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Deep space probes should be the fastest ones, due to incredible distances they are supposed to travel. AFAIK, Parker Solar (PS) will reach more than 600 000 km/h, incredible. On the other hand, Pioneer's/Voyager's speed is around 50 000 km/h, so about 10% of the PS.

I guess the Sun's gravity helps PS to reach this enormous speed (0.05% c), but deep space probes could get speed by Sun as well... or I don't know, but with this speed, new probes could catch Voyager/Pioneer in 5 years. And they could reach far beyond those current two furthest.

So why are they slow compared to PS?

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    $\begingroup$ I do not understand your contention that deep space probes should be the fastest ones. Voyager and Pioneer did not make close approaches to the sun. How would deep space probes "get speed by Sun?" $\endgroup$
    – Bob516
    Commented Dec 13, 2018 at 14:14
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    $\begingroup$ On your bicycle, you can go faster going downhill than uphill. $\endgroup$
    – Dan Pichelman
    Commented Dec 13, 2018 at 14:19
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    $\begingroup$ Thanks. Ok, so that high speed is mainly due to the Sun gravity, and obviously when going away, it will just pull back. $\endgroup$
    – Zotyi
    Commented Dec 13, 2018 at 15:36
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    $\begingroup$ You are correct: the Voyagers and New Horizons were some of the fastest spacecraft we've ever launched. They used the largest rockets available at the time. But after launch, gravity takes over as explained in the answer. $\endgroup$
    – Hobbes
    Commented Dec 13, 2018 at 15:43
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    $\begingroup$ And you do want deep space missions to be fast, to reduce the waiting time before you get results back. $\endgroup$
    – Hobbes
    Commented Dec 13, 2018 at 15:43

2 Answers 2

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Physical

First and foremost, the physical reason is that objects accelerate as they approach massive bodies and decelerate as they recede:

Parker Solar Probe achieves its peak orbital speed (almost 200 km/s eventually) at its closest approaches to the Sun - as it falls inwards towards the Sun on each orbit it speeds up then slows down again on the way back out. At its aphelion, however, its speed drops to less than 20 km/s.

On the other hand the Voyagers, Pioneers and New Horizons are all moving away from the Sun. Since their final respective gravity assists, they have been gradually losing speed - note they will not come to a halt and fall back to the Sun though because they exceed escape velocity.

Practical

All of the deep space probes had primary missions to explore the outer planets in our Solar System. Because of this, your assumption that

Deep space probes should be the fastest ones, due to incredible distances they supposed to travel

isn't really correct. They weren't designed to travel vast (interstellar) distances as quickly as possible; they were designed to reach the outer planets intact and relay data back to Earth. Since completing this task, they have been essentially drifting off into deep space. They obviously are still transmitting very valuable data, but this is a secondary objective.

Parker Solar Probe, however, was designed to get as close to the Sun as possible (within technical limitations) and, as a result of its trajectory, achieve very high speeds.

Further reading:

For reference, compare the speed plots of Parker Solar Probe, showing its increasing peak speed with successive orbits, and Voyager 2, showing its decreasing speed as it moves away from the Sun (note - the Voyager plot in particular is very approximate):

enter image description here Image credit: Phoenix7777, Wikimedia

enter image description here

Image credit: Cmglee, Wikimedia

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    $\begingroup$ Thanks. It makes sense. They weren't designed to travel vast (interstellar) distances as quickly as possible - this is pity... $\endgroup$
    – Zotyi
    Commented Dec 13, 2018 at 15:34
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    $\begingroup$ @Zotyi - Voyager 1 was launched in 1977 and it's primary mission - study the outer planets - ended in 1980. That's 41 years of continuous operation. Not bad for a tape deck strapped to an antenna! $\endgroup$
    – Robotnik
    Commented Dec 14, 2018 at 0:47
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    $\begingroup$ @Zotyi Voyager 1 and 2 were designed for 5 years of operation: They work 41 years now, about 8 times longer. They are expected to keep operating until 2025 or longer, by that time it would be 10 times longer. Though Pioneer 10 worked for 16 times longer than expected 21 months of operation (You may want to check out my question.) We should be proud and happy about such reliable space probe. You know, they are destined to voyage our galaxy - The Milky Way endlessly. $\endgroup$
    – not_Prince
    Commented Dec 14, 2018 at 1:59
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    $\begingroup$ @Zotyi the major reason we haven't is that space is big. Interstellar distances are orders of magnitude larger than interplanetary, meaning the speeds required would be orders of magnitude larger as well. Even if we could get a probe on an escape trajectory with PSP's speed (very difficult), it would still take ~10,000 years to cover the distance to our nearest star with very little to look at (as far as we know) in between - see here for more comparison $\endgroup$
    – Jack
    Commented Dec 14, 2018 at 12:25
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    $\begingroup$ @Jack I think the "as far as we know" part is the point ;) $\endgroup$
    – Lightness Races in Orbit
    Commented Dec 14, 2018 at 14:11
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GravityWellFunnels

You have probably seen funnels like the above in shopping malls. Drop a coin in the funnel and it will move slowly at the edge and move faster as it nears the center.

This is a good model of a gravity well. Stuff moves a lot faster in the inner solar system.

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    $\begingroup$ If you are my daughter, you can play god and capture the spacecraft just as it is about to drop into the black hole. A spacecraft saved is a spacecraft earned. $\endgroup$
    – user25936
    Commented Dec 13, 2018 at 18:38
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    $\begingroup$ @jdv If some gigantic hand grabs the Parker probe just before it falls into the sun, it will be the biggest discovery NASA ever makes. But I'm not sure we're ready for the knowledge that we're just a physics demonstration in a hyper-universal shopping mall. $\endgroup$
    – David Richerby
    Commented Dec 14, 2018 at 10:48
  • $\begingroup$ I, for one, welcome our giant STEM-curious child overlords. $\endgroup$
    – user25936
    Commented Dec 14, 2018 at 15:45

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