Monday, June 6, 2016

Red Dragon and Planetary Exploration

A few weeks ago, as I’m sure most people reading this blog know, Elon Musk, the CEO of SpaceX announced plans to land their Dragon spacecraft, largely at the company’s expense, on Mars.  While this plan is audacious enough, Musk has previously positioned SpaceX’s Dragon capsule as an all-purpose lander suitable to explore almost the entire solar system. 

Since Musk’s announcement, I’ve been doing research and thinking about what the availability of a commercial planetary lander might mean for planetary exploration.  Even if landing the company’s Dragon spacecraft on Mars proves to be a one-time event, it will demonstrate that the technologies for planetary missions have become widely available. 

What if, though, SpaceX’s Dragon spacecraft becomes a standard catalog item that could ordered, the way a launch vehicle is?  What might the impact be on planetary exploration?  As I thought about this, I concluded that three questions are key: How flexible will the Dragon spacecraft be as a payload delivery vehicle?  How far afield can it operate in the solar system without design changes so massive that it becomes necessary to essentially redesign it?  And how will be missions it might fly be paid for?

Mock up of the Dragon version 2 capsule for Earth orbital missions.  Credit SpaceX.

First, some background on the Dragon spacecraft.  SpaceX currently is using the first version of the uncrewed Dragon spacecraft to deliver supplies to the International Space Station.  It is also being being funded by NASA to design a crewed version of the spacecraft to deliver astronauts. (I couldn’t find out what proportion of the Dragon development is being funded by NASA versus by SpaceX’s own funds.)  Musk has ensured that this second version incorporates key technologies such as heat shields that can survive atmospheric entry from interplanetary speeds and built in thrusters that allow soft landings.  (Phil Plait at Slate magazine has an excellent article on the technologies involved.  Wikipedia has additional information.)

Designing a craft for interplanetary flight requires numerous differences from a craft designed to operate in low Earth orbit for short periods of time and then return to our world.  Systems must be able to function for months to years.  Electronics must be able to function in the harsher radiation environment outside the Earth’s magnetosphere.  The communications system must be able to transmit and receive data over distances of hundreds of millions of kilometers.  There will be hours to days between communications periods, requiring the craft to be able to operate autonomously.

Artist's conception of the Red Dragon capsule on the surface of Mars.  Credit: SpaceX.

To operate as a long-lived lander on the surface of a planet, the spacecraft must deploy solar panels to generate power after touch down.  (I’ve not heard of any plans to use plutonium-based power generators for the Dragon spacecraft.)  Batteries must be carried and recharged to keep the spacecraft operating during the night.  And especially on frigid Mars, the spacecraft will need insulation and heaters to keep it warm.

These problems are well known, and we can presume that SpaceX’s engineers have designed the Dragon spacecraft and their subsystems with these issues in mind.  It’s worth pausing for a moment to consider the kind of commitment this implies to SpaceX’s commitment to revolutionize Martian exploration because these enhancements likely aren’t cheap.

By making these investments in the basic Dragon spacecraft design, referred to as the Red Dragon for Mars missions, SpaceX can take advantage of what is likely to be a low volume assembly line building these craft.  While NASA has designs of its own it can reuse to land on Mars, flights are likely to be infrequent enough that their components may gradually become obsolete and the expertise to rebuild and test them may fade.  SpaceX presumably will be building several Dragon spacecraft a decade, allowing it to gradually update the design and keep its expertise intact.  The result will likely be a lander that may be substantially cheaper than NASA reusing its existing designs for some Mars missions.

How Flexible is the Interplanetary Dragon Lander?

I know of two proposals for scientific missions that would use the Red Dragon spacecraft.  One, IceBreaker, led by NASA’s Chris McKay would return to the northern polar plains of Mars to further investigate the subsurface ices found there for habitability and signs of life.  The appeal of the Dragon spacecraft appears to have been its expected low cost.  McKay has also proposed the IceBreaker mission using a near copy of the much smaller NASA Phoenix and InSight landers.  The instruments would have weighed a few tens of kilograms, barely taking advantage of the payload that the Red Dragon could deliver.  The Dragon lander (dubbed the IceDragon for this concept), however, could have carried a much larger drill than the Phoenix-derived lander, allowing samples to be collected from much further below the surface.  (The one published abstract for the IceDragon mission from several years ago proposed a 2 meter drill.  Technology development since then might allow much deeper drilling.)

Conceptual design for the IceDragon version of the Red Dragon capsule that could be sent to sample the icy plains of Mars' arctic regions.  Human figure shown for scale.  Credit: NASA.

