How fast could we go back to Uranus and Neptune?

Question

Since SpaceX is making everything so much cheaper to launch into space. If we launch a pair of orbiters to Uranus and Neptune, how fast could they get there? They could be really light and small, so they could use less fuel and just go faster. Also what kind of data could we learn from them?

Answer 1

Uwe says that a "really light and small" orbiter for the outer planets won't work, and I agree. So would Alan Stern and David Grinspoon, whose book Chasing New Horizons describes the history of that mission to (and now beyond) Pluto.

Early in the book they discuss a 1990s proposal to send a craft to Pluto weighing only 35 kilograms for a "Pluto Fast Flyby." But when the mission designers went to work on it, they discovered that even with only a couple barebones scientific instruments, the intended 35 kg mass ballooned to over 100 kg even without redundancies. With that costs and fuel requirements also rose, and the idea self-destructed -- one of the many false starts that came and went for Pluto exploration in the 1990s.

New Horizons ultimately succeeded, but the moral is that going to the outer planets, even for a flyby, is not going to be cheap or technically simple. Getting an orbiter, as distinct from a flyby, to those outer worlds is unavoidably more complex and expensive than anything we have yet launched that way.

Answer 2

https://yellowdragonblogdotcom.files.wordpress.com/2018/03/full-report-ice-giants.pdf

here is NASA study of mission to ice giants Uranus and Neptume conducted in 2017. ( pdf, 14 MBytes)

Trajectory study is in appendix A, begins on page 177.

3 launch vehicles were considered - Atlas 5, Delta 4 Heavy and SLS. Falcon 9 was not considered because of its low effectiveness for high C3 trajectories (high escape velocity). Falcon Heavy had no certain specifications in this time.

We can notice from graphs that use of SLS reduces the fight time from 10-12 years (for Atlas 5 rocket) to about 6 years. Not so game-changing advantage given size and cost of SLS several times bigger.

Shoud be noted that these trajectories are for orbiter. The faster it flies to an ice giant planet, the more fuel it should have for insertion burn to become orbiter.

For flyby probe (like New Horizons) bigger rocket is more effective. My guess that by SLS with additional kick stages a probe could reach Uranus in 2-3 years.

For Falcon Heavy and Staship - in vanilla version they are not well suited for high-velocity trajectories needed to reach the ice giants. They have big and heavy upper stages (for Starship the upper stage is Starship itself). Of course we can dream about Starship launching 100+ tonnes hydrogen kick stage (better two nested stages) - if it would become reality, than...

Also should be noted that NASA prefers to keep balance in exploration program. There are many planetary science groups lobbying for different destinations. So the better compromise is usually to launch two missions with cheaper launchers and longer flight times than one mission with expensive launcher and shorter flight time.

Answer 3

Preamble:

I used my interplanetary trajectory "software" (described elsewhere on this site) to determine the best trajectories to Uranus and Neptune starting 01-Jan-2023. I also explore the available orbiter mass made possible by various launch vehicles.

Launch vehicle performance is generally taken from the NASA Launch Services Program (LSP) Performance Web Site and augmented with a method described in another post here when more performance is needed (i.e., STAR 48B kick stage).

For both destinations I looked at a direct transfer and one using a Jupiter gravity assist. A simple capture orbit (C3=0) is assumed at each destination, plus a conservative 300 m/s buffer based on the initial insertion orbits of the 3 prior outer planets orbiters:

Mission:	Insertion Orbit C3 (km^2/s^2):	$\Delta V$ from C3=0 (m/s):	Orbiter Dry Mass (kg)
Juno	-31	267	1593
Galileo	-12.8	215	1880
Cassini-Huygens	-7.5	122	~2200

(Based on orbital data accessed from JPL's HORIZONS in first orbit)

Uranus:

The direct Uranus transfer takes about 12 years at optimal efficiency of ~9.5 km/s from low Earth orbit (LEO):

(Personal work)

Broken up into the launch and arrival segments it looks like this:

(Personal work)

At a reference C3 of 155 $km^2/s^2$ and spacecraft $\Delta V$ of 1.5 km/s current launch vehicles could send a "light and small" orbiter:

Launch Vehicle (w/ STAR 48B kick):	Total Throw Mass (kg):	Orbiter "Dry" Mass (kg):
Falcon Heavy (Expen. / Recov.)	1000 / 440	600 / 265
Vulcan VC2	450	270
Vulcan VC4	690	415
Vulcan VC6	880	530
SLS Block 1	1740	1045

(assumes Isp of 300s for spacecraft main propulsion system)

A better idea is to use a gravity assist at Jupiter, if the phasing of Jupiter and Uranus are correct during the desired period. This turns out to be not the case for my arbitrary 2020's launch period:

(Personal work)

You can see the Jupiter-Uranus phasing error in the lack of shorter, lower energy trajectories. The phasings of the outer planets have extremely long timescales, like the 175 years between "grand tour" alignments. There are more efficient Earth-Jupiter-Uranus trajectories, but they arrive at Uranus later than direct transfer architectures for this launch period.

Neptune:

The direct Neptune transfer takes about 18 years at optimal efficiency of ~10 km/s from low Earth orbit (LEO):

(Personal work)

Again segmenting launch and arrival looks like this:

(Personal work)

The phasing with Jupiter is better for Neptune, but still not ideal:

(Personal work)

Significant savings are afforded with a Jupiter gravity assist at the expense of only a couple of years time of flight.

(Personal work)

At a reference C3 of 120 $km^2/s^2$ and spacecraft $\Delta V$ of 1 km/s current launch vehicles could send a much more substantial orbiter to Neptune:

Launch Vehicle (w/ STAR 48B kick):	Total Throw Mass (kg):	Orbiter "Dry" Mass (kg):
Falcon Heavy (Expen. / Recov.)	1605 / 720	1140 / 510
Vulcan VC2	720	510
Vulcan VC4	1085	770
Vulcan VC6	1380	980
SLS Block 1	2850	2030

(assumes Isp of 300s for spacecraft main propulsion system)

Conclusion:

You could send a half ton orbiter to Uranus by the late 2030's, and a ~1 ton orbiter to Neptune by the late 2040's using current technology.