A Lander for NASA’s Europa Mission
Posted by Van Kane
05-01-2016 7:03 CST
This article originally appeared on Van Kane's blog and is reposted here with permission.
“This Act includes $1,631,000,000 for Planetary Science. Of this amount, $261,000,000 is for Outer Planets, of which $175,000,000 is for the Jupiter Europa clipper mission and clarifies that this mission shall include an orbiter with a lander that will include competitively selected instruments and that funds shall be used to finalize the mission design concept with a target launch date of 2022.”
“…$175,000,000 is for an orbiter with a lander to meet the science goals for the Jupiter Europa mission as outlined in the most recent planetary science decadal survey. That the National Aeronautics and Space Administration shall use the Space Launch System as the launch vehicle for the Jupiter Europa mission, plan for a launch no later than 2022, and include in the fiscal year 2017 budget the 5-year funding profile necessary to achieve these goals.”
- Final budget law for Fiscal Year 2016 regarding NASA’s Europa mission
While there’s at least eight years until it launches, this has been a pivotal year for developing NASA’s Europa mission. Last spring, NASA selected a rich and highly capable instrument set. This summer, following a design concept review, the mission moved from concept studies to an official mission. And just last week, Congress directed NASA to expand the mission by adding a small lander as well as launch the mission by 2022 and use the Space Launch System. These latter aren’t just suggestions: they are the law.
There’s been almost no official information on the lander. What we know comes from a long article from Ars Technica’s Eric Berger on the then possible addition of a lander and a dedicated plume flyby sub-satellite. Berger is a long time space reporter and has developed a good relationship with House Appropriations Subcommittee Chairman John Culberson (R-TX). (I make sure I read all of Berger’s articles.) As Berger describes in detail in his article, Culberson has been the driving force behind the aggressive funding for this mission.
In addition to an earlier launch, Culberson also has wanted to see the mission carry a lander in addition to the mother craft that would make at least 45 close flybys of the moon. In prior years, Culberson added funding to NASA’s budget specifically to study a lander option, and the Jet Propulsion Laboratory’s engineers have been studying options. Berger’s story is focused more on Culberson, but it does provide a number of facts about the possible design for the lander:
The leading concept for the lander would be a small lander, perhaps about 230 kg with 20–30 kg for instruments. For comparison, the 1996 Mars Pathfinder lander had a mass of 265 kg.
The lander would be delivered to Jovian orbit by the main spacecraft and then released in a high parking orbit well outside the intense radiation fields at Europa’s orbit. The main spacecraft would study Europa’s surface for two to three years during its flybys to find the best combination of a scientifically interesting and safe landing spot.
The actual landing would use the same skycrane approach used by the Curiosity Mars mission to deliver the lander safely to the surface.
The lander would likely last perhaps 10 days on the surface using battery power. During the lander’s lifetime, it would investigate the chemistry of the surface using a mass spectrometer and possibly a Raman spectrometer.
A lander could add $700M or more to the mission cost. The last cost estimate I heard for just the main spacecraft was $2.1B. We don’t know how firm the lander cost estimate is.
Adding a lander would delay launch from a possible 2022 to 2023.
This description is pretty bare bones, but with a little legwork, it is possible to flesh out these ideas with some informed speculation. It helps that a number of previous studies have been published that examined concepts for a Europa lander.
The primary goal of any lander would be to sample material from the interior ocean to see if the chemicals needed to support life are present and whether complex organic molecules suggesting biotic or pre-biotic activity exist. We lack the technology to drill through the kilometers of ice to reach the ocean directly. However, in many locations the icy shell appears to be fractured and water from below has spilled onto the surface and frozen, and in certain locations may be actively venting into space. The goal will be to set the lander down in one of these zones.
Our current knowledge of Europa’s surface is too poor to select the scientifically most interesting sites that are also safe to land in. The main spacecraft will spend three years circling Jupiter and flying low over Europa 45 times. One of its prime goals will be to for its cameras and spectrometers to find the optimal combination of evidence of ocean material on the surface with a safe landing zone. Any landing will need to wait for scientists to build their high resolution maps.
One aspect of this proposed lander concept is different than those I’ve seen before. Most lander studies have looked at small spacecraft (and this proposal would count as a small spacecraft) that would be carried by the mother craft until just before landing. For the design Berger reported on, lander and its descent stage would orbit Jupiter on their own for months to years before landing. This means that together they are a fully functional independent spacecraft with its own solar arrays for power, propulsion, navigation, and communications. Apparently the cost and mass of adding these functions to the descent stage and lander is a better bargain than adding the radiation hardening that would be required if the lander were carried past Europa 45 times.
Once on the surface, the lander could be well-protected from radiation. The rotation of Jupiter’s magnetosphere causes the radiation to slam into Europa’s trailing hemisphere. The leading hemisphere has Europa’s bulk as a very effective radiation shield, and radiation there is fairly low. Past proposals have focused on putting a lander on the leading hemisphere. As a result, the lander likely would run out of power before radiation would fry its electronics. Fortunately, there are several regions on the leading hemisphere where the icy shell appears to have been recently (in geologic terms, anyway) fractured.
Berger’s article states that the lander would likely be powered by batteries, limiting its life to around 10 days. Solar panels apparently are being considered, but I can see why they might not be attractive. Sunlight at Jupiter is weak, and solar panels large enough to harvest a meaningful amount of that light might be too bulky and heavy for the mission.
