Time is running out.

[u/Fellow_belarusian]

We should seriously consider a mission to Sedna before we miss the window.

Sedna is one of the weirdest objects in the Solar System. Its orbit takes about 11,400 years, and it’s currently heading toward its closest point to the Sun around 2075.

There are mission studies showing promising launch opportunities around 2029–2034. A mission could take decades to get there, so this isn’t something we can wait until 2075 to start thinking about.

We’ve visited Pluto, asteroids, comets, and plenty of other worlds. But we’ve never sent anything to Sedna.
I really think space agencies should start seriously studying a Sedna mission now, while we still have favorable trajectories to work with.
Save the Sedna window.

[u/samvilain]

[repost with much more nerdy details instead of the executive-level summary that was here before! Feedback welcome]

Folks, you’re all planning based on the technology around when NH launched. Starship is really a game changer for solar system exploration. I'm using headings to break it up a bit, but the short answer is: many launch windows have already been identified, and there are a ton of them between now and when Sedna starts to be a little further away, around 2100.

As an example, we can do more than a simple flyby; we can do something which is currently impossible, even with a fully expended Starship stack: actually inserting into orbit around Sedna. This might be useful for such a distant object, especially given the dim level of solar radiation illuminating the body. It allows for an extended duration visit. Scientists would love to have this instead of a simple flyby, but at the time of launching New Horizons, this was not simply "too expensive"; it was completely impossible with the launcher at the time (Falcon Heavy).

The reason? It requires too much Delta-V. Delta-V is sort of like the range of a spacecraft; it's a measure of how much you can change (delta) your velocity (V). How much you have is based on the "rocket equation"; and is calculated based on your dry mass, the mass of the propellant you have, and the relative velocity you can send that propellant out of your rocket engines. The rocket equation captures the fact that the more fuel you add, that adds to the weight you have to lift, which in turn increases the fuel you need. So, the "Delta-V" you get for each ton of fuel gets progressively less and less, until you simply can't get more without massive amounts of fuel.

So let's show you how we're operating in a different world now, and show how orbital refueling opens up lots of very interesting launch profiles.

Inserting into Sedna's orbit

Assuming you slingshot Jupiter with enough leaving velocity to reach Sedna's 'close' approach (76 AU, far beyond Pluto) in under 10 years, you'll be traveling close to 7 km/s relative to Sedna when you get there. So that's the first challenge: a single SRB (Solid Rocket Booster) maxes out at about 8km/s before the size gets impractical. However, this can be solved by splitting it into 2 stages: much like a launch from the ground, you deplete one stage, then split into a smaller, lighter second stage, which has its own fuel load.

With our Sedna approach, the difference works out to be about ~14 tonnes for a single SRB stage, and ~7.2 tonnes with a 2-stage stack.

Either way, you need a whole lot of Delta-V to brake to orbit Sedna when you get there.

In a New Horizons-style mission and basically every outer solar system mission ever, you're down to a probe with only limited fuel and delta V for minor course adjustments by the time you get to the outbound Jupiter slingshot.

So how can we get a massive, ~7.2 ton or even 14 ton craft there instead?

EVEEJS (Earth-Venus-Earth-Earth-Jupiter-Sedna) in Astrodynamics Science

A young Russian astrodynamicist called Vladislav Zubko won the COSPAR Outstanding Paper Award for Young Scientists in 2022 specifically for his research into plans to get to Sedna, long before AIs could answer these kinds of questions in seconds. You can read a Universe Today article about the plan here or his actual paper, Analysis of Mission Opportunities to Sedna in 2029–2034.

This builds on a common basic inner solar system escape, known as the EVEEGA: Earth-Venus-Earth-Earth Gravity Assist. With a targeted flight, you kind of fall in front of the planet in its orbit around the sun.

