Comment by areoform | Hacker News Reader

Comment by areoform | Hacker News Reader

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At its launch, the Parker Solar probe surpassed all prior records of speed. And it will continue to break its own record over the 7 years after 7 consecutive encounters with Venus that will lower its trajectory to 9.86 Solar Radii

While a lot of it will be eventually achieved due to orbital mechanics, most of the push was given to it by the Delta IV Heavy. For those who may not be familiar, the launches are always spectacular due to the pre-flushing of Hydrogen through the system. NASA has an HD video that you can download, https://www.youtube.com/watch?v=AlyuSwRSVHU

The PSP's speed is impressive without context, but it's even more impressive in context. Here are phenomena in the same range as the PSP.

The Earth rotates around the sun at a speed of,

    29,800 m/s or 0.01% the speed of light

Our galaxy approaches the Messier 98 https://en.wikipedia.org/wiki/Messier_98 galaxy at a speed of,

    140,000 m/s or 0.04% the speed of light.

PSP is faster than both of these at,

    163,000 m/s or 0.05% the speed of light.

Due to the quirks of orbital mechanics, by the end of its life, the probe will be even faster and it will reach,

    192,000 m/s or 0.06% the speed of light

It's 8,000 m/s shy of our solar system's orbital velocity around the galactic center.

The design is a stunning accomplishment of engineering and daring. If you would like a detailed overview of how the mission trajectory was designed, the navigation team has written an excellent explainer that you can read here, https://trs.jpl.nasa.gov/handle/2014/47575

I was confused at first because the article mentions that "a spacecraft bound for the center of the solar system would need to slow itself down" and "engineers must take Parker past Venus seven times throughout the mission, so that the spacecraft can use that planet’s gravity as a brake", but you mention that PSP is traveling very fast (and indeed I've always understood that items orbiting closer to the center travel faster). Is this just a dumbing-down in the article's verbiage, or is it common to think of shrinking the perihelion as "braking"?

Think about it as a vector. Imagine if you were a god, and you placed a particle in the same orbit as Earth. Now imagine you wanted to smash it into the sun. How would you do it?

For the particle to fall straight "down" to the sun, in a straight line, you would have to cancel all of its orbital velocity.

That's why it's difficult to get close to the sun and it requires a lot of energy. You have to subtract out a large part of Earth's orbital velocity.

But that's just one part of the explanation. It's not intuitive per se (I imagine it as a play between kinetic and potential energies), but the closer you are to a body, the faster your orbit. The farther out the "slower" your orbit.

It's the opposite of a disc.

Ah, thanks.

If I get you right, the braking slows the orbiting object down. But that slow-down causes it to move to a lower orbit, losing potential energy in exchange for increased orbital velocity. And the velocity you drop in high orbit, you gain with interest as you move to low orbit.

So braking makes you go faster.

Passing by Venus is lowering the spacecraft's periapsis, increasing the orbit's eccentricity. The lower the satellite goes, the faster it will be at periapsis. Those speeds the GP comment is refering to are, in all likelihood, velocity as periapses.

Yes, indeed! It's the magnitude of its velocity around the sun at closest approach.

There we go, thank you, that's the mental model I needed to bring it together

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