The Probe & the Scientific Method

Take aim...fire!
 
The Probe into the Jovian upper atmosphere was the Greatest Rifle Shot in History. AKA "the shot heard around the Solar System."

You have to remember that the Probe had no independent guidance or propulsion but was spun off the larger Orbiter some 80,000 kilometers from its point of entry into the atmosphere. There had to be precise aim. A deviation of 1.5 degrees higher or lower (relative the entry point into the atmosphere) would have meant failure: either diving into the atmosphere too steeply and burn up - or too shallow and glance off to an infinite journey back into space. They say that this is like launching a ship from Los Angeles across the Pacific Ocean and having it sail dead down the middle of Sydney Harbour in Australia.

Pretty hard to top that shot!


80,000 kmeters... why so far?
 
The question is asked, why didn't they do the probe launch closer to Jupiter - 80,000 kilometers seems like a pretty far distance? Well, the Orbiter itself had to be aimed right at Jupiter, that is, flying straight toward the spot where the Probe would enter the atmosphere: that's how the Probe was going to be aimed. The idea was to impart the axial spin of the spacecraft to the Probe. Obviously, after launching the Probe the Orbiter was going to have change its trajectory, it didn't want to follow the Probe into Jupiter, rather swing around the outside of Jupiter so that it could achieve an orbital pattern.

Remember, they could only send so much propellant fuel with the spacecraft and there was going to be a terrific "burn" when it got to Jupiter, to slough off velocity so that Jupiter's gravity could capture it and turn it into an artificial moonlet. They didn't have the luxury of getting up close to the target and then blow propellant to do a major course correction and remember, the closer they would get to the target entry point, the greater the angle to have to course correct. Not to mention fighting the monstrous Jupiter gravity well. No, there wasn't any way around it - it was just going to have to be a terrific aim.

Orbit Deflection Maneuver

The early release of the Probe saved Orbiter propellant, and saving propellant--a limited resource for Galileo--was important. 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.


So how did they aim the probe?
 
Galileo was built so that it could triangulate on particular stars with absolute precision and the computer program would cause activation of the thrusters so that directioning with enormous precision could be achieved. They had better than one degree accuracy on each of the three axes. The construction of the mechanism housing the Probe itself had to be precise relative the Galileo spin axis and the part having to do with the release of the Probe had to be ultra-precise so as not to impart any local acceleration to the Probe mass.

Think about it - 50,000 years ago, our best tool was a sharp stick and a nice piece of rock.


Hey, who turned up the heat?
 
When the Probe start entering the Jupiterian atmosphere, it was traveling at a speed of 106,000 miles per hour. How fast is this? - well, you could circle the Earth at her equator about four times every hour - of course, you'd have to stop and replace the tires twice an hour!

It didn't stay going this fast for long. In four minutes, it had slowed to one-hundredth of that speed, to 1500 km/h. Wow! talk about G-force, the ultimate roller-coaster - you like squashy-face? The probe had to withstand a structural load 230 times Earth-surface gravity.

Some like it hot

The aeroshield, the oblately conical shell designed to protect the instrument module, heated to 22,000 degrees Fahrenheit.

This is twice as hot as the surface of the Sun. Ouch!


1-800 CALLNASA
 
Back on Earth, the scientists at JPL sat in stone-cold silence as they waited for the message from the Orbiter signaling that it was in communication with the Probe. The Probe's radio was no where near strong enough to be picked up on Earth. The plan all along had been for the Orbiter to be just "overhead" as it were, swinging by Jupiter getting ready to do the JOI burn - it would pick up the signals from the Probe and store them on the Tape Recorder for downlink to the DSN later on. Still, the Orbiter would give a thumbs-up when the Probe started sending.

One minute goes by, a minute and a half - what is happening half a trillion kilometers away? Finally, on minute number two, Galileo reports in: the Probe is sending.

Having survived its fiery entry into Jupiter's atmosphere, the Probe has deployed its parachute and is falling into the gas giant's upper layers.

The probe transmitted its data to the orbiter at 128 bits per second per string (channel). There were reports on the sunlight and heat flux, pressure, temperature, lightning activity, winds, and composition and structure of the atmosphere, as well as energetic particles measurements acquired during pre-entry.

For fifty-two minutes the Probe's radio crackles with humankind's first up-close- and-personal view of the King of Planets. Finally, at a depth of 160 miles from the top of the clouds, the Probe finally falls silent, a victim of the increasing pressures and temperatures encountered.


The Case of the Strange Data
 
When the scientists were finally able to retrieve the data from the Probe, they were shocked. Scientific observation is always approached, at least in part, from the perspective of previous observation, hypothesis and theory and what the scientists expected to see coming back in the Probe's data was confirmation and fine-tuning of well-supported ideas of gas giant planetary formation, physical structures like cloud layers and and the like. Scientists are always prepared to have these ideas challenged by new data but the stuff coming back from the Probe was just downright wierd. Where was the percentage of chemical H2O (water) that had to be there if Jupiter formed from the original planetary nebula? Where were the distinct layers of clouds that had been so meticulously detailed from observations ranging from Pioneer and Voyager to Hubble?

Only traces of the anticipated ammonia and ammonium hydrosulfide upper cloud layers were detected. Similarly, there was no data from a theoretical layer of water cloud.

It didn't seem to make any sense. Could there be something wrong with the instruments? Could the theories be that wrong? A little wrong was to be expected but this much? It just didn't make any sense.

"Perhaps," someone suggested, "perhaps, the Probe went into an anomalous area, something different from what the rest of Jupiter is like." You can almost hear the skeptical response to this: "Oh sure!! just call it an anomaly...sweep it under the rug."

But you know what? - that's exactly what it turned out to be. When they got around to taking some photographs of the exact entry point of the Probe, what they saw was ... a hole. A hole in the clouds. A small area relative the hugeness of Jupiter: you couldn't have hit it if you tried (well, maybe those Rocket Scientists could've for all we know!). But plain dumb luck was enough.


A hole is a hole is a hole...
 
I can just see Mr. Planetary Scientist, a little bummed out because the Probe didn't go where it was supposed to, then two minutes later poring over the data going, "hmmm, isn't *that* interesting..."

Turns out that the hole in Jupiter's clouds *is* interesting. They have done quite a bit of study of the hole and its surroundings. All around it are the towering, billowing gas clouds but as you get closer to the hole itself, it's easy to see that there are powerful winds flowing into it. Doppler analysis, which can show direction of movement, confirms this.

It looks to be a meterological phenomenon that we know from Earth's atmosphere. At Earth's equator, warm moist air rises, creating a distinctive cloud banding - this air, having lost its moisture, then descends as cooler, dry air over Earth's deserts. This can explain the relative absence of water in the environment that the Probe encountered and the lack of cloud layers.

Now there's an unexpected return from our trip to Jupiter. I guess no matter where you go: weather is weather. In a more serious vein, it's a practical lesson in scientific method, a lesson that tells us that the universe will have surprises for us at every turn but if we just keep our wits about us, we'll be able to make sense of it in the long run.


The End of the Probe
 
The probe continued to transmit data for 57.6 minutes until the 24-bar level (152 degrees Celsius and 140 kilometers below the 1-bar pressure level). At this point, the radio failed and it is assumed that the probe melted finally in the intense pressure and heat of Jupiter's atmosphere (at the 5000-bar level, 1700 degrees Celsius). It vaporized and merged with Jupiter's atmosphere.