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Comment by gtirloni | Hacker News Reader

I'm a complete newbie about space travel so when the article says it takes decades to reach planets like Mercury, I went searching for the fastest way we can/could travel today.

From the Wikipedia article about the Solar Probe+, it seems the spacecraft should achieve 200km/s as it passes by the Sun. At that speed, a trip to Mercury (57.91 million km) would take 80 hours?

Could someone with more knowledge about space travel weight in on the difficulties of achieving this and/or any glaring mistakes I have made in my assumptions and calculations?

To clarify, I'm interested in how fast we should ship super advanced robots to these planets. So things like human survival while it passes by the Sun aren't important in this case (I think? Is melting a huge problem for shipping robots that way?).

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You don't go direct, and the target is also moving. But the main limitation is "delta V", or the effective speed change you can achieve with a particular quantity of fuel.

The Mariner 10 trip did it in 5 months, which is reasonable. The article doesn't mention Mercury at all. A minimum-fuel trip to Mars is estimated at 3 years, I believe.

Wikipedia:

> Another reason why so few missions have targeted Mercury is that it is very difficult to obtain a satellite orbit around the planet on account of its proximity to the Sun, which causes the Sun’s gravitational field to pull on any satellite that would be set into Mercury's orbit. Furthermore, spacecraft naturally accelerate as they approach the greater gravitational pull of the Sun, but must slow down for orbit insertion, so this entails considerable fuel requirements. This is different with superior planets beyond Earth’s orbit where the satellite works against the pull of the Sun. Therefore, reaching an orbit around Mercury requires especially expensive rocketry. Mercury's lack of an atmosphere poses further challenges because it precludes aerobraking or the use of a parachute type device.[3] Thus a landing mission would have even more demanding fuel requirements.

> From the Wikipedia article about the Solar Probe+, it seems the spacecraft should achieve 200km/s as it passes by the Sun. At that speed, a trip to Mercury (57.91 million km) would take 80 hours?

If you read the Wikipedia article more closely (https://en.wikipedia.org/wiki/Solar_Probe_Plus) you'll note that it takes seven flybys of Venus over the course of six years to get into that orbit, and the resulting orbit has an 88 day period.

Plus, if you want to do anything more than whiz past Mercury, you need to bleed off that 200km/s speed to orbital velocity, which takes an enormous amount of fuel.

The speed as it is passing by the sun is the highest that it will reach in its route (the closest part of its elliptical orbit around the sun) You couldn't change that momentum to be going in the right direction (Towards mercury's gravitational influence, which is small compared to the sun) or slow it down enough to orbit mercury after it gets there without expending a crazy amount of fuel. So you could do it for a flyby, but trying to land something on the surface wouldn't work.

The other problem is since 200km/s is the fastest speed, you have to get to that point at the normal one.

Yes, the moment Earth and Mercury are the closest is when the Sun, Mercury and Earth are aligned, so going from Earth to Mercury at this moment is heading towards the Sun, but then gravity from the Sun will be too strong to be able to stop at Mercury.

Although the other explanations in this thread are correct, I fear an analogy might be better.

Yes, you can reach mercury quite quickly, but in order to land on it (or get captured into its orbit) you need to match its speed and its direction of travel. (speed and direction together are velocity, remember).

It's like I'm a stuntman jumping from one car to another on the freeway. If one car is going 60mph and one is going 20 mph it's going to end badly.

You'll start with a certain velocity on earth, and end up with a target velocity when you reach your destination. That's the delta-v. The type of rocket and how much propellant you have determine the delta-v.

Once you've made your rocket burn, you just have to wait until your paths cross. So, that's why it can take a long time for these missions. That's why people always talk about a journey time of 235 days to mars.

This is not the only possible way to do it though, it just uses the least fuel. If you're willing to use a lot more fuel, you can speed things up significantly, but generally we don't do that.

Your calculations are pretty much meaningless. Orbital mechanics is not that simple.

Here's the path we actually used to send an orbiter to Mercury. Interestingly, it did a bit of solar sailing to make some fine adjustments to its trajectory by angling its sun shield.

http://www.theplanetstoday.com/messenger_flight_path.html

More direct transfers are possible, but they require more dV on the spacecraft, and the rocket equation means increasing dV increases the ratio of fuel mass to craft mass exponentially.

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