Probe Squash

Feb. 97 - The Results and wrap-up have been updated

Introduction

The other planets of our Solar System are just as alien to us humans, but we're starting to figure out what those other planets are really like. On December 7, 1995, a 5-foot tall, blunt-nosed probe will dive into Jupiter's atmosphere, allowing us for the first time to get direct measurements of the atmospheric and weather conditions in the giant planet. What, exactly, is the atmosphere composed of? How deep do the winds go, and what powers them? Is lightning a frequent occurrence?

For up to 75 minutes, the Galileo Probe will automatically carry out carefully planned experiments that might help scientists answer these questions, and will then radio the data to the Galileo orbiter flying safely overhead.

And then, the Probe will fall silent. If everything works perfectly, the Probe will have talked to the Orbiter for 75 minutes. But, there are many things that we don't understand about Jupiter, and the Orbiter could possibly lose contact with the Probe even earlier.

Once a week, you'll be able to read about various dangers that the Probe must overcome--a firey entry, tremendous pressure, and more. At the end, your class will be able to make its own prediction about how long the Probe will last, and then see how your estimates compare with the real thing.

Finally, we'll talk about what we think will happen to the Probe after the end of its mission. Will it get squashed like a bug? Will it burn up in flames? Will it float somewhere in Jupiter's insides? Tune in and see!

Usage Suggestion

Installment #1: Launch Delays

Galileo's atmospheric Probe went through a similar type of retrofit. Due to delays in the Galileo launch and lengthening of the mission were replaced or rebuilt. These are the parachute, the mortar cartridge for chute deployment, and the lithium-sulfur dioxide batteries. The Net Flux Radiometer instrument also was rebuilt for improved performance. In addition, in the years before launch, all scientific instruments and subsystems aboard the Probe underwent detailed performance tests.

Be sure to tune in to the next segment of "Will the Probe Get Squashed," when we'll discuss the extreme entry conditions the Probe must survive before it can start upon its scientific mission!

Installment #2: The Probe's Entry Into Jupiter: Trial By Fire

Never before has a spacecraft experienced such intense conditions, and to simulate the entry environment and the response of the Probe, scientists at NASA's Ames Research Center had to build special high-speed arcjet and laser facilities. In addition, a complex computer code was developed by NASA-Ames, NASA-Langley, and contractors to determine response of the Probe to severe entry temperatures. What resulted was a spacecraft composed of two sections: a virtually impenetrable outer shell (deceleration module) for protection during entry and an inner capsule (descent module) containing the delicate electronics and scientific instruments.

The outer shell, which will surround the capsule through entry and then drop away, includes thick heat shields and their supporting structure, the thermal control hardware that will be used through entry, and a pilot parachute. Nested inside of this shell is the inner capsule, that carries the payload and which alone will descend through Jupiter's atmosphere. The payload carries the main parachute and the science instruments, plus the systems that support the experiments and transmit their data back to the overflying Orbiter for relay to Earth (kind of like an outfielder hitting the cutoff man in the infield).

By comparing test results to the calculations and allowing a 30 to 44 percent safety margin at various places along the Probe's body, scientists are confident that the shield is thick enough (about 6 inches at the nose) to withstand these severe entry conditions. The total weight of the forebody heat shield is 335 pounds, of which 193 pounds are expected to be vaporized during entry. What remains of this heat shield after entry will separate from the Descent Module when the main body of the outer shell drops away.

Additional detailed technical information on the heat shield is available at the end of this section.

The Probe is aimed to strike the atmosphere at an angle of 8.5 degrees to the horizontal. If that entry angle was a mere 1.5 degrees shallower, the Probe would skip off back into space; increasing the angle by 1.5 degrees means that the entry would overheat the spacecraft and destroy it.

In the next installment of "Will the Probe Get Squashed?", we'll talk about the Probe slows itself down even further. Plus, it's not only students who take tests--the Probe had its own tests to pass!

In about 2 minutes of deceleration, an ablative heat shield of carbon phenolic material dissipates the enormous kinetic energy of entry, reducing the Probe velocity to Mach 1. For the expected entry conditions, the maximum dynamic pressure will be about 5.0 x 10^5 N /m^12 (1.0 x 10^4 lb/ft^2) and the maximum deceleration level will be about 225 g. At a Mach 1 altitude of about 49 km (29 miles), the Probe deploys a parachute and jettisons the heat shield.