The second proposal would be to use the Dragon spacecraft to deliver a launch vehicle to return samples directly from the surface of Mars to the Earth.  This concept takes full advantage of the payload mass and volume offered by the Dragon.  Other concepts proposed to return samples to Earth envision two missions.  The first would land on Mars and launch a sample canister into Martian orbit.  The second would retrieve the canister and perform the voyage to Earth.  The large ascent vehicle enabled by the Dragon would, the proposers argue, combine these two functions to lower costs, complexity, and risk. 

Unfortunately, this proposal requires that a rover already be on Mars that could deliver a canister with samples to the Dragon spacecraft.  While NASA’s planned 2020 Mars rover will collect samples, it will leave them on the surface for a later rover to collect and return to an ascent vehicle.  There isn’t the payload mass for the proposed Dragon mission to carry its own fetch rover and a direct-to-Earth ascent return vehicle, although it could launch samples into Martian orbit if it has to carry its own fetch rover according to the proposers.

Conceptual design for the Red Dragon used to carry a Mars Ascent Vehicle (MAV) and an Earth Return Vehicle (ERV).  Credit: NASA.

There’s much we don’t know about the Red Dragon design.  Will it be suited only to missions where the scientific investigations are limited to the immediate landing site where robotic arms could reach such as these two proposals?  That would suggest missions to locations where homogenous conditions exist over large areas such as the polar plains of Mars. 

A variation of the proposal to use the Dragon design to return samples from Mars would be to use it to return samples from different regions of the moon or from some of the larger asteroids.  Since the scientists proposing lunar sample returns are interested in broad regional differences in composition, grabbing samples with an arm in the immediate vicinity of the lander would meet their goals.

However, many of the goals for lunar and Martian exploration require rovers that can reach and study or sample multiple locations across within a much larger study area.  I have not seen any analysis about whether delivering a capable rover would be a feasible extension of the Red Dragon’s design.  Could the Dragon design host a moderately large rover (smaller than the Curiosity rover but perhaps bigger than the Opportunity rover) and deploy it to the surface by including a large hatch and a ramp or crane? 

NASA already has two current, proven designs for Martian landers.  The first, used for the Phoenix and InSight missions, delivers a few tens of kilograms of instruments using a small, stationary lander.  The key advantage of this platform is that it provides a soft landing, and its low stature provides easy access to the surface.  The second system used parachutes and a rocket-powered skycrane to deliver the Curiosity rover to the surface (and the design will be reused for the 2020 rover).  It also places up to 930 kilograms of rover directly on the surface, eliminating the need for ramps to get the rover off the lander.  The landing system is also designed with the myriad of safety requirements needed for a rover/lander to use a plutonium power supply.

The Red Dragon will be able to deliver a slightly more massive payload, about 1000 kilograms.  While the payload space appears large by the standards of most planetary landers, it doesn’t appear large enough to carry a duplicate of the Curiosity rover.  Unlike the skycrane landing system, the payload will sit inside a large spacecraft, well above the ground.  Long robotic arms or drills would be needed to bring surface samples to instruments inside the spacecraft or place small instruments on the surface.  Delivering a large package such as a rover to the surface would require ramps or a crane.  I expect that these problems are solvable, but they create a level of complexity that NASA’s skycrane system was invented to avoid.

Comparison of the sizes of the Dragon capsule (left) and the Curiosity rover entry and descent system.  The Dragon capsule would not have the width to carry a Curiosity-sized rover, but likely could carry a smaller rover to the Martian surface.  Credit: NASA.

Beyond the Moon and Mars

Musk has been quoted saying that the Dragon capsule can be used to explore almost any world in the solar system: “With Dragon launched on a Falcon Heavy, it can go pretty much anywhere in the solar system, because that’s a heck of a big rocket…  Dragon 2 is capable of transporting scientific payloads to anywhere in the solar system, with a liquid or solid surface, with or without an atmosphere. So Dragon is really a crew transport and science delivery platform…  Dragon, with the heat shield, parachutes and propulsive landing capability, is able to land on a planet that has higher entry heating, like Mars. It can also land on the Moon, or potentially conduct a Europa mission.”

Musk’s claim appears to touch on three separate issues.  The first is whether the Falcon Heavy could propel a Dragon spacecraft to any location in the solar system.  The answer likely is yes.  For our moon, Mars, and Venus, the launcher will be able to send the Dragon directly to these worlds.  If the Falcon Heavy cannot send a Dragon directly to Mercury or the outer planets, it certainly could launch it on a trajectory that would use Venus and/or Earth gravity assists to provide the additional velocity needed.

