Juno just passed its closest approach to Jupiter, going 58 km/s because it was so deep in Jupiter's gravity well. Ironically, it did this while retrofiring its engines, to slow down.

That sounds like a record... Is it? They mentioned the speed only in passing during the broadcast.

But - it was also retrofiring for 21 minutes in the solar system's second deepest gravity well, much closer in than anything else has ever been. And it had to shed the velocity needed to enter orbit during that time, it couldn't come around again and aerobrake like a probe can at Mars.

So did Juno also brake harder than anything ever made by human hands?

One thing that is interesting to me is it was retrofiring for 21 minutes to get into the right orbit. How much velocity did it bleed off during that process, since it was an Oberth maneuver so deep in a gravity well?

  • 1
    $\begingroup$ IIRC Helios 1 and 2 were going ~70 km/s at perihelion, but Juno is likely the next fastest after them. (And, assuming everything goes well, Solar Probe Plus will have them both beat in 2024, it's supposed to reach ~200 km/s or so.) $\endgroup$ Jul 5, 2016 at 4:58
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    $\begingroup$ Certainly it didn't brake harder than Luna 2. $\endgroup$
    – SF.
    Jul 5, 2016 at 6:28
  • $\begingroup$ Not even close to a big deceleration. I read somewhere the total speed reduction was around 500 metres per second. This was enough to guarantee capture. But any earth re-entry gets something like 7000 m/s through re-entry braking, and I think a moon landing would need a total rocket braking of around 1600 m/s. $\endgroup$
    – Andy
    Jul 5, 2016 at 7:45
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    $\begingroup$ Wouldn't the biggest deceleration by a spacecraft be the probe Galileo dropped into Jupiter? $\endgroup$ Jul 12, 2016 at 21:10

3 Answers 3


Jonathon Mcdowell of Jonathon's Space Report fame reports:


NASA's Juno probe entered Jupiter orbit on Jul 5. The probe's UK-built Leros-1b engine fired for 35min 2s starting at 0230 UTC, slowing the vehicle by 0.542 km/s to reach a 3900 x 8029000 km x 89.8 deg orbit around the giant planet.

The engine-induced velocity change was, of course small compared to the velocity change caused by Jupiter's gravity as Juno passed through perijove, but enough that as it arced upwards again, it no longer had enough speed to escape the planet's pull - although it won't start falling back down again until Apojove 0 on around Jul 31.

At 0000 UTC Juno had a velocity of 28.1 km/s relative to Jupiter at a height of 261000 km above the cloud tops. As it fell inward, by the start of the burn at 0230 UTC this had increased to 53.9 km/s at a mere 19000 km above the planet at 47 deg N latitude. Maximum jovicentric velocity, 57.95 km/s, was reached at 0248 UTC, with Juno only about 4400 km over the Jovian equator. By the end of the burn Juno's speed relative to Jupiter had dropped to 54.2 km/s. On Jul 12, a week after orbit insertion, Juno was travelling only 4.5 km/s relative to and away from the planet, at a distance of almost 5 million km from it.

The almost 58 km/s perijove velocity appears to be the record speed at periapsis relative to the central body during an orbit insertion. As it happens, the probe's heliocentric velocity was almost the same, 59.3 km/s. Relative to Earth, the probe was travelling at 61.7 km/s. On its third perijove later this year (Aug 27) the velocity vectors of probe and Earth will be better aligned and although it will again be travelling at 58 km/s relative to Jupiter, relative to Earth it will be going 73.7 km/s according to NASA (the predicted trajectory on Horizons atually reaches 76 km/s), and this is the maximum expected geocentric velocity during the mission.

Fastest ever? - Not so fast!

So, Juno is orbiting its host planet faster than any planetary orbiter ever. However, this is not, as some media outlets have reported, the fastest ever spacecraft relative to the Earth.

The largest geocentric velocity reached by a spacecraft was 98.9 km/s, by Helios 2. The Helios 2 mission was a joint German-US probe to study the solar wind, placed in an elliptical solar orbit of about 0.28 x 1.0 AU.

My media contacts tell me that JPL claims (I haven't heard from JPL directly) that the record was only 164000 mph - i.e. about 73 km/s - and was set in Apr 1976. It is true that the HELIOCENTRIC velocity record was set on 1976 Apr 16 by Helios 2, reaching a velocity of 68.6 km/s, beating the 66.1 km/s record of its sibling Helios 1. And it is true that on that day the geocentric velocity of Helios 2 was 73.4 km/s, the record quoted by NASA.