Thermal protection during entry is provided by a carbon phenolic forebody heat shield and a phenolic nylon afterbody heat shield. Although these materials have been used extensively for Earth re-entry vehicles, on the Galileo Mission they will be subjected to environments never before experienced in flight. Entry velocity relative to the atmosphere is 48 km/sec, far higher than any atmospheric entry attempted to date.

The shield is also subjected to mechanical erosion. The shield is subjected to a hot atmospheric shock layer (16,000 K, or 28,000 degrees F). The heat transfer at the nose of the vehicle at peak heating exceeds 42 kW/cm^2. The approximate mass of the forebodyheat shield is 152 kg (334 pounds), of which about 87 kg (191 pounds) is expected to be lost by ablation during entry.

Installment #3: The Probe's Parachute

The main parachute is 8.2 feet in diameter, and is made of Dacron and Kevlar.

And now, the real test of the Probe's longevity begins!

Installment #4: The Big Five (Spacecraft design issues)

• Jupiter's high pressure
• temperature
• the Probe's battery lifetime
• maintaining a radio link between the Probe and Orbiter through the increasingly dense atmosphere
• or the constraints of the Orbiter's mission which will force it to stop listening for the Probe's signal

General Design Issues

When designing a spacecraft, engineers and scientists work together to say what limits they'd like the spacecraft to reach. For example, an atmospheric scientist might say that she would like to be able to have the Probe reach down to a pressure level 14 times that of Earth. The engineers assigned with building the actual probe will then design a probe that can take that much pressure--and more, just as a safety margin. This is the design limit.

Next, engineers need to check that the hardware that they've built can actually stand up to real-use conditions. This means that they must test it under conditions that are more severe than are expected in the flight. One good rule of thumb is to test to 125% of the expected flight use, which is enough to guarantee that the design of the hardware will be able to survive the real mission. However, you don't want to run your test all the way out to the design limit, since that's where it's more likely that the hardware will fail! This process is called qualification testing. Any hardware that goes through a qual test does not get flown because the testing itself over stresses the hardware. But having passed this test, the hardware design is assumed to be qualified and any hardware that uses this same design is qualified as well.

Finally, the actual piece of hardware that will be flown into space gets tested as well, to 80% of the expected mission load. This checks that the equipment doesn't have any flaws, while at the same time not putting enough stress on the equipment to possibly cause it to fail at a critical time during the space flight! This is the actual acceptance test. A piece of hardware that passes this level is assumed to be okay for the actual flight.

To summarize,

1) Design limit is most stringent, but no hardware is tested to this level

2) Qualification test level is less stringent than the design limit, but exceeds the expected mission load. The hardware design is tested at this level, but the actual pieces of hardware that are tested are not used in flight.

3) Acceptance test level is the least stringent, falling slightly below the expected mission load. Hardware is tested at this level, and can then be used on a mission.

A GENERAL WARNING: the Probe, or any one part of the Probe, won't suddenly stop performing once the Probe passes beyond the acceptance or qualification limits. Engineers on the Probe Engineering Team expect the Probe to keep performing beyond these limits, beyond the battery voltage cut-off, and beyond the nominal (or expected) radio frequency link performance value. However, how far the Probe can go beyond those limits is an entirely different question--and one that we haven't tested for!

Installment #5: Keeping the pressure on

That's not the way the Probe mission is going to end--in fact, scientists would be disappointed if the Probe did stop moving down deeper into Jupiter's atmosphere, since gathering data at higher pressure levels is part of the Probe's mission! The Probe is intended to survive to at least the 10-bar pressure level (one bar is equal to the pressure that we feel here on Earth at sea level). At this point, the Probe would be 90 kilometers below the cloud tops. Under nominal (expected) conditions, this will happen about 38 minutes after entry, but possibly as late as 41 minutes after entry. It is certainly likely that the Probe will continue to function at much deeper levels (and, consequently, at higher pressures).

The Probe had to pass an acceptance test (13 bars) , and a qualification test (16 bars). The actual design limit on the Probe is 20 bars, when we expect to be 60 minutes into the Probe mission. At 75 minutes into the mission, when the receiving of the Probe data at the Galileo Orbiter ends (effectively ending the mission) due to Orbiter mission constraints, the pressure will be up to 30 bar--10 bar over the Probe design limit! All of the flight units and the entire descent module were tested to the 13 bar limit in the descent-pressure-temperature chamber. One of the Probe's instruments--the Neutral Mass Spectrometer--was actually pressure tested until it failed, at 21 bars. Will the Probe be able to handle the pressure until the end of the data relay? Or will there be a tremendous squash? And another problem to think about before the next installment: the pressure and temperature of a gas depend on each other. Is it fair, then, to make assumptions about how the Probe will fare looking at pressure levels alone? Remember the warning!

