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 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," Andrew Ingersoll, a study leader and planetary scientist at the California Institute of Technology, said 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-style 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," David Atkinson, a University of Idaho professor and the principal scientific investigator for the second study, explained 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 makeup [of an 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, or RTGs, a long-lasting battery fueled by plutonium, for its electric power. The Cassini orbiter currently at Saturn, for example, uses RTGs for power because the vast distance makes solar panels unpractical.
"Yes, we'd need RTGs, and yes, RTGs carry plutonium," Ingersoll said. He added that the power source can only pose a danger if it is vaporized over a city — a very unlikely case, because 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 that 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 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, or 396 degrees below zero Fahrenheit), 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, Neptune was certainly the most photogenic of the ice giants."