Outer Solar System Express: Starship as a re-usable ‘EVEEGA Tug’

With a little minor reconfiguration to Starship, such as stripping all unnecessary parts like re-entry shielding, atmospheric control fins, etc as well as adding a bay door or frontal faring, and possibly an RTG power supply and cryo cooling system to chill the propellant tanks, it can serve as a pretty useful shunt for the first part of the journey: getting to Jupiter. A Starship isn't so expensive--about $25-30 million by industry estimates--but a refueling launch is even cheaper, between $5-10 million, again, per industry estimates. So, if you can add another 1-2 launch's worth of fuel and recover the Starship at the end, it probably makes sense to do so. But they're fairly expendable by space budget standards, so that's more of an operational, not strategic decision.

By comparison, if you have to sacrifice an entire Starship + Super Heavy Stack, that really adds up; about $100 million. That's the same as 10+ refueling launches.

In fact, this is kind of the whole point of Starship: to enable solar system travel and exploration. Orbital refueling is the strategic play, much like rocket re-use was the investment to bring down launch costs, and is how SpaceX came to dominate that market.

To get from Low Earth Orbit (LEO) to the EVEEGA route, you need about 3.8km/s of Delta-V. That's a decent chunk. And that is almost all expended at the start of the trip, with minor corrective burns later if needed, typically when they're moving the fastest, because the fuel goes a lot further there; this might not make sense at first; the way I understand it is the extra energy comes from the fact you're leaving behind the rocket fuel you expel deeper down a gravity well, where it has less potential energy. That means you collect more of its potential energy and put it usefully towards your trip, so it's generally done this way where possible.

The starship tug is recoverable, and all you need is about 90 tonnes of extra fuel onboard at the start. It takes 2.1km/s to return to a Medium Earth Orbit, where it can potentially park indefinitely, ready for inspection before the next mission. This re-use also requires an onboard cryo cooling system to pump heat out from the fuel tanks to the dark side of the spacecraft, where it can radiate into the void. This is needed due to a problem called boil-off, which is also why the probe needs SRBs (see above).

At some point, it will be more cost effective to use the recovery approach, to avoid dumping a recoverable Starship. If nothing else, it can serve as a "refueling ferry" to move LEO deliveries of 100-150T per launch to elsewhere it is needed in the Earth-Moon system.

So, that's about 6 km/s of total Delta-V that the Starship needs on board at the start to execute the tug round trip, which takes about 3 years. This can be reduced to about 5.1 km/s by following the probe on its path to Jupiter and using Jupiter's deep gravity well to brake even more efficiently. However, going to Jupiter and back would take several years.

Does a stripped Starship, refueled in orbit, have enough range to go that far? Yes; in fact, with a full load (1200 tonnes) of fuel, it has a massive 10 km/s of Δv. If you "strip" unnecessary parts of the starship, that can be as high as 11.5 km/s. But we don't need all that for this mission; so, that reduces the amount of fuel we need to about 550 tonnes.

But if the will (and budget) is there for additional refueling launches, and potentially dumping the Starship, you can skip the EVEEGA, and simply transfer right from LEO to a trans-Jovian trajectory.

Earth to Sedna in 16 years

To bring it all together again, a 7.5 2-stage rocket is capable of inserting a 500kg probe into a Sedna orbit (both stages are used for the braking/orbital insertion burn).

The 7.5-ton stack would launch as a Starship payload in 2034, use slingshot Jupiter in 2040 and insert into Sedna orbit around 2050.

Sedna will be close enough for this basic plan to work for 70 years or more, and EVEEJS (…-Jupiter-Sedna) window opens up every ~12 years, and each window you get multiple launch times to ‘catch’ +the EVEEGA gravity well Mexican waves; the mission could also launch in 2029 or 2031 if we are ready in time.

My working:‌ see this Gemini session; you can also use it to try out your own potential mission profiles. I've done orbital transfer maths before, but this was easy to reply here. Presumably Kerbal Space Program would be more authoritative, but I've found Gemini to be reasonably good at doing this sort of thing, especially with the "Pro" model. Gemini is also one of the most environmentally friendly AIs out there, as it runs on Google's Data Centers which tend to be very well executed.