How to Conquer Titan With a Nuclear Quad Octocopter
In December, NASA announced two finalist concepts for a robotic
mission that will launch in the mid-2020s. The first is the Comet
Astrobiology Exploration Sample Return (CAESAR), which would send a
fairly conventional spacecraft over to a comet to grab a chunk of its
nucleus and bring it back to Earth. That's cool and all, but we're much
more excited about the second finalist concept: Dragonfly,
from the Johns Hopkins University Applied Physics Lab (APL), a quad
octocopter that would explore Saturn's moon Titan from the air. The
idea is that it would work like a planetary rover, except that it would
fly instead of drive, allowing it to cover much more ground at the risk
of, you know, crashing.
We've seen lots of drones that can do amazing things, and also lots
of drones that crash very, very badly while trying to do amazing things.
Sending a fully autonomous flying robot to an alien world over a
billion kilometers away and expecting it to fly around for a couple
years without any human intervention seems extraordinarily ambitious, so
we checked in with APL to see exactly what they're working on.
Titan is an appealing place to visit because in some important ways,
it's more like Earth than any other place in the solar system. It's a
bit chilly, with an average surface temperature of just 94 Kelvin, and
the atmosphere is thick (four times thicker than Earth's) and made up of
nitrogen and methane. It also has one-seventh the gravitational pull of
Earth. But, what's so interesting about Titan is that it's got a
methane cycle much like Earth's water cycle, with liquid methane forming
lakes and rivers and clouds and rain. There's also a bunch of organic
compounds thrown into the mix, which makes it an intriguing place to
look for very primitive, and very weird, life.
The Cassini mission to Saturn included a little probe called Huygens,
which was dropped on Titan in January of 2005. Huygens was mostly
designed to
This image from the Huygens probe shows Titan’s surface.
measure atmospheric conditions, but it managed to survive landing on
the surface of Titan for a little over an hour anyway, and sent back a
picture of the surface.
When considering how the surface of Titan might be explored, the
moon's exotic characteristics open up many more creative options than
would be available for a planet like Mars. In particular, the low
gravity and high atmospheric density of Titan favor flight, which would
allow an exploration robot to visit many more sites of scientific
interest much faster than a stationary lander or a rover. In the past,
NASA has considered things like helicopters and hot air balloons and
airplanes, but over the last decade, multi-rotor drones have become the
standard for maneuverable and dependable robotic flight. According to
APL, Titan is in fact “the easiest place in the solar system to fly a
quadcopter.”
Dragonfly is based around a 300 kilogram-ish "quad octocopter"
design, which is a quadcopter that has motors and propellers that are
doubled up. There's a small aerodynamic penalty for arranging things
this way; strictly speaking, a conventional octocopter with eight
separate propellers would be more efficient. But Dragonfly has to fit
inside a hypersonic aeroshell for delivery to Titan, and the quad
octocopter is much more space efficient while still maintaining a
generous amount of redundancy.
The reason that Dragonfly is as bulky as it is in the first place is
that it needs to bring along its own power source. Titan is far too hazy
and far too, uh, far from the Sun for solar power to be a viable
option, so Dragonfly will rely on the same kind of power system that the
Curiosity rover is using on Mars: a radioisotope thermoelectric
generator (RTG) that converts heat generated by plutonium-238 into
electricity. RTGs can operate for decades, and are especially useful for
deep space missions because the 'waste' heat they generate can be used
to keep things nice and warm....MUCH MORE
