropriations subcommittee that funds NASA and a strong advocate for Europa exploration.
As currently conceived, Europa Clipper will launch no earlier than June 2022. The mission is keeping open its launch options, considering both the use of the Space Launch System as well as alternative vehicles, including the Delta IV Heavy, Falcon Heavy, and Vulcan.
An SLS would permit Europa Clipper to travel on a direct trajectory to Jupiter, arriving, in one scenario, on Christmas Eve 2024. The other vehicles would require the use of gravity assists, taking nearly three times as long—7.4 versus 2.5 years—to arrive at Jupiter. Once there, though, the missions are the same: flying in orbit around Jupiter, making between 30 and 45 flybys of Europa over two and a half years.
Europa Clipper will be a large spacecraft. With an estimated mass of about six metric tons, Goldstein said it will be the heaviest mission built by JPL to date, slightly larger than Cassini. With its solar panels unfurled, Europa Clipper will be 27 meters from tip to tip.
|“Frankly, the thing that keeps me up at night most is getting all those instruments working,” Goldstein said. “It’s quite a complex group of highly capable instruments.”|
“One question that I’ve received many, many times is, ‘Why did you go solar?’” Goldstein said. He argued that the use of solar power is much more flexible in terms of power-versus-mass trades the mission has to make, as opposed to the use of nuclear-powered radioisotope thermoelectric generators (RTGs). “If we were to fly the same mission as we have now, and use RTGs, we are now pushing above nine RTGs for this mission.”
NASA has also selected a payload of instruments for the mission. The nine instruments include cameras and spectrometers operating at ultraviolet, visible, and infrared wavelengths, as well as magnetometer, plasma, and dust instruments and an ice-penetrating radar.
The mission team has already started to plan how that suite of instruments will be used. At the committee meeting, project scientist Bob Pappalardo showed a simulation of one of those flybys, including what instruments would be used at each phase of the flyby. “We received inputs from instrument engineers and investigation scientists, working with the [principal investigators] to put together a sample flyby,” he said at the committee meeting. “We are trying to keep things as simple and repetitive as possible.”
The early, generous funding has helped the mission avoid many of the technical issues that strike other projects, Goldstein said. “We have suffered many times from the inaccuracies of our estimates and our understatement of the risks as we move forward,” he said of past missions without the same level of support.
That doesn’t mean Europa Clipper doesn’t have any issues. “Without question my biggest concern is the necessitated tight integration of the radar with the solar arrays as an integrated element,” he said. That includes power and interference issues between the arrays and the radar.
A second issue, he said, is a lack of information about the launch environment of the SLS. That risk, he said, is going down “quite a bit” after technical meetings with the SLS team. “They have done some significant wind tunnel testing and other modeling and testing,” he said. “We know our loads have come down, and we have better knowledge of what the environments are.”
“Frankly, the thing that keeps me up at night most is getting all those instruments working,” he said. “It’s quite a complex group of highly capable instruments.”
The lander, intended to be a follow-on to Europa Clipper, is still in its earliest phases. In February, NASA released the report by the proposed mission’s science definition team, laying out the goals of the lander and the instruments needed to achieve those goals.
First and foremost, according to the science team, is the search for life on Europa. “Europa may hold the clues to one of NASA’s long standing goals—to determine whether or not we are alone in the universe,” the report stated. “The highest-level science goal of the mission presented here is to search for evidence of life on Europa.”
The team also developed two other goals. One is to study whether Europa is, in fact, habitable, separate from the search for evidence of life itself. A third goal is to characterize the properties of the moon’s surface to support future missions there.
|“Like in ‘Mission Impossible,’ we’ll hit the self-destruct button and sterilize,” Goldstein said of plans to incinerate the lander for planetary protection.|
To achieve those goals, the report included a baseline suite of five science instruments. They include an organic composition analyzer to look for evidence of organic materials, a microscope to look for microbial cells, and a spectrometer to characterize organic and inorganic compounds. The lander would also carry color stereo cameras and a seismic instrument. Supporting those instruments is a system to collect samples from the surface and to a depth of up to 10 centimeters below it.
Carrying out that mission will require a spacecraft far larger and more complex than Europa Clipper. “This is a large, large vehicle,” Goldstein said. He noted that Europa Clipper, at about six metric tons, is the largest spacecraft JPL will build to date—until it builds the lander. “We are 16.6 metric tons,” he said of the lander.
“Almost all of that mass,” he said, is in the form of propellant: first to put the spacecraft into orbit around Jupiter, then around Europa, and finally for the landing itself. That mass, he said, means that the only option for launching the mission is the SLS, and even that will require a gravity assist to get to Jupiter.
The mission’s current design calls for a launch no earlier than October or December of 2025 (Goldstein said there are two separate launch windows for a late-2025 launch.) A launch then would see the spacecraft arriving at Jupiter in July 2030, with a landing no sooner than December 2031.
At Europa, the lander will separate from an orbiter spacecraft based on the Europa Clipper design that will serve as a communications relay between the lander and Earth. The lander will employ a “skycrane” system similar to that used for the Curiosity Mars lander; the skycrane lowers the lander to the surface while staying high enough that its thruster plumes do not significantly contaminate the landing site.
Original plans for the lander, Goldstein said, called for the use of landing petals, like the Mars Pathfinder lander, to stabilize the spacecraft on the surface. However, an analysis of the conditions on Europa’s surface, a jagged terrain with outcrops and fissures, forced them to rethink it. “The team looked at that quite a bit and realized… that it was very susceptible to ‘pathological’ landing cases that would end the mission,” he said.