Installment #6: Temperature issues: Helping the Probe to Keep Its Cool

How do we protect the Probe from these extreme temperature changes? Before entry, thermal control is provided by hardware in the outer shell. After entry, layered blankets made of a material called Kapton slow down the rate of temperature changes in the Probe capsule, protecting the interior from rapid heating or cooling. Titanium was used for parts of the Probe structure because it doesn't conduct heat well (it would be a poor choice for cookware).

As you can imagine, testing the Probe's ability to withstand high and low temperatures is vital--but measuring the actual atmospheric temperature where the Probe might fail isn't easy. It seems like it would be simple: stick the Probe in the Descent-Pressure-Temperature testing chamber, and heat up the chamber.

The problem is that while some parts of the Probe and its instruments are designed to take high heat, other, more sensitive parts have to be protected from the surrounding high temperatures. Although the Probe is examining the Jupiter environment, we still have to isolate the Probe from that very environment! Essentially, the Probe is like a big thermos bottle. When the Probe was heated up to test limits, it then took several *days* until the Probe cooled down enough for the next test! Engineers even brought in fans to blow air across the Probe's nose (which didn't end up helping much).

While this helps to keep the Probe working, it makes it very difficult to figure out what the temperature will be *inside* the Probe for a given temperature * outside* the Probe. Engineers can look at predictions of the Probe's trajectory, and scientific models that predict what the gas temperature will be at different distances below the entry point, but they can't predict the temperature inside the Probe without an awful lot of effort.

Here's what the engineers do know: The probe has a unit temperature qualification value of 60 degrees Celsius (140 degrees Fahrenheit). While building and testing the Probe, a test was run in 1983, where the Probe was put into a large test chamber and the engineers tried to simulate Jupiter's atmosphere while the Probe sampled the atmosphere in the chamber. Just like in the mission, the Probe started quite cool and then was suddenly given a very cold dose of helium which then started to warm up. Slowly at first, but faster and faster as the test went on. Although the test didn't go the whole 75 minutes, the engineers could still use it to guess what will happen. Can you? Here are some of the important results of the test showing how the different parts of the Probe warmed up:

Time from entry        0 min   10 min    20 min    30 min    40 min    47 min

External Atmosphere    -76 C    -50 C      -9 C      51 C      92 C     119 C

Transmitter             -9 C      0 C       7 C      15 C      27 C      40 C
Exciter A              -13 C    -18 C     -18 C     -10 C       6 C      24 C
Exciter B              -10 C    -14 C     -11 C      -7 C       9 C      27 C
Data and Cmd Processor  -2 C     -4 C      -5 C      -1 C       5 C      11 C
Subsystem Power Supply  -3 C     -6 C      -7 C      -5 C       2 C       9 C
Instrument Power Supply -3 C     -8 C      -1 C      -2 C      14 C      32 C

Science Instruments
   ASI                  -2 C     -8 C     -10 C      -4 C      11 C      29 C
   NEP                 -10 C     -8 C      -9 C      -5 C       7 C      21 C
   HAD                 -10 C    -13 C     -13 C      -4 C      11 C      28 C
   NFR                  -7 C    -11 C     -12 C      -5 C      12 C      31 C
   NMS                   0 C     -1 C      -4 C       0 C      16 C      31 C
   LRD                  -3 C     -6 C      -8 C      -5 C       9 C      25 C

Obviously none of the parts of the Probe failed in the 1983 test because the test was stopped before anything got too hot. But what will happen when things keep getting hotter? To help you with this, here is how we expect the atmosphere of Jupiter to warm up during the Probe's mission:

Entry (0 minutes)   -8 C
      10 minutes   -94 C
      20 minutes   -23 C
      30 minutes    28 C
      40 minutes    71 C
      50 minutes   107 C
      60 minutes   140 C
      70 minutes   170 C
      75 minutes   184 C

Does this make you revise last week's estimate of the Probe lifetime?

And, some thing to think about before the next installment: without power, the Probe will be unable to send any data to the orbiter. Does the Probe's battery have enough power to carry out the entire mission?

