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NASA Wants To Probe Deeper Into Uranus Than Ever Before
Up until now, NASA has never paid too much attention to Uranus – but now the space agency wants to take a good, long look. And one of the things it might be investigating is all that gas. A NASA group outlined four possible missions to the ice giants Uranus and Neptune.
These missions include three orbiters and a possible fly-by of Uranus. The planned probes would take off in the 2030s, New Scientist reports.
[...] One of the proposed missions includes a fly-by of Uranus, which would include a narrow-angle camera – and a probe which would drop into Uranus's atmosphere to measure gas and heavy elements. 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.
(Score: 1, Informative) by Anonymous Coward on Saturday September 29 2018, @07:27AM
Launch windows are getting less critical as high-impulse/low-thrust propulsion (ion engines of various types, and with various power sources) moves into maturity. The currently-proposed missions only include SEP (solar-electric propulsion -- like Dawn, but scaled up to 25kW) for a Neptune mission; because of decreasing solar intensity, they're building it as a discardable stage to be ditched at 6AU -- for this mission profile, there is one Earth assist and one Jupiter assist. They also developed SEP profiles for Uranus missions, with two Venus assists and an Earth assist, but no Jupiter/Saturn assist, and thus no worries about Jupiter-Uranus orbital alignment. They were deemed too expensive (compared to the chemical-only, Venus-/Earth-/Earth-/Jupiter-assist programs) for only shaving one year off the flight time. (The Neptune mission, on the other hand, was impossible without SEP, given the Delta-IV Heavy as the best launch vehicle under consideration.)
What are our other options? How could we get beyond 6AU with ion propulsion, to reach Neptune with no Jupiter flyby?
Where our current favorites, 238Pu and 241Am, only offer a single α decay (~5 MeV) before hitting a long-lived (t½ > 105y) daughter isotope, we could go with something like 227Ac -- the immediate decay (0.05 MeV β-, t½=22y) is underwhelming, but then there's a whole cascade of short-lived (t½<20 days) descendants going through no less than 5 α decays (6-7 MeV each), and a couple β- (~1.4MeV each).
Now to be sure, 227Ac's t½ is a little short for the outer planets -- I imagine you'd have to start with more heat than your thermoelectrics can handle, and a radiator to ditch the surplus heat, and progressively diminish the effect of the radiator (by collapsing it or reducing coolant circulation) as the decades pass. And the various β decays, while not essential for power, do pose a serious shielding problem -- though if you place the RTG(s) on a boom, most of that shielding can be jettisoned once the spacecraft leaves Earth's vicinity, keeping just enough to shade the spacecraft. But it's just an example of how limited 238Pu and 241Am are; with 6x the energy per atom, and 4x the decay rate, you save enough in mass to make up for whatever shielding and variable heat management you need to do.
Another potential is 248Bk; similarly short t½ but shorter decay chain (only 2 α decays) before it stalls on a long-lived isotope.
There's also another direction, that of using lighter elements to get more power per mass. For instance, 44Ti (electron capture, t½=60y), decays to 44Sc (1.5 MeV β+, t½=4h). Again, there's a β shielding issue, and this time, as with light elements generally, the bulk of the energy comes through β decay, so you don't get significant thermal power if you let them escape. But it delivers about 1/3 the power of 238Pu for less than 1/5 the mass, or approximately double the power per unit mass. Not sure that's enough gain to cover the mass of shielding at a currently-useful scale, but keep in mind there's a square-cube relation between a sphere of radioisotope and a given thickness of shielding; it gets better as you scale it up.
For these proposed missions, we're not quite far enough along, so launch windows still matter, and will probably still matter by the time it would fly. But by 2040 or so, I think these once-in-decades launch windows will be a thing of the past.