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
Plots of the Helios 1 and 2 geocentric velocity versus time can be seen
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