Berger’s article lists just two possible instruments for the lander. Based on his language, the core instrument would be a mass spectrometer that would “weigh” the molecules and atoms in samples scooped, cut, or drilled from the surface. Extremely complex molecules could suggest life, especially if they are rich in elements, like carbon, which are the basis for life on the Earth. A second instrument under consideration would be a Raman spectrometer, which would illuminate samples with lasers and use the resulting “glow” to measure composition including complex organic molecules. (For those who understand Raman spectroscopy, please forgive this simplification of a complex subject; here’s a link to a Wikipedia article for more on this technology.) I’ve also heard through other sources that the lander would carry an imager to examine the terrain around the landing site.
Once on the surface, the lander would use a sample acquisition system to collect a sample of ice from the surface. As Berger points out, at Europa’s surface temperatures, the ice is as hard as rock, so the cutting or drilling mechanism will need to be robust. After the sample is collected, it would be delivered to the instruments to measure its composition. If the lander touches down near an active vent, the mass spectrometer could also measure the composition of the particles and gases in the plume.
Previous studies have typically proposed at least two other instruments. Europa’s icy shell is constantly being stressed by the tides induced by Jupiter, which should produce high seismic activity. A seismometer would give scientists a rich data set on the interior structure of the ice. Europa also sits within Jupiter’s intense magnetosphere, which causes an induced magnetic field in the moon’s interior ocean. How this induced field varies as Europa orbits Jupiter would provide valuable clues to the size and salinity of the ocean. A magnetometer on the lander could provide continuous measurements for the life of the lander. Berger’s article was silent on whether or not these instruments are under consideration for this version of the lander.
(On a side note, a magnetometer plus a simple plasma probe would allow the lander to conduct useful science while it orbits Jupiter waiting for landing. Scientists would like to study Jupiter’s magnetosphere from multiple locations at once. The lander while in orbit around Jupiter could complement similar measurements from the main spacecraft, and depending on the timing, also from Europe’s JUICE spacecraft that will enter Jovian orbit in the late 2020s.)
Berger’s article is silent on how data would be returned to Earth. Two possibilities are obvious – low data rate transmissions directly from the lander to Earth or high data rate transmissions from the lander to the orbiter for later relay to Earth. Data relay from the mother flyby spacecraft likely would be possible, but the rapidly changing relative locations of the landing site and the orbiter circling Jupiter may limit how much data could be returned and when communication relay is possible. A recent European study for a Europa lander assumed that the mother spacecraft would have just one chance to directly receive data from the lander in a 10 day period. One argument for excluding a seismometer is that this instrument would produce large amounts of data that may be difficult to return directly to Earth. The European study found that the brief relay between lander and orbiter would have enabled the return of seismic data. Magnetometers, on the other hand, produce only small amounts of data that likely could be directly relayed to Earth (assuming the lander would have that ability).
A major challenge for any Europa lander will be that the scientifically most interesting places to study also appear to have extremely rugged terrain, which makes landing risky. Berger’s article briefly mentions that the lander would use an autonomous landing system to examine the terrain below it to pick out safe spots to put down. Technologies to allow a lander to image its landing site during descent have been studied for years and were implemented on the Chinese Chang’e 3 lunar lander and are under consideration for NASA’s 2020 Mars rover. During final descent, these systems use images taken by the lander in real time to analyze the terrain below to identify safe landing zones. With an autonomous guided descent, scientists can target an area that overall is rugged but has small safe zones.
What I conclude from the clues Berger supplies is that the Europa landing would be much like the Philae comet lander (although with Europa’s higher gravity, the lander will not bounce across the surface after touchdown as Philae did). The lander would have just a few days to conduct its operations and return the data to Earth. On Mars, we have become accustomed to landed missions that last years with plenty of time to carefully consider where to sample and then conduct follow up studies. A Europa lander will be a mad dash to complete the science goals before the batteries die.
By the end of its life, the lander will have returned our first data directly from the surface of an ocean world that may harbor life that arose independently from the life on Earth.
Editorial Thoughts: I of course want to see a lander delivered to the surface of Europa, but I have mixed feelings about the inclusion of a lander on NASA’s first dedicated mission to Europa for two reasons. First, as I will explore in more detail in my next post, adding a lander to the existing Europa mission will push its costs up, perhaps to the $3.5B range when including a launch on the SLS. Congress will need to substantially increase the planetary budget to prevent the Europa mission from crowding out the smaller planetary missions that provide balance to the program. While Congress can pass budget laws directing year to year spending, meeting these aggressive goals will require that the President’s Office of Management and Budget (OMB) accepts the new plan and allows NASA to sign the necessary multi-year contracts with its vendors. In the past, OMB has resisted prioritizing the Planetary Science budget to accommodate a Europa mission.
Second, the driving force behind the expanded mission depends on one Congressman and his continued re-election, his political party’s continued control of Congress, and his health. The alternative approach would be to run the exploration as NASA has run the Mars program by spreading costs out over a sequence of missions. This would be in the vein of the proposed “Ocean Worlds” program currently being shopped to NASA and Congress.
I expect that in the next few months that we will learn considerably more about the lander’s design and NASA’s plans on how it will fit into its overall planetary program.