Sen—More than two decades after Voyager 2 conducted its flyby of Uranus and Neptune, NASA hinted earlier this week that a dedicated mission to an ice giant planet may be in the pipeline.
The announcement came at the Outer Planets Assessment Group conference in Laurel, Maryland. Jim Green, NASA’s head of planetary sciences at the Jet Propulsion Laboratory announced the agency would look at a large flagship mission to place an orbiter around the planet Uranus, with a possible duplicate mission to Neptune.
Planners expect both missions to cost in the range of $2 billion dollars each for development. The Neptune Orbiter is the least likely of the two, as NASA’s proposed Argo mission to the outermost planet will miss its 2015-2020 window for a Jupiter and Saturn gravitational assist. The New Frontiers-class New Horizons mission to Pluto was also set to include a second duplicate spacecraft slated for a dedicated Neptune flyby, which was cut due to a tight budget.
A hypothetical timeline for a proposed Uranus Orbiter mission. Image credit: NASA
But getting there won’t be easy. Even with gravitational assists, it will take a mission several years to arrive at Uranus or Neptune. It took Cassini almost seven years to arrive at Saturn, and New Horizons took nearly a decade to fly past Pluto. Uranus and Neptune orbit the Sun at a distance of 19 and 30 AUs respectively (where one AU, or Astronomical Unit, is the Earth-Sun distance of 150 million km), and any orbiter would need both a decent amount of velocity to reach an ice giant in a mortal researcher’s lifetime, but must also slow down to achieve orbit.
The 2013 decadal survey placed the possibility of a Uranus Orbiter as third priority, behind the Mars 2020 rover and the Europa Clipper mission to explore Jupiter’s moon. The pipeline from proposal to launch pad is a long one, and this week’s announcement is a tentative first step towards development, peer review and project assessment, all of which must occur before a Uranus Orbiter mission even hits the drawing board.
On the plus side, mission planners for a Uranus Orbiter may have a wealth of new launchers to choose from in the next decade, including SpaceX’s Falcon Heavy and NASA’s SLS heavy lift rocket. Even then, we wouldn’t expect to see the launch of a Uranus Orbiter until the mid-2020s, with an arrival at Uranus in the early 2030 timeframe.
Exploration in a time of plutonium scarcity
The ongoing plutonium shortage may affect any future mission to the outer Solar System as well. Every outer Solar System mission except Juno—which arrives in orbit around Jupiter next year—has employed a nuclear Radioisotope Thermoelectric Generator (RTG) as its primary power source. The production of the isotope Pu-238 for space exploration was a fortuitous result of the production of fissile Pu-239 for nuclear weapons, mostly in Russia and the United States. This ended with the Cold War, though NASA announced in 2013 it was working with the U.S. Department of Energy to restart plutonium production. Just how much plutonium NASA has is classified, but it’s thought to have perhaps enough remaining for one large Flagship and one smaller New Frontiers-class mission. NASA bought the plutonium powering the Mars rover Curiosity from the Russians, and the Mars Rover 2020 will also be nuclear powered. Cassini has 32.7 kg of plutonium aboard, and a Uranus Orbiter would probably carry a comparable supply.
The U.S. Department of Energy's new advertized target rate of plutonium production in the U.S. is 1.5 kg per year starting in the early 2020s, with the 1st 600 grams produced by the end of 2019. NASA halted its development of a new generation of nuclear Sterling generators in 2013, which would have been four times as efficient. As a result, a Uranus Orbiter would employ the older proven MMRTGs (Multi-mission) for a power source.
Why Uranus? Well, we’ve only seen these outer worlds up close once and very briefly, when Voyager 2 flew past Uranus in 1986 and Neptune in 1989. If we really want to study them, we need to stop and monitor these worlds over time. Ice giants seem to be in a class of their own, between true gas giants such as Jupiter and Saturn, and terrestrial planets like Earth. We’re finding ‘super-Earths’ orbiting other suns now, and here’s the chance to study two such massive worlds that blur the lines close up. The Galileo and Cassini missions both carried atmospheric probes or landers, and a Uranus Orbiter might feature one as well, as it represents a once in a lifetime opportunity. The icy Uranian moons Titania and Oberon might be good lander targets. Plus, technology has improved immeasurably since the Voyager days, and if this summer’s flyby of Pluto is any indication, there’s a huge thirst for space exploration out there.
Voyager 2 looks back at a crescent Uranus: a scene unwitnessed by human eyes. Image credit: NASA/JPL
But there’s another good reason to explore Uranus and Neptune. With the exception of ESA’s Huygens Titan lander—which hitched a ride on Cassini—only NASA has fielded missions to Jupiter and beyond. These have proven to be a grand legacy of exploration for the human species. The alternative to launching new missions is to watch as our eyes in the outer Solar System ‘go dark,’ as missions such as Cassini, New Horizons and even the venerable Voyagers fall silent.
Which future for space exploration would you choose?