Galileo and Juno are the only two spacecraft to have entered orbit around Jupiter.
Did either of them use one of the Galilean moons for a gravity brake before in order to enter a stable orbit around Jupiter?
Or did they just directly insert into Jupiter's orbit using deep space delta-v maneuvers?
I checked JPL's Horizons with the following setup. It looks like Juno was Galilean-agnostic but Galileo had some definite close encounters, especially with Io.
Gravity has no specific range, it just falls of as $1/r^2$ so it definitely had a significant interaction with Io but whether it can classified as an assist or just an effect will need further investigation, but I think whatever effect there was must have been extremely small compared to all the velocity gained by falling into Jupiter's huge gravity well.
These spacecraft were "flying fuel tanks", mostly because of the huge deceleration necessary to fall into orbits around these massive bodies to compensate the gravitational acceleration.
above: Galileo capture at Jupiter, below: same for Juno
note: "relative speed" is really the rate of change of range (distance)
Io Europa Ganymede Callisto
------- ------- --------- ---------
Juno 348,720 675,054 1,022,210 1,897,202
Galileo 2,719 34,560 1,191,685 792,388
import numpy as np
import matplotlib.pyplot as plt
names = 'Io', 'Europa', 'Ganymede', 'Callisto'
datas = []
for name in names:
fname = 'Juno ' + name + ' horizons_results.txt'
with open(fname, 'r') as infile:
lines = infile.readlines()
print(len(lines))
a = [i for (i, line) in enumerate(lines) if 'SOE' in line][0]
b = [i for (i, line) in enumerate(lines) if 'EOE' in line][0]
lines = lines[a+1: b]
data = [[float(x) for x in line.split(',')[-3:-1]] for line in lines]
datas.append(data)
datas = np.swapaxes(np.array(datas), 2, 1)
print(datas.shape)
fig, (axr, axv) = plt.subplots(2, 1, figsize=[7, 7])
for (r, v), name in zip(datas, names):
t = np.linspace(0, 2, len(r))
axr.plot(t, r*1E-06, label=name)
axv.plot(t, v, label=name)
print('Juno closest to ', name, np.round(r.min(), 1), ' km')
axr.set_ylabel('distance (million km)')
axv.set_ylabel('relative speed (km/sec)')
axv.set_xlabel('time since 2016-Jul-04 00:00 (days)')
axr.legend()
axv.legend()
axr.set_ylim(0, None)
axv.plot(t, np.zeros_like(t), '-k', linewidth=0.5)
plt.show()
datas = []
for name in names:
fname = 'Galileo ' + name + ' horizons_results.txt'
with open(fname, 'r') as infile:
lines = infile.readlines()
print(len(lines))
a = [i for (i, line) in enumerate(lines) if 'SOE' in line][0]
b = [i for (i, line) in enumerate(lines) if 'EOE' in line][0]
lines = lines[a+1: b]
data = [[float(x) for x in line.split(',')[-3:-1]] for line in lines]
datas.append(data)
datas = np.swapaxes(np.array(datas), 2, 1)
print(datas.shape)
fig, (axr, axv) = plt.subplots(2, 1, figsize=[7, 7])
for (r, v), name in zip(datas, names):
t = np.linspace(0, 2, len(r))
axr.plot(t, r*1E-06, label=name)
axv.plot(t, v, label=name)
print('Galileo closest to ', name, np.round(r.min(), 1), ' km')
axr.set_ylabel('distance (million km)')
axv.set_ylabel('relative speed (km/sec)')
axv.set_xlabel('time since 1995-Dec-06 12:00 (days)')
axr.legend()
axv.legend()
axr.set_ylim(0, None)
axv.plot(t, np.zeros_like(t), '-k', linewidth=0.5)
plt.show()
From Cassini horizons_results.txt header, in response to comments
UPDATED with final trajectory prediction prior to atmospheric entry and
end of mission. Schedule of events for final day:
2017-Sep-15
Event time at Saturn Signal receipt time at Earth
5:08 UTC (10:08 pm PDT - Sept. 14)
High above Saturn, Cassini crosses the orbital distance of Enceladus
for the last time
7:14 UTC (12:14 am PDT) 8:37 UTC (1:37 am PDT)
Spacecraft begins a 5-minute roll to point instrument (INMS) that will
sample Saturn's atmosphere and reconfigures systems for real-time data
transmission at 27 kilobits per second (3.4 kilobytes per second).
Final, real-time relay of data begins
7:22 UTC (12:22 am PDT) High above Saturn, Cassini crosses the orbital distance of the F ring
(outermost of the main rings) for the last time
10:31 UTC (3:31 am PDT) 11:54 UTC (4:54 am PDT)
Atmospheric entry begins; thrusters firing at 10% of capacity
10:32 UTC (3:32 am PDT) 11:55 UTC (4:55 am PDT)
Thrusters at 100% of capacity; high-gain antenna begins to point away
from Earth, leading to loss of signal.
https://history.nasa.gov/sp4231.pdf
Galileo definitely did gravity assist at Io. See the link (3.7 Mb pdf), pages 202-203.
quote:
Two of the first events of Arrival Day were the Orbiter’s flybys of the moons Europa (at 3:09 a.m. PST) and Io (at 7:46 a.m. PST). The Orbiter passed 32,500 kilometers (20,200 miles) from Europa, but only 890 kilometers (550 miles) from Io, closer than originally planned (see figure 8.4). The Orbiter’s trajectory had been altered to make maximum use of Io’s gravitational field for decelerating the spacecraft, thereby conserving propellant for later in the mission.
The saving of propellant because of the gravity assist was rather significant. Galileo insertion burn has reduced the velocity by ~600 m/s (ibid, page 208). Without the assist delta-v would be closer to 900 m/s.
(Deduced from similar numbers for future ESA Juice mission, 7.5 Mbytes, page 85).
EDIT: ok, 175 m/s from another answer by @kwan3217, but still significant.
Juno mission could not use gravity assist from the moons. It was inserted on polar orbit around Jupiter. Galilean moons orbits are coplanar to equator of Jupiter, so Juno has flow too far from them to use gravity assist.
Other answers discuss how close Galileo got. Here is a source identifying the intent, Galileo Messenger v37:
https://planetary.s3.amazonaws.com/galileo_messenger/GM037.pdf
"The Io flyby is also critical for JOI: Passing in front of Io in its orbital path delivers a gravity-assised brake on Galileo's trajectory. The reduction in the Orbiter's speed (its ΔV) by 175m/s accouts for about a fifth of the total change needed for the capture orbit, the first in the orbital tour."
I remember watching this on the early World Wide Web, when I was growing up -- back in the day, NASA had one of the stronger presences on the web. I remember thinking that the Io pass, probe relay, and Jupiter orbit insertion was way too much to expect to work within just one day. As it happened, the tape recorder got stuck and they had to cancel all of the Io observations.
