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Comment by elliekelly | Hacker News Reader

Somewhat related I saw it’s going to take NASA’s new Perseverance rover about 7 months to get to Mars and it’s traveling at something like 25,000 MPH. Since NASA had to take the planetary orbits and launch timing, etc. into account I’m assuming that’s the fastest ideal velocity for the rover but not necessarily the fastest possible.

So I guess my question is, given that Ceres is so much further away than Mars, what’s the constraining factor for faster space travel? Safety? Fuel? Hardware? Technology? If Elon was going to spend every penny he has to get to Ceres as fast as possible, how would his billions best be spent?

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Faster is normally not all that important: it's much better to save fuel and send more stuff than to get there a few months earlier. Fuel costs vary roughly with delta-V, the total amount you have to accelerate or decelerate your spaceship. Because of the tyranny of the rocket equation, they aren't linearly proportionate to delta-V, but in general, if it takes more delta-V it's going to be harder and more expensive to send a spaceship there.

Here's a map of the solar system by delta-V [0]. Good news is it's roughly equally difficult to get to Ceres as to get to Mars! It's further away, but it's smaller which means less decelerating when you get there.

For your actual question...you can get faster routes with more fuel. We're pretty close to the limits of how big a rocket we know how to build and still get it to Mars (or Ceres) with a 2-ton payload (enough for a lander, a rover and a little fuel) using the most fuel-efficient route. If you want to get there faster, which might be important for manned missions, you need a smaller payload or a bigger rocket. Or the most likely solution - you launch lots of rockets and join up the payloads into a bigger spaceship in Earth orbit. If Elon's goal was to make a one-way trip to Ceres as fast as possible, he'd be doing mostly the same as he is now - focusing on being able to build lots of rockets and launch them cheaply.

[0] https://imgur.com/WGOy3qT

> It's much better to save fuel and send more stuff than to get there a few months earlier.

Yes. For cargo and probes/rovers. Not for humans. The faster we can arrive, the less supplies are required and the risk of something going wrong while the nearest help is light minutes away is lessened.

> For your actual question...you can get faster routes with more fuel

Not fuel, but more Delta-V. Which may or may not imply on more fuel. Options are: more fuel (rocket equation bites here), less weight, more efficient thrusters, aka higher ISP (see the nuclear propulsion sibling comment).

What this gives you is the ability to use other trajectories besides pure Hohmann Transfers - which are efficient, but slow.

Ideally we would send supplies ahead of time with the most efficient transfers possible, then get humans there as fast as we possibly can. Otherwise this would take a while, around 400 days, one way. So over two years just to get there and back, plus wait time until the next launch window.

Whoa! Thanks for the awesome answer and the map! This blew my mind:

> Good news is it's roughly equally difficult to get to Ceres as to get to Mars! It's further away, but it's smaller which means less decelerating when you get there.

As someone without any background in science I only ever hear about the journey to moon and mars so it’s super interesting to eyeball the marginal difficulty of getting to different planets on that map. Thanks for sharing!

Based on the linked (very cool) map, I don’t think what you say is accurate? A flyby of Ceres would be slightly harder than entering Mars orbit, and entering Ceres orbit would be harder than entering Neptune orbit.

Yes for a flyby or orbital mission. I was thinking of a landing mission which costs more delta-V. In practice, you can get that delta-V by aerobraking when going to Mars but not to Ceres, though on the other hand you need to build the spacecraft to withstand the Mars atmosphere. So maybe the answer is "it's complicated" but either way it's reasonably close, not the 5x further you might think from looking at the closest approaches of the orbits of Earth, Mars and Ceres.

The 7 month trip is not the fastest, it's the most fuel efficient because it needs the least amount of dV. Hoffman transfer orbits are what you should google about( and they exist for every planet).

Nuclear propulsion. I think a development program could be started from a planetary protection standpoint then hardened for standard missions.

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