The Next Pluto Mission: Part III

Continued from Part II ...


Now let’s have some fun.  Suppose, this coming July, New Horizons were to discover something truly wild as it flashed past Pluto.  What if it revealed a bizarre surface chemistry that - like the oxygen in Earth’s atmosphere - could only be the result of some biological process?  What if its imager recorded a clearly artificial set of markings on its surface - a giant pyramid, the ruins of an alien civilization.  (What if the cameras revealed a large, goofy-smiling dog?)

In light of such a monumental discovery, we might very well skip the next logical step of a robotic Pluto lander, and instead mount a manned mission.  I’ll put aside questions of cost for now, and assume that for the sake of this speculation, a manned Pluto mission - like the Apollo program - is just something that we were going to do, no matter what.  Is a manned Pluto mission within our near-term technological grasp, at any cost?

The most advanced propulsion systems we have today require 10 - 15 years to deliver a 1.6 kilogram spacecraft into Pluto orbit.  The international space station, though lacking significant propulsion, has been continuously orbiting the Earth, manned, for 14 years, since 31 October 2000.  There is, of course, an enormous difference between the ISS and a manned Pluto spacecraft.  The ISS has been resupplied and occupied by rotating crews from Earth’s surface several times per year for the past 14 years.  The Pluto astronauts would be utterly isolated; their life support systems would have to be completely self-contained.  The longest period one human being has ever spent in space is 437 days.  And no small, closed, self-contained biosphere capable of supporting human life has survived more than two years.

Tracy Caldwell Dyson aboard the International Space Station (ISS).

What if we put our Pluto-bound astronauts into hibernation?  Aside from the possibility of the mission control computer becoming homicidal during wakeup phase, there’s another objection: we don’t currently know how to hibernate human beings for more than a decade and have them come back alive.  For that reason, I’m forced to relegate hibernation scenarios to science fiction, and rely on technologies which are known at the present time.


Is there any known spacecraft propulsion technology capable of delivering a multi-hundred-ton manned mission to Pluto within a year?  It turns out that the answer is yes, and that the technology has been with us since the 1950s.  Science fiction buffs reading this piece will probably have guessed that the answer is Project Orion.  For everyone else, the Wikipedia article on that topic gives a good overview.  Briefly, the concept is to propel the spacecraft by exploding thousands of small nuclear bombs behind it.  Each detonation drives a “pusher plate” attached to the spacecraft by an enormous set of shock absorbers.  The exhaust velocities are tens to hundreds of kilometers per second, but with millions of tons of thrust.

An artist's conception of the NASA reference design for the Project Orion spacecraft powered by nuclear propulsion.

The original Project Orion physicists worked out the essentials in the early 1960s.  NASA revisited the concept again in 2000, this time under the name “External Pulsed Plasma Propulsion”.  The smallest Orion nuclear spacecraft have a mass of about 900 tons.  The original team developed an “advanced interplanetary” configuration capable of delivering a 10,000-ton spacecraft to Saturn and back again in three years.  While such a spacecraft could be launched directly from the Earth’s surface, nuclear fallout concerns would make this course of action untenable.  Instead, it would have to be constructed in Earth orbit - like the ISS - and depart for Pluto from there.

A year or two later, our nuclear-bomb-firing mothership would decelerate into orbit around Pluto, and turn its engines off.  A manned descent to Pluto’s surface would take place using more conventional chemical rockets.  Pluto’s surface gravity is about 1/12 of the Earth’s, or half of the Moon’s.  Landing on Pluto’s surface from a low orbit at 100 kilometers’ altitude requires half the delta-V of a landing on the Moon from the same height (800 meters/sec vs. 1700 meters/sec.)

Landing any spacecraft - let alone a manned spacecraft - on Pluto would present some unique challenges.  Unlike the Moon, Pluto has a very thin atmosphere of nitrogen, methane, and carbon monoxide.  Its surface pressure is varies from 6.5 to 24 micro bars - about as thick as Earth’s atmosphere 50 miles up, or about 1/1000th the density of Mars’s atmosphere at its surface.  This is probably just enough to require some kind of heat shield, but not enough to provide any useful aerobraking capability (like a parachute).  Elon Musk’s Dragon V2 capsule combines a heat shield with propulsive landing rockets, and is probably a step in the right direction.  The Dragon V2 stores enough fuel for 300 meters/second delta-V, so extra fuel tanks would be needed to land, take off, and rendezvous with the orbiting mothership.  But the technology seems feasible.

The SpaceX Dragon V2, during a test of its abort system.

There might be other hazards.  The Moon’s surface is mostly made of silicate rock.  Pluto, on the other hand, is covered with ice - not just water ice, but frozen methane, carbon monoxide, and nitrogen.  On contact with hot rocket exhaust at several thousand degrees, there’s a real danger that the landing site might vaporize.  Some care would have to be taken to land our first Pluto explorers on a stable, rocky outcropping.


Imagine you’re one of those first human Pluto explorers, stepping out of your lander.  Pluto’s moon Charon would hang motionless in your sky.  The two are tidally locked, always presenting the same face to each other as they orbit over a 6.37 day period.  But at only 19,600 kilometers away - closer than our geosynchronous satellites - Charon would appear nine times larger in Pluto’s sky than the full Moon appears from Earth.  Pluto’s other four moons Nix, Hydra, Kerberos, and Styx would be visible as slowly-moving stars, gradually rising and setting, while Charon remained fixed in the heavens.

Charon as seen from the surface of Pluto.

The Sun would be the brightest object in the sky, but would look nothing like it does in ours.  Pluto’s Sun is only an arc minute across, and would appear starlike.  But what a star!  At magnitude -19, it would appear 650 times brighter than our full Moon, will all that brightness packed into an icy, diamond-like point.

Jupiter would be the brightest planet in your sky, around magnitude 2.5, somewhat fainter than the stars in the Big Dipper.  Saturn would vary in and out of naked-eye visibility, from about magnitude 4.5 to 8.5.

And if you looked carefully, appearing about three full-Moon diameters away from the starlike burning Sun, you might notice another, much fainter, bluish “star”.  That pale blue dot would be the Earth: at magnitude 3.7, still visible to your unaided eye, but difficult to pick out from the Sun’s glare.  That’s home.  You’ve come a long way to this cold, lonely outpost at the edge of the Solar System.  And unlike New Horizons, you’re coming back.

Science fiction?  Possibly.  But let’s not forget that Pluto was discovered only 85 years ago.  Today, a spacecraft carrying the ashes of its discoverer is speeding toward that planet: a fact unimaginable in 1930.  What will the next 85 years hold?  If there’s anything you should count on, it’s not to count anything out.

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