The second issue is whether the Dragon could land on these more distant worlds, which breaks into two parts.  The first is how the Dragon capsule would kill the high approach velocity when it reaches its destination.  The atmospheres of Mars, Venus, and Titan can kill much of the speed.  It’s less clear to me whether the Dragon spacecraft would carry enough fuel to first brake into, say, Jovian orbit and then kill the remaining velocity to land on Europa.  (A direct landing on Europa without entering Jovian orbit also would need to kill a lot of speed.)  Musk’s statements suggest it could, but for now we lack details.

The third issue is whether the Dragon spacecraft could deal with the special environmental issues at various worlds.  The moon and Mars are reasonably straightforward challenges both because of their proximity and their comparatively (for planetary destinations) benign environments. By contrast, while the Mercurian poles are cool, the spacecraft would need to deal with intense solar heating on the way to this world.  The surface of Venus is intensely hot and has a crushing atmospheric surface pressure.  Jupiter’s moon Europa sits deep within an intense, electronics frying, radiation belt.  Titan’s surface is as bitterly cold as Venus’ is broiling.  Landing on a small asteroid or comet may require special adaptations such as harpoons to hold the capsule on the surface.  Traveling to any world beyond Jupiter will require a radioisotope generator for power (Saturn might be an exception). 
It is possible to design spacecraft to handle any of these challenges.  Missions have been proposed to land on each of these worlds, but using custom designs that take into account the unique challenges of each environments.  Would it be cost effective to modify the Dragon spacecraft to handle the challenges of any of these worlds, or more cost effective to design a custom spacecraft?  

There’s also the question of whether a prior scouting mission would be needed.  For the moon and for high priority locations on Mars, existing high resolution images would allow mission planners to identify precise locations of safe terrain within otherwise rugged but scientifically interesting terrains.  The same will be true for selected sites on Europa following NASA’s planned Europa Multiple Flyby mission.  What about a world with only coarse resolution mapping or none at all?

Example of the rugged terrain the salt deposits of Ceres are found on.  Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

As a thought experiment, I considered a Dragon mission to land on the asteroid Ceres to explore the salts on the surface that have erupted from a likely (now frozen?) subterranean ocean.  We have moderate resolution (35 meters) images of the surface from the Dawn spacecraft currently orbiting that world.  The images of the terrain where the exposed salts are found that I’ve seen, however, appear quite rugged at 35 m resolution.  And a lot of lander-killing ruggedness can hide within that scale of resolution.  For assessing a potential Martian landing site, by comparison, NASA likes to have images with sub-meter resolution.
I’m sure that the Dragon capsule or its service module could physically carry a high resolution camera to scout for safe landing sites.  But would the spacecraft have the precise pointing capability to accurately aim the camera and the stability to prevent image jitter?  Again, solutions to these problems are well known, but is this something that would have to be added to the Dragon spacecraft?

Who Will Pay for Interplanetary Dragon Missions?

Until we know more about the capabilities of the Red Dragon capsule, it is hard to know what its advantages and disadvantages will be compared to existing lander designs.  It may provide a significant cost advantage – remember that likely assembly line of Dragon spacecraft.  However, the lander hardware is just one part of the cost of a mission.  There is still the cost of the launch, the instruments, potentially a rover or a launch vehicle for sample return, and operations. 

Even if the cost of a Red Dragon landing were free, these other costs would drive the total mission costs to several hundred millions of dollars.  This puts a Red Dragon-based mission in competition for funding with all the other missions planetary scientists would like to conduct.  A lunar or Martian landing mission may be selected by NASA only once a decade, and the Red Dragon may or may not be the most suitable design. 

As an alternative, perhaps SpaceX will schedule a Martian landing every two years or so and will sell payload space to space agencies, universities, and even private companies to cover costs.  Then the cost to any individual space agency, university research group, or company might be relatively small – a few million or tens of millions of dollars. 

Or Musk may use the profits from SpaceX or a portion of his personal fortune to drive his own program of Mars robotic exploration.  From his statements we know he wants to eventually take humans to Mars.  Perhaps Red Dragon is a stepping stone to that grander vision.

Red Dragon is possible because of the vision and drive of its founder, Elon Musk.  He made a strategic decision to build a capsule that could land on Mars as well as meet NASA’s needs in near Earth orbit.  We will need to wait to learn what types of landed missions that vision will encompass and which worlds beyond Mars Musk wants to explore.