Installment #7: Battery lifetime: electricity is vital

Before the Probe was released, it was able to draw power from the Orbiter. Now that it's on its own, it relies on a battery. And not just any battery--the Probe's lithium/sulfur dioxide batteries have to stand up to conditions far tougher than anything that D cells from the supermarket are designed for.

Once the Probe separated from the Orbiter, the batteries provided the only power source for the Probe during its almost 5 month long journey to Jupiter. (a side note--one of the things that the Probe engineers had to be very careful about while getting ready for Probe separation from the Orbiter was to make sure that the batteries weren't accidentally discharged!) Therefore, it was important to conserve energy on board the Probe, so everything was turned off, with the exception of a timer that would "wake" the probe six hours before entering the atmosphere. If the battery ends up running down faster than expected, the Probe mission can't last as long. But we won't know the battery's status until the Probe wakes back up.

When they're not being used (when they're not "under load"), the lithium batteries have a voltage of 39 volts. Fresh out of the factory, they have a total capacity of 21 ampere hours (an "ampere hour" is enough electricity to keep a one ampere current flowing in a circuit for an hour), of which one ampere hour will be lost as the battery ages (the battery, after all, is over six years old!); during the descent portion of the mission, an ampere-hour of energy is used every 7 minutes). The Probe mission calls for the battery to function for an hour after the Probe enters the atmosphere; by that time, the Probe should use up about 18 ampere hours. With the remaining two ampere hour margin, there should be enough power to last for at least 75 minutes after Probe entry, assuming that the Probe powers up no earlier than scheduled.

(There are other batteries on board as well--two thermal batteries are used for pyrotechnic events. You can hear the thermal batteries igniting, and smell them after they ignite! Specifically, thermal batteries are used for 1) the mortar fire that will send out the pilot parachute, 2) the release of the Probe heat shields, 3) extending the Nephelometer (or cloud sensor) arm, and 4) activating the sharp cable cutters that will sever wires connecting the Probe's outer protective shell to its inner capsule.)

To test the battery, a spare battery that was built at the same time as the flight one (and therefore is just as old), and has been kept at the same temperature as the flight battery, was hooked up to run under similar loads and temperature. The loads are more severe than the actual Probe battery will face. The test was run using the higher loads because this had been the estimated load that, years earlier, before the exact loads on the spacecraft were known, engineers had predicted the battery would face. To make it easier to analyze the test results, the testing conditions weren't changed from test to test over the entire lengthy history of the program.

Under this test, the battery lasted 68.5 minutes. After adjusting for the fact that 1) the flight battery will experience smaller loads, and 2) the flight battery will be some ten months older when called upon to do its job than was the battery used in the test, Probe engineers are predicting that the battery will be able to keep the Probe powered for 75.7 minutes. That will take the Probe down to almost 30 bars of pressure, or about 193 degrees Celsius.

Do you regard this as good news or bad news for the Probe's total lifetime? And, something to think about for next time: so far, we've talked about how to keep the Probe "alive." But how do we get the Probe's data from the hostile Jovian environment back to Earth?

Installment #8: Radio link through the dense atmosphere

There's a well-known philosophical question that asks "if a tree falls in the forest, with no one around to hear it come crashing down, does it make a sound?" Probe engineers and scientists might update this to say "if the Probe collects all the data we've hoped for, but doesn't send that data anywhere, does it really matter if the Probe survives entry?" The Probe itself does not send data directly to Earth--all of the Probe's precious data must be transmitted by radio to the Orbiter waiting overhead. If that radio link is never established, all of the Probe's science is lost forever. And if the radio link is disrupted, there might be "gaps" in the data that *is* returned to Earth.

Since the radio frequency link is so crucial to the success of the Probe Mission, it has a backup, or redundancy, built in: not one, but two parallel and simultaneous data "streams" are sent from the Probe to the Orbiter. On the other end, the Orbiter has two digital relay radio receivers that "acquire" (basically, tune into), track, and process the Probe data along with radio science and engineering data. The Probe's computer puts the data in the format needed for it to be transmitted to the Orbiter flying high above, which will then send the data back to Earth.

There are other problems that can also affect the Probe's relay link. For example:

*Keeping the Probe signal "in view": The relay antenna on board the Orbiter has to be repointed four times during the Probe relay so that the Probe's signal "hits" the antenna relatively near the center. This helps to keep the Probe's signal as loud and clear as possible, which means that we can keep collecting Probe data for as long as possible. Once the Probe has descended below the water clouds, there will be a pointing update every 10 minutes.

