A European Spacecraft to Accompany NASA’s Europa Spacecraft?

Last year, NASA’s managers invited the European Space Agency (ESA) to propose a small spacecraft to explore the Jovian system.  The small craft would be carried to Jupiter by NASA’s own, large Europa multi-flyby spacecraft.  This daughter mission could add to the exploration of Europa or study another target within the Jovian system.

ESA has recently posted the results of studies for two possible spacecraft that might be carried by NASA’s Europa spacecraft to the Jupiter system.  One would land on Europa and the other would fly by the volcanic moon Io.  While these were concept studies and not an actual proposal from ESA to NASA, they give an idea of the possible capabilities and limitations on an ESA contribution. 

In the coming year, Europe’s scientists can make actual proposals for spacecraft to be added to NASA’s mission.  They can do so through ESA’s competition to select it’s fifth medium sized (~550M Euros or somewhat more in dollars at the current exchange rate) science mission.  Proposals for the Europa mission will be pitted against other planetary and astrophysics missions.  However, because NASA would cover the costs of launch and delivery to the Jupiter system, proposals for the Europa mission could have a leg up in the competition.

A small ESA spacecraft would need to find a scientific angle not already taken by its larger cousins.  Next year, NASA’s Juno spacecraft will study the Jupiter itself from just a few thousand kilometers above the top of the cloud deck.  ESA’s JUICE spacecraft will arrive in the late 2020s to study Jupiter from afar, flyby Europa and Callisto multiple times, and then orbit Ganymede.  NASA Europa Mission (apparently no longer called the Europa Clipper) will flyby Europa 45 times as well as flyby Ganymede and Callisto.  These missions will carry suites of extensive and highly capable instruments.

Under NASA’s proposal, the American space agency is reserving space and 250 kg of mass to host the European spacecraft.  (NASA is also separately reserving mass for the equipment to connect the two spacecraft.)

One concept for an ESA daughter probe (in blue) shown attached the NASA’s Europa Mission spacecraft.  Credit: ESA

So if you had a ride to the Jupiter system for 250 kg, what could you do with it?

Two obvious possibilities were mentioned at the time that the offer was announced: build a small lander for Europa or a small spacecraft that would fly through any plumes erupting from Europa’s surface.  The ESA team looked at both.

For a lander, the European study group considered a type of hard lander known as a penetrator.  Shaped roughly like a cannon shell, penetrators smack into the surface and then travel a few meters into it before coming to a stop.  (Think of a bullet shot into the ground.  The friction between the bullet’s surface with the soil slows and eventually stops the bullet.)

Concept illustration of a Europa penetrator (blue) embedded in the icy surface.  An after body (green) remains on the surface with the antenna to communicate with NASA’s Europa mission spacecraft.  A cable would connect the two sections.  Credit: ESA

Penetrators have been used on the Earth to deploy sensors from planes into remote locations such as ice shelves.  Penetrators have the advantage of not requiring expensive landing systems – the penetration into the surface supplies the braking.  A unique advantage on Europa is that once buried, the surrounding ice protects the probe from the radiation fields around Europa.

The concept studied would use the penetrator to deliver two sets of instruments in to the ice.  The first set would study the chemistry of the ice and materials within it.  A habitability package would use chemical reactions to look for conditions such as pH consistent with possible life, a mass spectrometer would analysis the composition, and a microscope would image the sample.  A seismometer would record Europa-quakes to study the level of activity within the icy crust and to gather clues about its structure.

The penetrator with its deployment stage.  Credit: ESA

The penetrator would be delivered by a deployment stage that would essentially be a small spacecraft with a substantial retrorocket.  The main NASA spacecraft would target the daughter craft to pass just 35 km above the landing zone.  Just before that distance, the European spacecraft would fire its rocket, reducing its speed to zero relative to the Europan surface below.  The delivery spacecraft with the attached penetrator then begin their free fall to the surface.  The delivery craft would have 231 seconds to reorient itself so that the penetrator points straight down and then release it.  NASA’s Europa spacecraft passing overhead would have a short few minutes to listen for a radio confirmation that the landing succeeded.

The penetrator deployment.  SRM burn is the rocket burn that removes all velocity between the penetrator and the surface of Europa.  The diagram uses the older name ‘Clipper’ for the NASA spacecraft.  Credit: ESA

Approximately ten days later, the NASA spacecraft would return to Europa and receive the data collected by the penetrator’s instruments.

While the penetrator concept is exciting, the devils are in the details.  Planetary penetrator missions have been studied for many potential missions.  Russia launched two on a mission intended for Mars but which never left Earth orbit.  NASA delivered two tiny penetrators to Mars, but they were never heard from after they were released by their carrier spacecraft.  Japan spent years developing penetrators for the moon but eventually cancelled the project because of development problems. 

