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In the 21st century, why is there is no orbiter for Uranus or Neptune or at least their moons? It would definitely tell us more about the structure and composition of these planets.

I know it must be in the public interest in order to create and finance such mission, but I don't think it would be so expensive (considering the fact that we have created loads of orbiters for other planets). Exploring them would provide us valuable information about their moons as well.

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    $\begingroup$ Because Cassini-Huygens to Saturn cost $3.3 billion and because the economic Hohmann transfer trajectory would take 16 years to reach Uranus and 40 years to Neptune. $\endgroup$
    – LocalFluff
    Commented Apr 4, 2017 at 10:29
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    $\begingroup$ A finnish researcher has been proposing a "Uranus Entry Probe" for years (link) Maybe someone should tell him to change the name ;) $\endgroup$
    – JollyJoker
    Commented Apr 4, 2017 at 14:05
  • $\begingroup$ Creating an orbiter for planets closer to earth is much cheaper and easier. Comparing orbiters for Mars and Uranus is like comparing strawberries to watermelons. $\endgroup$
    – Uwe
    Commented Apr 5, 2017 at 7:29
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    $\begingroup$ "must be in the public interest"? Most of the public would absolutely disagree. Many don't want any money "wasted" on space. In fact if it wasn't for Elon raising popular interest we would probably be winding any space programme down in the western world. $\endgroup$
    – Rory Alsop ♦
    Commented Apr 5, 2017 at 18:02
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    $\begingroup$ @Tom11 You should believe that orbiters to Mars and Uranus are very different. It is also very different to transport an orbiter to these planets and to insert them into an orbit. Uranus and Neptun are not only very far away, they are also totally different to Mars. $\endgroup$
    – Uwe
    Commented Apr 5, 2017 at 19:03

2 Answers 2

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Orbiters are expensive, and the further away its target is, the more expensive it gets.

  • you want a high transit speed, to get there in a reasonable time
  • then you need to brake to get into orbit, this takes lots of fuel (more fuel due to the high transit speed)
  • because of the long mission duration, personnel cost is high
  • for the outer planets, solar panels don't generate enough power, so you need a radiothermal generator (RTG).

RTGs use Pu-238 as a heat source. Pu-238 is rare and radioactive, so RTGs are very expensive and only available in small numbers. Production of Pu-238 for RTGs has been restarted, but volume will be low: 1.5 kg/year starting in 2019. The New Horizons RTG contains 11 kg of plutonium and produced ~300 W at mission start. At that speed, 2 large probes or 3-4 smaller ones are all you can provide in a decade.

With current rockets, an orbiter to Mars is doable. For planets further out, you quickly run into mass constraints. Cassini required the largest rocket available at the time.

With aerobraking, you need less fuel, but that technique is still experimental, making it less likely to be used on a flagship-class mission. And it's a risky technique to use for capturing a spacecraft into orbit: you only get one shot that has to be just right. Too little braking and you don't achieve orbit but keep going, too much braking and you burn up. Aerobraking depends on detailed knowledge of the atmosphere (and its variability), and we just don't have enough of that for Uranus or Neptune.

Orbiter missions to Uranus and Neptune are being considered (this thread gives lots of detail on tradeoffs, probe size calculations etc.), but they have to compete with other scientific missions. You can only do so much on a given budget.

The selection process for science mission also plays a role, I suspect. Because we've had several missions to Mars already, there is a large, active and experienced community of Mars scientists who can propose new missions with a degree of certainty. For Uranus and Neptune, we have far less data, and a much smaller scientific community with less experience running missions. So during every mission selection process you see lots of focused mission proposals with a high degree of budgetary confidence from the Mars community and one proposal with lots of uncertainty from the Uranus/Neptune community.

This question about an orbiter for Pluto goes into more detail, including calculations on how large the spacecraft would need to be.

Regarding "loads of orbiters":

  • Mercury: 3 orbiters
  • Venus: 5
  • Mars: 14
  • Jupiter: 2
  • Saturn: 1

These numbers also give a rough indication of the combination of how expensive a mission is (more expensive/difficult=fewer missions) + how interesting the target is (chance of detecting life = more missions).