*Fading signal strength: As the Probe falls deeper into Jupiter's atmosphere and penetrates the clouds, the radio signal loses strength as the distance that the signal must travel increases, and as the radio signal is absorbed by the atmosphere. There is also a background noise coming from the high energy charged particles in Jupiter's radiation belts.

*The Probe's wild ride: Jupiter's winds and turbulence could cause the Probe to move about violently, making it harder to stay in lock (because of distortions to the signal).

Probe engineers have run through a worst possible case situation, which includes **dozens** of different factors that might cause problems with the radio link. Even then, it's estimated that there would still be a radio link until after the Probe descended below a pressure level of 10 bars (which would take no more than 42 minutes).

Technical Details:

After taking all of the above into consideration, each relay radio receiver has been tested and designed to meet the following requirements: It will acquire the Probe signal within 50 seconds, with an acquisition probability of 99.5 percent, and a probability of reading a false signal of only .001 percent. (That is, the odds are 99.5 in 100 that the Orbiter will tune into the Probe's signal within 50 seconds of when the Probe's transmission starts, and there is only a 1 in 100,000 chance that the antenna would mistakenly pick up the wrong signal.) The minimum signal strength at acquisition is 31 dB-Hz. The system is required to track a signal strength as low as 26 dB-Hz. If either receiver loses the Probe signal because it's not quite tuned in (rather like an FM radio station drifting in and out on your car stereo while you're driving in the mountains) , the software is built into the system which allows the signal to be reacquired within 60 seconds. The unit also measures Probe signal strength and Doppler rates for radio science use (wind and atmosphere absorption measurements).

At the time that the receiver was built, it was truly a unique design: among other features, the receiver can do "comm on the move," meaning it can respond to rapid environmental changes in the radio link. Considering that the Probe's frequency is going to change dramatically as a result of tremendous changes in the way that the Probe and Orbiter are oriented, and also the velocity between them, it's absolutely necessary for the Orbiter's receiver to be able to respond to these changes and keep the signal locked up!

When we started this activity, you were told that Probe relay would last no longer than 75 minutes. In the next installment, we'll learn why that's so.

Installment #9: Is the Orbiter available to talk with the Probe?

Since the Probe's data is so unique and important, you might think that Galileo's scientists and engineers would arrange to keep the Orbiter "listening" for the Probe's signal until the Probe had gasped its last.

Nope. The Orbiter will receive Probe data until 78 minutes after the Probe enters the Jovian atmosphere, giving a **maximum** relay duration of 75 minutes. This corresponds to reaching a depth of about 164 kilometers below the cloud tops, and a pressure of 30 bars. After that 75 minutes is up, the Orbiter will stop listening to the Probe, even if the Probe is still working.

Although the Probe's mission will have ended, the Orbiter will be getting ready to spend the next two years in orbit around Jupiter. Just over an hour after the end of Probe relay, the Galileo Orbiter will fire its main engine so that the spacecraft enters orbit, and doesn't just go flying by the giant planet! As you can imagine, this is a tremendously important event, and there's a lot of concern with making sure that the spacecraft is ready for this maneuver. It takes time to get ready for the Jupiter Orbit Insertion burn, or JOI: the relay antenna used to listen to the Probe must be stowed away, and the Orbiter must start spinning faster (10 revolutions per minute, instead of the usual 3.5) to make the spacecraft more stable. So, since it's predicted that the Probe's signal strength will quite probably be too weak to be picked up by the Orbiter by the 75 minute mark, it makes sense for the Orbiter to end Probe relay, and start preparations for JOI.

This is the next to last installment of ProbeSquash. At this point, you know about the different challenges facing the Probe, ranging from a delayed launch to high temperatures in Jupiter's atmosphere. We've given you a look at how experts have tested the Probe, and the test results. Just like many people working on Galileo, you've made your predictions about how long the Probe will last.

And now, we wait. The final installment of Probe Squash will come after December 7, when we'll all find out if the Probe had a successful flight. As a bonus, we'll also learn about the ultimate fate of the Galileo Probe, lost forever in the swirling winds of our solar system's largest planet.

Final installment #10: Results and wrap-up

For more information, visit the Quick-look Science Summary.