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Comment by Ankaios | Hacker News Reader

Flying close to the Sun is hard, both because of the thermal environment and the difficulty of achieving trajectories that get close. A mission that launched two years ago, Parker Solar Probe, is actually going to get closer. Parker and Solar Orbiter have different objectives and should complement each other well.

http://parkersolarprobe.jhuapl.edu/

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Ya, at first glance it seems easier to get to the sun, but due to orbital mechanics it's actually harder than it seems.

Probes bound for deep-space destinations like Mars can piggyback off Earth’s momentum to fly faster. For a spacecraft to launch toward the sun, on the other hand, it must accelerate to nearly match the Earth’s velocity—in the opposite direction. With the planet’s motion essentially canceled out, the spacecraft can surrender to the sun’s gravity and begin to fall toward it. https://www.theatlantic.com/science/archive/2018/08/parker-s...

Notably it requires more delta-V - i.e. more fuel - to reach the Sun than it does to leave the Solar system entirely.

Hmm so a probe can't sling-shot around another planet to do a U-turn?

It's possible, Parker Solar Probe is using Venus for gravity assists to get very close to the Sun. http://parkersolarprobe.jhuapl.edu/The-Mission/index.php

It will take 7 passes of Venus though.

The reason that PSP isn't taking these images despite going much closer is that it doesn't have a camera for imaging the Sun.

Interesting article thx.

I dont get why the speed up works only towards the outer solar system though. Why does it have to loose speed to control direction?

The direction is "towards" or "away" from the sun, the only way to get closer to the sun is to make your orbit smaller, the only way to do that is to slow down in your current orbit. You can't reach a lower orbit by accelerating. (I'm simplifying a bit, it's all acceleration, what matters is if it's in your orbital velocity, prograde, or against it, retrograde)

>You can't reach a lower orbit by accelerating

Ah thanks this made sense. I guess if I was spinning something on a string I would have to slow it down for the orbit to shrink.

You can accelerate so slow down. The flyby of a planet creates an accelleration that, if in the correct direction, results in a slower orbital velocity in the sun's reference frame. In really simple terms, if you approach the planet from the right (in a counterclockwise solar system) you speed up, if you aim to the left you slow down. The velocity is transferred to/from the planet.

https://youtu.be/hKjcbmlAaYY

Notice how the probes orbit gets more eccentric but still smaller each time it approaches a planet on the left. The gravity assist is slowing it down, again in the solar reference frame.

Gravitational slingshots are a method were you accelerate and end up slowing down but I don't think it's fair to say that you "just accelerate to reach a lower orbit", since your acceleration ultimately ends up being either retrograde for very low fly-by orbits or on the normal/radial axis. Or possibly even prograde.

Basically, using a flyby to apply retrograde speed is not much more than accelerating prograde and then using the gravity of a planet to A) move your orbital position without a speed change and/or B) redirect your acceleration.

It's all relative. You can use gravity assists to get closer to the sun, but there are a lot more planets above our orbit than below it (and mercury is too small to be useful). ...so timing gravity assisted deceleration orbits is not just hard, but it takes forever and the only reasonable solution is a very eccentric orbit.

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