Galileo FAQ - The Probe
![[Spacecraft showing Antenna and Probe]](images/hga_sc.gif)
The Probe was nestled on the "bottom" of the spacecraft, below the area where the camera scan platform is mounted.
How fast was the Probe going when it enters Jupiter's atmosphere?
170,700 kilometers per hour (106,000 miles per hour), or 47 kilometers per second (29 miles per second) That's the highest impact speed of any man-made object ever; it's about 100 times the muzzle velocity of a bullet fired from a .45 caliber gun.
What science did the Probe instruments return?
Galileo's Probe incorporated experiments to measure temperature and pressure along the descent path, locate major cloud decks, and analyze the chemistry of atmospheric gases. In addition, the Probe attempted to detect and study jovian lightning both by looking for optical flashes and by listening for the radio "static" they generate. The latter detector also measured high-energy electrons close to Jupiter just prior to atmospheric entry.
Once the Probe separated, what woke it up many months later just before
entry? Did it just have a built-in programmable timer, or did the Orbiter
beam the Probe a command to wake up? Or did it remain fully "awake," doing the
same thing (that is, taking data) from the time it separated until the end of
its mission?
Yes, the Probe did have a built-in, programmable timer, which was set by ground
command shortly before the Probe separated from the orbiter. The Probe designers
wanted to have some flexibility in starting the timer to accomodate any late
changes in the Probe release schedule. If Probe release was delayed for any
reason, the timer could be reset appropriately. The timer was the only thing running
on the Probe during its five month long cruise to Jupiter.
Like an alarm clock, the timer was set to wake up the Probe 6 hours before entry
into Jupiter's atmosphere so that the Probe could 1) take measurements of the
inner magnetospheric energetic particle environment, and 2) listen for radio
emissions characteristic of lightning (these actually sound like long,
descending whistles).
This "pre-entry" phase ended when the Probe's accelerometers detected signs that
the Probe was being decelerated by Jupiter's atmosphere. At this point, the
Probe started its entry/descent phase.
Although Jupiter is a planet, it is very different from Earth. In fact,
scientists refer to hard and rocky planets like Earth, Mercury, Venus, and
Mars as "terrestrial," while planets like Jupiter, Saturn, Neptune, and
Uranus are called "gas giants," since they seem to be, essentially, huge balls
of gas and liquid with a small rocky core. So, Jupiter doesn't really have a
"surface" in the sense of its being something that humans could walk
around on, or that a spacecraft could land on.
Galileo's atmospheric Probe traveled between 130 and 160 kilometers
below Jupiter's cloud tops, deep enough to help answer questions such as
what's in Jupiter's yellow clouds, or how strong are the winds below the
cloudtops. However, the Probe didn't come anywhere near seeing the
"surface" of Jupiter's rocky core, buried roughly 60,000 kilometers
underneath the cloud tops.
Why didn't we take an image of the Probe as it drifted
away from the Orbiter after separation?
Imaging the Probe as it drifted away from the Orbiter was contemplated both
for engineering assessment (that is, looking for any problems with the
Probe hardware) and
optical navigation. In order to assess the external
condition of the Probe, detailed pictures would be desireable, but, because
the SSI is focused on infinity, objects up close would be out of focus. In
fact, by the time the Probe was in focus (at about 18 km away from the
Orbiter), it would only be about 2-3 resolution elements
(about 4-6 pixels) across--not
very detailed!
Taking a picture of the Probe for Optical Navigation purposes appeared to be
feasible
, but, because the Probe delivery knowledge requirements are being
met, and because of the operational costs of trying to return large data sets
(such as images), it was decided not to pursue optical navigation.
We need to consider two different questions: (1) why not wait until Galileo was in orbit around Jupiter before releasing the probe? and (2) if the probe had to be released before Galileo was in orbit around Jupiter, why not wait until Galileo was quite close to Jupiter?
Galileo's approach trajectory to Jupiter had the spacecraft flying by Jupiter, much like Voyager. The spacecraft could only enter orbit by performing a braking maneuver known as Jupiter Orbit Insertion, or JOI. JOI was a huge maneuver that, unlike any other maneuver, required the use of Galileo's 400 Newton thrusters (the Orbit Deflection Maneuver, discussed below, used the 400N thrusters as a "practice run" for JOI). Since the Probe was attached to the Orbiter right below the 400N engine (see illustration), the 400N engine could not be fired until the Probe was released, and, therefore, the Orbiter and Probe could not possibly enter orbit around Jupiter linked together.
Even if entering orbit together were possible, keep in mind that the combined Orbiter/Probe would have been much more massive than the Orbiter alone. This means that it would have taken much more propellant to perform JOI successfully.
Assuming the Probe would be released before Jupiter arrival, there were two factors that would favor a late Probe release. The Probe had no propulsion or control capability of its own. Once released from the orbiter, it followed a precise ballistic trajectory into Jupiter's atmosphere. Hence, meeting the Probe's aimpoint requirements (primarily, the angle that the Probe's flight path would make with Jupiter's atmosphere) argued for a later release, since any errors in the Probe's final targeting or from the release itself wouldn't have very much time to act before the Probe hit the atmosphere. Navigation uncertainties would also decrease as the Orbiter and Probe got closer to Jupiter, making it easier to target the aimpoint more accurately. If you were sailing a ship the roughly 10,000 miles from Los Angeles to Australia, and you made a slight error in your course heading as you left Santa Monica Bay, that slight error might well cause you to miss your destination. However, that same small error, made when you were only a few hundred miles from your destination, would have less effect on your final landing point.
Also, the Probe's battery had a limited lifetime; once the Probe separated from the Orbiter (and, hence, from the Orbiter's power supply), the battery would start running down. Late Probe release would increase battery lifetime margin.
On the other hand, an early release of the Probe saves Orbiter propellant, and saving propellant--a limited resource for Galileo--is usually a good thing. Remember that the Probe had no trajectory control of its own. That means that at the time that the Probe was released, the Orbiter was also on a collision course with Jupiter! So, shortly after the Probe release, the Orbiter changed its trajectory (by firing its main engine) to avoid sharing the Probe's fate. Since the Orbiter was still a long distance from Jupiter, a small trajectory change--and, consequently, a small amount of propellant--was all that was needed to properly "aim" the Orbiter. If Galileo's navigators waited to release the Probe until the spacecraft was much closer to Jupiter, the Orbiter would have had to use much more propellant to carry out the "Orbital Deflection Maneuver" as the spacecraft gets closer to Jupiter, so the earlier the probe could be released, the better.
The final decision on the time of Probe separation resulted from an evaluation of all of the above factors.
Both the orbiter and the probe are regarded as "the" Galileo spacecraft, rather like siblings in a family having the same last name. So the official names are "Galileo Orbiter" and "Galileo Probe."
Return to Galileo Frequently Asked Questions
Submit your own question