While a pair of
NASA rovers explore Mars and the Cassini-Huygens mission peers close
at Saturn, two research teams are targeting a more distant planetary
quarry, the ice giant Neptune.
In the
separate studies, planetary scientists and engineers are drawing up plans to
send an orbiter laden with atmospheric probes and landers to Neptune, the eighth
planet from the Sun. While each mission has its own way of reaching Neptune,
both seek a better understanding of the planet and its surrounding 13 known
moons, especially the oddball Triton.
"It's
all part of the history of our solar system," said Andrew Ingersoll, a study
leader and planetary scientist at the California Institute of Technology
(CalTech), in a telephone interview. "Neptune and Uranus are ice giants, and
made mostly of heavier stuff than Jupiter or Saturn."
Ingersoll and his colleagues
envision a Cassini-like mission that
could use conventional rocket propulsion and gravity assists to reach Neptune.
Meanwhile, another version
of the Neptune mission, which features the use of a nuclear fission reactor and
ion
propulsion to reach the ice giant and a timescale that spans
two decades, is also under scrutiny.
"What
makes Neptune unique is Triton," explained David Atkinson, a University of
Idaho professor and the science principal investigator for the second
study, in an e-mail interview. "It is speculated that Triton is actually a Kuiper
Belt Object that was captured by Neptune."
Boeing
Satellite Systems' Bernie Bienstock leads the second study with
Atkinson.
Both Neptune mission
efforts are part of NASA's Vision Mission program to develop long-term
space exploration goals.
The funny thing about
Neptune
Neptune
sits about 2.7 billion miles (4.4 billion kilometers) from the Sun. Voyager
2 swung by the planet in 1989 and Pluto, because of its
orbit, periodically wanders inside Neptune's orbit.
Since
Neptune is an ice giant, a dedicated mission to the planet would yield not only
more about its formation and evolution, but also how such planets fit into the
solar system, Atkinson said.
"The
chemical make-up [of and ice giant] is different from gas giants like Jupiter,"
Paul Steffes, a radio scientist and member of the nuclear
electric Neptune team, told SPACE.com. "It's less affected by the
inner solar system bodies and more representative of the primordial solar
nebula."
While
the same case for exploration can be made for Uranus, a fellow ice giant, the
kicker is Neptune's largest moon Triton, which astronomers believe is a
non-native captive of its parent planet. It circles Neptune in a retrograde
orbit, in the opposite direction of Neptune's rotation. It has a gossamer thin
atmosphere where parachutes would be useless for any landing
probe.
"Triton
is just a really interesting object," Ingersoll said, adding that Neptune's
partial ring arcs add to planetary system's draw.
Getting there
sooner
Ingersoll's Neptune-bound
craft would take a page from many of NASA's far-flung planetary exploration
missions and rely on radioisotope thermal generators (RTGs), a long-lasting
battery fueled by plutonium, for electric power. The Cassini orbiter currently
at Saturn, for example, uses RTGs for power since the vast distance makes solar
panels unpractical.
"Yes, we'd need RTGs and
yes RTGs carry plutonium," Ingersoll said, adding that the power source can only
a danger if it is vaporized over a city, a very unlikely case since most launch
scenarios would have them dropping into the ocean in an emergency. "There's been
a lot if irrationality about nuclear power and fuels."
Ingersoll's team estimates
their spacecraft would take about 12 years to reach Neptune, but stopping once
it arrives may be a challenge. His team is studying how to use aerocapture,
a maneuver that allows a spacecraft to enter orbit around a planet using
the atmosphere and no fuel. While NASA has experience with aerobraking, a
gentler, fuel-burning maneuver, it has yet to use aerocapture in a mission.
"The most challenging thing
technologically for us is to fly an aerocapture mission to Earth of Mars to
demonstrate that it can be done," Ingersoll said, adding that the method occurs
at higher speeds and digs deeper into a planet's atmosphere than aerobraking. It
may also require a heat shield for thermal protection, he
added.
Neptune's
Prometheus
A nuclear electric
propulsion Neptune flight would build on NASA's plans for the Jupiter Icy Moons
Orbiter (JIMO) mission under Project
Prometheus, which is expected to use a nuclear fission reactor to
power an ion engine.
The method is slow. An ion
engine propelled Neptune mission launched around 2016 would take time to build
up enough thrust to reach the planet, entering orbit around 2035, researchers
said. But once there, the spacecraft would still have a large fuel and power
supply for a long-duration stay.
"Since this mission may
very well be the only mission Neptune this century, it is important the complete
Neptune system be studied in detail," Atkinson said, adding that the sheer power
provided by a Prometheus-type spacecraft would provide that
opportunity.
But finding a way to
integrate enough science instruments, detectors, cameras and other equipment,
not to mention daughter spacecraft designed to separate and explore on their
own, is still a large challenge in order to justify the 20-year
mission, Atkinson said.
"At the present time, there
is not a launch vehicle with enough capacity to launch a single Orbiter
spacecraft capable of transporting Neptune Entry Probes and two Triton landers
to Neptune," he added.
Probes and
landers
In
addition to an orbiting spacecraft that would make the rounds of the Neptunian
system, NASA called for researchers to address the need for probes and landers
in their respective studies.
Both
teams envision sending a trio of atmospheric probes plunging into Neptune, each
at different latitudes in order to provide a diverse look at the planet.
Ingersoll's team favors the shotgun method, unleashing all three of its probes
in one blow. Bienstock and Atkinson's study, however, plans to release the
probes sequentially.
"The
plan is to use identical probes...to learn from each deployment," Steffes
said.
Both
studies are also looking at sending a pair of Triton landers, though
setting the spacecraft down on the icy moon may be tricky. The surface is an
extreme 35 degrees Kelvin (-238 degrees Celsius), and since Triton sports
geysers and possibly seismic activity, landers would have to operate for long
periods of time to monitor it.
"Landing on Triton is not trivial,"
Atkinson said, adding that conventional landing rockets could contaminate
Triton's surface near spacecraft so some other method is required.
"The atmosphere is too thin for parachutes and it is unlikely a
rocket-controlled soft landing system can be used."
But
Atkinson has ample time to find the best way to stick a Triton landing. NASA has
funded his study with Bienstock, as well as Ingersoll's until mid-2005, when
their final recommendations will be submitted to the space agency.
"So
it's a pretty interesting place," Ingersoll said of Neptune and its satellites.
"For Voyager [2], Neptune was certainly the most photogenic of the ice
giants."
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