But: you don't get the maximum geocentric velocity by taking the date of the maximum heliocentric velocity and converting that one to geocentric (which is what JPL seem to have done) - the +/- 30 km/s modulation caused by the Earth's motion around the Sun means that the heliocentric and geocentric velocities don't peak at the same time. For a fixed elliptical Keplerian orbit around the Sun, the maximum heliocentric velocity always occurs at perihelion and always has the same magnitude. The maximum geocentric velocity will happen when perihelion happens on the opposite side of the Sun from the Earth, and since the orbital periods of probe and Earth are different, that will only happen once every many orbits.

In Apr 1976 the Earth was moving almost at right angles to Helios 2, so the geocentric velocity was not much bigger than its heliocentric one. In contrast, on 1989 Jan 12 I calculate Helios 2 was close to perihelion and moving in the opposite direction to the Earth, so a similar heliocentric velocity translated to a much larger geocentric velocity of 98.9 km/s. Caveat: I have extrapolated the 1980 orbital solution without including any perturbations, so the date is almost certainly wrong, but the magnitude of the maximum velocity probably isn't far off. I hope that GSOC and JPL can do a better job. My results, which use orbital elements obtained from NASA/NSSDC in 1993, are in good agreement with the SPK kernels of L. Wennmacher (2011) available at naif.jpl.nasa.gov for the period when they overlap.

But perhaps you don't want to count the 1989 Helios 2 record, because Helios 2 died in 1980. What is the maximum geocentric velocity of a working space probe? Helios 1 was still transmitting in 1985, and on 1980 Dec 5, it reached an impressive geocentric velocity of 96.2 km/s (215000 mph).

Plots of the Helios 1 and 2 geocentric velocity versus time can be seen at http://planet4589.org/space/jsr/Helios1Vel.jpg and http://planet4589.org/space/jsr/Helios2Vel.jpg

So to summarize:

  • Fastest geocentric velocity of human artifact: Helios 2, 1989 Jan 12?, 98.9 km/s
    • Fastest geocentric velocity of active probe: Helios 1, 1980 Dec 5, 96.2 km/s
    • Fastest planetocentric velocity of artifact in orbit around that planet: Juno, 2016 Aug (expected), 73.7 km/s
    • Fastest heliocentric velocity of (active or not) human artifact: Helios 2, 1976 Apr 16, 68.6 km/s

A new record is expected to be set in Dec 2024 when NASA's Solar Probe Plus mission, scheduled for launch in 2018, will reach the perihelion of an 0.04 x 0.73 AU solar orbit traveling at a searing heliocentric velocity of 205.0 km/s and an even more remarkable geocentric velocity of 234.8 km/s (525000 mph for the metrically impaired).

Supplement: sample Ecliptic1950 orbital elements for Helios 1 and 2

Helios 1 - Epoch 1980 Feb 24.00 0.310 x 0.985 AU i=0.006 Node=143.33 e=0.522 AOP=114.17 M=180.33 Helios 2 - Epoch 1980 May 12.73 0.291 x 0.986 AU i=0.029 Node=138.16 e=0.544 AOP=155.75 M= 0.00


Juno's main engine is fairly small. The craft reduced its speed by about 500 m/s with a 2000 second burn, so average acceleration was only 0.25 m/s2 -- about a 40th of a g.

Man-made objects brake at hundreds or thousands of gees all the time -- think of a bullet hitting a wall, for instance. Less destructively, ICBM warheads re-enter the atmosphere much more steeply than manned capsules, and might achieve around 100g for brief periods.

For deceleration done in space by rocket thrust alone, rather than atmospheric compression and drag, I'm not sure what the record is. Mercury used solid fuel retrorockets to terminate orbital flight, which would have produced about 1g acceleration if fired together. Most other braking burns are a fraction of a gee.

  • $\begingroup$ space.stackexchange.com/a/4055/25 has some other good examples. Cassini broke harder, for instance, as did Galileo. Viking missions had a delta v of more than 1400 m/s at Mars. Due to the Oberth effect, Juno's deceleration was pretty small. $\endgroup$
    – PearsonArtPhoto
    Jul 12, 2016 at 18:54
  • $\begingroup$ surely YM braked... $\endgroup$
    – Hobbes
    Jul 12, 2016 at 18:59

I would count a positron (an anti-electron) as a human-made object. That would be the fastest, with many, many of them having reached 0.999999999997 times the speed of light in the CERN Large Electron-Positron collider.

As for how hard they braked, yeah, really hard. When a positron hit an electron going the same speed in the other direction, it, umm, I don't even know how to calculate that deceleration.

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    $\begingroup$ Why would you consider a positron a human-made object? They're capable of being artificially produced, but are a naturally occurring particle. I don't know of any naturally occurring space telescopes though $\endgroup$ Jul 6, 2016 at 14:16
  • $\begingroup$ Because we made those positrons. $\endgroup$
    – Mark Adler
    Jul 6, 2016 at 14:26

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