|
|
|
In the realm of an ice giant
AS the twin rovers are exploring the Red Planet and the Cassini-Huygens probe is peeking at Saturn, plans are being made to explore yet another planet in our solar system. The distant ice giant Neptune is soon to be paid a visit by a mission from earth.
Planetary scientists are planning to send atmospheric probes and landers to study the icy world and its 13 moons, including Triton. In separate studies, two research teams are planning their own missions to the eight rock from the Sun. “It’s all part of the history of our solar system. Neptune and Uranus are ice giants and made mostly of heavier stuff than Jupiter or Saturn,” stated Andrew Ingersoll, planetary scientist and study leader at the California Institute of Technology (CalTech). He is working on a Cassini type mission with his colleagues, using conventional rocket propulsion and gravity assists.
At the same time, another mission is being planned — one that uses a fission reactor and ion propulsion to reach its destination in a time span of two decades. David Atkinson, who is the science principal investigator of the second mission and a professor at the University of Idaho, explains, “what makes Neptune unique is Triton. It is speculated that Triton is actually a Kuiper Belt Object that was captured by Neptune.” Observations of the planet over a six-year period suggest that Neptune has seasons like earth.
Since it takes 165 years to orbit the sun, a winter on the icy world would last for almost four decades. “Neptune’s cloud bands have been getting wider and brighter. This change seems to be a response to seasonal variations in sunlight, like the seasonal changes we see on Earth,” said Lawrence Sromovsky of University of Wisconsin-Madison.
According to Sromovsky, the sun is 900 times dimmer there than it is on earth. Thus solar energy may be too weak to drive the changes. He is of the opinion that internally generated heat from the planet may be responsible for seasonal changes. Both missions are being planned as part of Nasa’s Vision Mission program for long-term space exploration.
The ice giant lies 2.7 billion miles (4.4 billion kilometres) from the sun. Pluto has an orbit that sometimes takes it into Neptune’s. Voyager 2 zoomed close to the planet in 1989. According to David Atkinson, a mission to the planet will give us information about ice giants and how they form and fit into the solar system. “The chemical make-up of an ice giant is different from gas giants like Jupiter. It’s less affected by the inner solar system bodies and more representative of the primordial solar nebula,” stated Paul Steffes, a member of the nuclear electric Neptune team and a radio scientist.
As far as ice giants go, even Uranus is a good candidate but the main attraction, as far as Neptune is concerned, is Triton. Its orbit is retrograde — meaning it orbits Neptune in the opposite direction of its rotation and has an extremely fragile and thin atmosphere. This means that it’s a place where parachutes carrying any kind of probe, will have a problem landing.
Neptune’s partial ring arcs are another intriguing factor. Instead of going all around the planet, like Saturn’s rings, Neptune rings just go partially round it in half arcs. The spacecraft designed by Ingersoll, for the Neptune mission will rely on radioisotope thermal generators or RTG’s. These are long-lasting battery generators, fuelled by plutonium for electricity.
The Cassini spacecraft, already touring the Saturnian system, also uses RTGs as in these far-flung regions and so solar panels do not prove very useful. “Yes, we’d need RTGs and yes RTGs carry plutonium,” stated Ingersoll. But he clarified that they can only prove dangerous if they are vapourized over a city and as most launches would have them drop into the ocean in case of any emergency. Otherwise, it isn’t really a concern. “There’s been a lot of irrationality about nuclear power and fuels,” he says.
His team estimates that the spacecraft would take twelve years to arrive at Neptune but making it stop is the real challenge. Under study is “aerocapture,” a procedure that will let the spacecraft enter the planet’s orbit without any fuel, by using it’s atmosphere.
Though Nasa already has experience using aerobraking, which is a lighter, fuel-burning manoeuvre, aerocapture would be a novelty. “The most challenging thing technologically for us is to fly an aerocapture mission to Earth of Mars to demonstrate that it can be done,” said Ingersoll. He also added that aerocapture could be done at higher speeds and also digs much deeper into a planet’s atmosphere as compared to aerobraking.
The mission being planned for Neptune would bank on Nasa’s Jupiter Icy Moons Orbiter, JIMO, a mission for Project Prometheus. Plans have been made to use a nuclear fission reactor to power up an ion engine for this project. It would be a slow procedure, for a mission to Neptune propelled by an ion engine would take a long time to reach the planet.
For instance, a mission launched in 2016 would take a lot of time to build up the force to enter Neptune’s orbit, and would do so sometime in 2035. The good part is that once it reaches there, it will still have a great deal of fuel and power supply. “Since this mission may very well be the only mission to Neptune this century, it is important the complete Neptune system be studied in detail,” said Atkinson.
He also added that the Prometheus-type spacecraft would yield enough power to allow the opportunity. He outlined the challenge of fitting enough scientific instruments, cameras, detectors etc., including offspring spacecraft, which they could explore on their own during the twenty-year mission. “At present, 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,” Atkinson said.
Nasa has instructed research teams to study probes and landers intended for the mission in addition to orbiting spacecraft for the Neptunian system. Both teams plan to send three atmospheric probes into Neptune’s atmosphere at different latitudes to look at the diversity of the planet.
While the team headed by Ingersoll prefers the method of sending all three probes at the same time, Atkinson’s team favours sending them one by one. “The plan is to use identical probes…to learn from each deployment,” said Steffes. The teams also plan to send a pair of landers to Triton. Landing probes on the icy moon will be a tricky business with surface temperatures of -238 degrees Celsius.
In addition, Triton also has geysers and seismic activity going on so the landers will have to be operational for a long time to study it. According to Atkinson, “landing on Triton is not trivial. The atmosphere is too thin for parachutes and its unlikely a rocket-controlled soft-landing system can be used.” Also, a conventional landing rocket would contaminate Triton’s surface.
Though many challenges stand in the way, the teams have enough time to overcome hurdles as Nasa is providing funds for research till mid-2005 when they will have to submit their recommendations. According to Ingersoll, the Neptunian system is a pretty interesting place. “For Voyager 2, Neptune was certainly the most photogenic of the ice giants,” he said.
Triton, Neptune’s only large moon shares some of its characteristics with Pluto. Its diameter is 1,680 miles (2,700 kilometres), which is about 3/4th the size of our moon. Some scientists think that Triton was a small planet but was captured by Neptune’s gravity. Its surface is covered with ice consisting mostly of methane and nitrogen. The Voyager flyby also noticed fracture lines and depressions on the surface indicating the possibility of water ice under layers of methane and nitrogen.
Interesting features included “ice volcanoes” and geysers gushing nitrogen gas carrying dust particles. Scientists presume that solar heating could cause these eruptions. This is a unique scenario for any other body in the solar system.
Apart from Triton, Neptune has other moons like the small and rocky Proteus that circles the planet every 27 hours. Nereid is also small and has an extremely unusual orbit, in fact, the most eccentric orbit of any other moon in the solar system. It is at a great distance from the planet (almost 3 million miles) and takes almost one earth year to complete its orbit.
Larissa, on the other hand orbits very close to Neptune (30,000 miles) and speedily circles the planet in 13 hours and 19 minutes. Galatea, which lies just inside the rings, is thought to play a role in confining the ring arcs around Neptune. Despina, Thalassa and Naiad are other moons closer to the planet.
While both teams perfect their missions to the icy giant, we will have to wait for a few more years to get more information about this cold and distant resident of our solar system.
The writer
|
|
|
|
