Uranus and Neptune have never got much attention from us – we’ve only passed each once and never hung around. But that could change. A NASA group has now outlined possible missions to make it to one of these outer worlds to gather data on their composition. This should teach us about them and similar planets in other solar systems.
“The preferred mission is an orbiter with an atmospheric probe to either Uranus or Neptune – this provides the highest science value, and allows in depth study of all aspects of either planet’s system: rings, satellites, atmosphere, magnetosphere,” says Amy Simon, co-chair of the Ice Giants Pre-Decadal Study group.
There are four proposed missions – three orbiters and a fly-by of Uranus, which would include a narrow angle camera to draw out details, especially of the ice giant’s moons. It would also drop an atmospheric probe to take a dive into Uranus’s atmosphere to measure the levels of gas and heavy elements there.
The three must-haves for each orbiter mission are a narrow-angle camera, a doppler imager and a magnetometer, while an orbiter containing 15 instruments would add plasma detectors, infrared and UV imaging, dust detection and microwave radar capability.
The orbiter could be either a Neptune mission with an atmospheric probe, a Uranus probe of the same design, or a craft sent to a Uranus that ditches the atmospheric probe for the suite of 15 instruments.
The two biggest science priorities are determining the composition of the icy giant the probe visits, and determining its internal structure and the abundance of heavy elements.
Other goals include studying energy fields, weather and climate; in-depth studies of the moons; and finding out more about the composition and formation of the ring systems that orbit Neptune and Uranus like the rings of Saturn. A Neptune mission would also focus on its largest moon Triton, a likely captured Kuiper belt object with geysers and a tenuous atmosphere.
“Compared to Neptune, Uranus has a larger satellite system that likely formed in a disk around the planet (like the Jupiter and Saturn satellite systems) which is helpful for comparative planetary science, and I am excited to better understand the diversity of these small worlds,” says Jonathan Fortney at the University of California, Santa Cruz. He prefers the idea of a Uranus mission.
So why visit these ice giants at all? Of the exoplanets we’ve discovered to date, the Neptune-sized ones are the most plentiful. Simon says understanding how Uranus and Neptune formed could show how ice giants differ from gas giants, and why the former are more abundant.
“This might have implications for how you form a planet of that mass in exoplanet systems, for example,” she says.
Small launch window
There are a few mission hurdles. The journey would take at least 14 years, and would need to use nuclear power, because solar would be largely ineffective that far from the sun. The plutonium-238 powered atomic batteries used by NASA are in short supply, because international treaties prevented plutonium enrichment for many years. Starting in 2013, small amounts of enrichment were allowed again.
With luck, a Uranus missions would launch by 2034. But if NASA misses that launch time, the opportunity could be gone for a while.
“For Uranus, although the frequency of good launch dates falls off after 2034, there are still good opportunities through 2036. For Neptune, however, after 2030 it is pretty barren, owing to a lack of Jupiter gravity assists, for another 12 years, to about 2041 or so,” says Mark Hofstadter at the NASA Jet Propulsion Laboratory.
“So getting going now is helpful for a Uranus mission, and would be absolutely essential for Neptune mission,” he says.
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