A key problem with penetrators is that they need relatively flat landing sites for successful landings.  Europa’s surface is covered in slopes and rough terrain.  Also, Penetrators are built to tolerate high vertical velocities, but any lateral velocity can destroy the payload inside.  (Put another way, engineers can design for high G’s in one direction, but it’s hard to design for all directions.)  This means that the retro rocket must successfully kill all but the smallest lateral movement so the penetrator moves only vertically during its descent.

Another problem with penetrators is that the space inside is small and any instruments must be built to withstand high impact forces.  As a result, there’s usually significant instrument development required.  The penetrator concept report states that the instruments to study the chemistry of the ice are at a low state of technical readiness for use in a penetrator.

Issues such as these have kept penetrators as a great idea that has never been matured enough to become a reliable tool for planetary exploration.

My take on the report descripting the penetrator concept is that delivering a penetrator for Europa appears to be a high risk possibility both for completing the development in time for a launch and for actual delivery.  Another significant problem is that the concept craft would have a mass greater than 300 kg, well above the 250 kg NASA is offering.

The other concept studied by ESA’s engineers would be a daughter spacecraft that would be a straightforward use of existing technologies.  The original idea was for a small spacecraft that could fly through plumes erupting from Europa.  This idea seems to have lost its appeal.  First, diligent searches have failed to confirm the original observation of a possible plume (which was made at the limits of detectability).  Second, NASA’s Europa spacecraft is highly capable with cutting edge instruments, and it could fly through any plumes itself.

Between the Juno, JUICE, and Europa missions, almost all of the Jupiter system is already targeted for detailed study.  An exception, though, is the extremely volcanic moon Io that sits deep within Jupiter’s radiation field.  That moon became the target for the second study.

In this concept, NASA’s craft would release the European orbiter shortly after the two jointly enter Jovian orbit.  The ESA craft then would fire its own engine to lower the perijove of its orbit to encounter Io at least twice.

The Io flyby spacecraft would be so small that this top view engineering diagram uses millimeters for its length units.  The triangular main spacecraft body would be below the center main antenna dish.  Credit ESA.

The Io flyby spacecraft would carry four instruments.  A multi-color camera would image the surface at resolutions ranging from 2.2 km to 18 m per pixel, a thermal mapper would identify and measure the temperature of hotspots at resolutions ranging from 30 km to 50 m per pixel, a mass spectrometer would measure the composition of ions and particles ejected from Io, and a magnetometer would study the magnetic field around Io.

The trajectory for each of the two Io flybys (second flyby shown here) would be chosen so that the ground tracks pass over several areas of known volcanic activity so that these areas could be studied in high resolution.  The first flyby would pass 500 km above the surface at closest approach, the second flyby 100 km.  Credit ESA

Jupiter’s radiation is strongest in the plane of the equator where its major moons, including Io, orbit.  Since the ESA craft would be released from the NASA craft in an equatorial orbit, the ESA mission would receive the full radiation baking.  ESA’s spacecraft would have to traverse this radiation on both the inbound and outbound legs of its passes.  (A larger, fully dedicated Io mission such as the proposed Io Volcano Observer proposed for NASA’s Discovery program, would use a polar Jovian orbit instead, limiting its radiation exposure.)  The tiny ESA spacecraft potentially could perform more than two flybys if the harsh radiation close to Jupiter degrades the craft’s electronics more slowly than expected. 

The Io spacecraft study report does suggest that if the idea of an Io spacecraft is pursued, that the option of releasing it before the Jupiter orbit insertion burn is done should be considered.  That way, the Io spacecraft could do its own insertion burn and enter a Jovian polar orbit to reduce radiation exposure.

The Io flyby concept studies would come in on the heavy side by missing the 250 kg target mass by 17 kg.  To get that close, the concept design had accept a “more risky operational scheme”, that is, reduce backup systems and capabilities to minimize weight.

Either of these missions would be an exciting addition to the already planned JUICE and Europa missions.  The Io flyby craft seems to be less risky both to design and to fly.

When NASA first announced that it would offer room and mass for an ESA probe, ESA’s managers said they would let scientific teams propose which concepts would be considered as part of the next Medium science mission competition.  These two proposals are proof of concept studies that the proposing teams can use to inform their proposals.  We may be seeing even more interesting proposals from the science teams.  I can think of several alternative probes, and I’m sure the professionals can think of even more creative ones.

If ESA does contribute a small probe to NASA’s mission, the exploration of the Jupiter system may be more interesting than it already promises to be.