The revised approach is what Goldstein and others in the mission have nicknamed the “cricket” design, with four legs to support the lander on the surface. Or, rather, landing stabilizers. “I have to often fight off what teams calls what look like legs,” he said. “They are not legs. They are landing stabilizers. This vehicle does not walk.”
The lander may not walk, but it does have a sampling arm. The baseline mission calls for taking five samples from ice around the lander and feeding that into the spacecraft for analysis. The spacecraft’s overall scientific payload weighs in at 42.5 kilograms, including margin. “That is significantly up from previous Europa lander mission studies, where we were looking at 10 kilograms or 15 kilograms of science payloads,” said Kevin Hand of JPL, one of the leaders of the science definition team.
The lander will be battery-powered, rather than use solar panels or an RTG. That limits the mission’s life to about 20 days, although that could be extended to 30 days or more depending on how many samples are collected and the use of other instruments on the spacecraft. The strong radiation environment in Europa’s vicinity of Jupiter limits the spacecraft lifetime regardless of power source in any respect, making batteries the simplest approach to the mission.
And, at the end of the mission, the lander will literally self-destruct. Goldstein said a “thermite-based incinerator” will be triggered at the end of the mission, or at a set time after losing contract with the Earth, to destroy the spacecraft and greatly reduce the risk of any contamination of the surface by terrestrial microbes. “Like in ‘Mission Impossible,’ we’ll hit the self-destruct button and sterilize,” he said.
But that depends on the lander getting built in the first place. While the proposed mission has benefitted from the some of the same Congressional largesse that helped Europa Clipper, it’s now facing a roadblock from the White House.
The administration’s fiscal year 2018 budget request, issued March 16, offered a record-high $1.9 billion for NASA’s planetary science program, explicitly supporting the Mars 2020 rover mission and Europa Clipper. The lander, though, was specifically excluded from funding.
|“We do not know whether or not biology works beyond Earth,” said Hand.|
“To preserve the balance of NASA’s science portfolio and maintain flexibility to conduct missions that were determined to be more important by the science community, the Budget provides no funding for a multi-billion-dollar mission to land on Europa,” the budget “blueprint” document stated. More details won’t be forthcoming until the full 2018 budget request is released in May.
The language in the budget blueprint appears to reflect the priorities outlined in the latest planetary science decadal survey, published in 2011. That report highlighted a Europa orbiter mission as the second-ranked large, or “flagship,” mission for the coming decade, after a Mars rover mission to collect samples, which became Mars 2020. A Europa lander, though, didn’t make the cut.
This has created an added dimension of uncertainty about the lander mission, which some are reticent to talk about. At a town hall meeting about the Europa lander science report March 19, just before the start of the Lunar and Planetary Science Conference outside Houston, NASA officials sought to avoid talk about the budget document released just three days earlier.
“If you’re here to talk about something other than the science of this report, the science of this mission, you are unfortunately in the wrong town hall,” said Curt Niebur, the mission’s program scientist at NASA Headquarters, at the start of a five-hour meeting that avoided any discussion of budgets.
A day later, Jim Green, head of NASA’s planetary science division, did talk about the lander’s future in an interview after a town hall meeting at the conference about NASA’s overall planetary science program. He said the lander’s future depended on both the outcome of the 2018 budget request and what funding and direction it received in a final fiscal year 2017 appropriations bill that must be passed by April 28, when the continuing resolution funding agency programs at 2016 levels expires.
Little might happen, he said, if Congress simply extends the continuing resolution through the rest of the fiscal year. “If we get a continuing resolution this fiscal year, then we’re pretty much done with the Europa lander, because we’ve done everything we can do within the budget limitations that we have,” he said.
At the National Academies committee meeting, though, Goldstein said the did have funding to continue work through the rest of the 2017 fiscal year, including a mission concept review scheduled for June. “We still have enough funding to make it through the end of the year for development,” he said. “We’re going to pursue the mission concept review and let the chips fall where they may as we proceed.”
Given Rep. Culberson’s interest in a Europa mission—he attended a public lecture about Europa exploration at the National Academies later that day, briefly mentioning his support for missions to the moon—lander work may well be funded in 2017 and 2018 despite the administration’s request, keeping it on track for a launch in 2025 or later.
How NASA pays for it, though, remains to be seen. There is no price tag yet for the orbiter mission; even Europa Clipper doesn’t have a formal cost and schedule estimate, as it’s not far enough along in its development. Past studies, though, suggested Europa Clipper could cost on the order of $2 billion, plus launch and operations.
A Europa lander, much bigger and more complex than Europa Clipper, will likely cost much more. Keeping that on schedule for a 2025 launch means that NASA’s planetary science program will have to support several flagship missions simultaneously: in addition to Clipper and the lander, there will be Mars 2020 and future Mars missions to carry out other elements of the agency’s sample return strategy. That will put a strain on the agency’s budget, which also has to fund smaller Discovery and New Frontiers missions that scientists have argued NASA should fly more frequently.
A Europa lander, then, might have to wait, perhaps until after the next decadal survey report is completed in the early 2020s. But the interest, and the science case, for a lander mission remains strong, whenever it flies.
“We do not know whether or not biology works beyond Earth,” said Hand. “So, this search for life beyond Earth, as made possible by missions like this Europa lander, really opens up a new doorway into our universe.”