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    $\begingroup$ At least the RTG constraint seems to be alleviated already. Production is ramping up to cover foreseen NASA demands for the outer planets and Mars rovers. $\endgroup$
    – LocalFluff
    Commented Apr 4, 2017 at 13:56
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    $\begingroup$ As an addendum to the answer: NASA has a budget, and they strive to learn as much about our universe as possible on that budget. As long as there is more to be learned by going to a close planet, they won't waste money going to one that is further away. $\endgroup$
    – Cort Ammon
    Commented Apr 5, 2017 at 4:30
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    $\begingroup$ @MartinSchröder: Cassini operations cost is listed as $710 million, most of that will be personnel cost over 25+ years. That's not negligible. saturn.jpl.nasa.gov/mission/about-the-mission/quick-facts $\endgroup$
    – Hobbes
    Commented Apr 5, 2017 at 11:39
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    $\begingroup$ @MartinSchröder - Re "Surely we are talking about single-digit millions." Probably a good deal more than that. The two Voyager satellites still receive multiple Deep Space Network contacts per week, each of which costs several thousands of dollars to tens of thousands of dollars. Then there are people. The most recent Voyager Space Flight Operations Schedule lists eleven JPL employees. I'd be shocked if a fully-loaded FTE JPL engineer costs less than $250K/yr. Plus there are researchers in other NASA centers and in universities who are supported by the Voyager budget. $\endgroup$
    – David Hammen
    Commented Apr 5, 2017 at 15:59
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    $\begingroup$ @MartinSchröder What makes time on the DSN so expensive? $\endgroup$
    – user
    Commented Apr 6, 2017 at 12:00
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Using my Hohmann spreadsheet you can get an idea of approximate delta V and trip times. The spreadsheet assumes circular, coplanar orbits. So it's ballpark, not exact estimates.

LEO to Mars capture: 4.3 km/s, .71 years
LEO to Uranus capture: 8.5 km/s, 16 years
LEO to Neptune capture: 8.6 km/s, 30.6 years

For a capture orbit I assume a 300 km altitude periapsis and an apo-apsis at the edge of the Sphere of Influence (SOI). This is a loosely bound capture orbit, but apo-apsis can be lowered by passing through the planet's upper atmosphere at periapsis. However the periapsis speed of either Neptune's or Uranus' capture orbit is more than 20 km/s so the periapsis drag passes may be tricky. The scale height of the ice giants is much smaller than Mars' scale height so there is need for greater precision if using their atmospheres to shed velocity. Go a little too low and the orbiter won't come back out.

enter image description here

Also the sunlight is much less. At Mars distance, sunlight is 43% what we have on earth. Uranus .3%, Neptune .1%. So we'd need nuke power sources.

With Jupiter assist

In the comments LocalFluff brought up the possibility of a Jupiter gravity assist. And in fact this is often used for missions to the outer solar system.

LEO to Trans Jupiter Insertion: 6.3 km/s
Trip time to Jupiter: 2.73 years

So just the burn to get us on the way to Jupiter is still substantially more than Mars. I'm not sure what the delta V budget would be after the Jupiter gravity assist.

The 2.73 years it takes to get from earth to Jupiter need to be added to trip times from Jupiter to ice giant. If it's a Hohmann path, trip times from Jupiter are:

Jupiter to Uranus: 21 years
Jupiter to Neptune: 37 years

So total trip time to Uranus would be about 24 years. Total trip time to Neptune would be 40 years.

So while a Jupiter gravity assist may help with the big delta V budgets, it makes the long trip times even longer.

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  • $\begingroup$ So you say that it is doable with nuke power sources? $\endgroup$
    – Tom11
    Commented Apr 6, 2017 at 7:15
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    $\begingroup$ Nuclear RTG production now no longer seems to be a bottle neck for planetary exploration. The Hohmann calculation of course has to be greatly reduced, not only by aerobraking, but also by the Jupiter (and maybe Venus) gravity assist and the Solar electric propulsion that a Uranus mission actually would use. $\endgroup$
    – LocalFluff
    Commented Apr 6, 2017 at 10:23
  • $\begingroup$ @Tom11 I believe orbiters around gas giants are doable now. But NASA lacks funding to do every worthwhile project. Lack of sunlight is just one of the things that make such a mission more difficult and expensive. The long trip times are a major obstacle, in my opinion. Not only do these increase mission cost, it also makes it less likely those who planned the mission will live to see the fruits of their efforts. $\endgroup$
    – HopDavid
    Commented Apr 6, 2017 at 14:54
  • $\begingroup$ @LocalFluff Your comment led me to look at a Jupiter gravity assist. Your RTG links (both on your comment below my answer and Hobbes' answer) lead to directories. Would it be possible to link directly to the pages you have in mind? $\endgroup$
    – HopDavid
    Commented Apr 6, 2017 at 14:57
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    $\begingroup$ @oscar Lazi New Horizons didn't use a Hohmann transfer to get to Pluto. $\endgroup$
    – HopDavid
    Commented Sep 4, 2022 at 20:52

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