QUESTION:
How long do you expect the probe to last in Jupiters atmosphere?

ANSWER from Nadin Cox on December 28, 1995:
There are a number of stages in the probe's demise.  For a detailed
description, please see the "Fate of the Galileo Probe" on the Galileo
Probe homepage (http://planetary.org/galprobe). 

Basically, we were recording data from the probe for 81 minutes.  The 
probe batteries gave out minutes after that time. The probe then sank 
further and further into the Jovian atmosphere until the high pressures 
and temperatures crushed some of the instruments and melted the aluminum 
and titanium in the probe.  

We believe the probe finally vaporized approximately 10 hours into 
the mission.

ANSWER from 
THE PLANETARY REPORT, vol. 15, no. 6, November/December 1995, p. 14
The Fate of the Galileo Probe
Jonathan Lunine and Rich Young

On December 7, the Galileo probe will enter the atmosphere of Jupiter
and beam back to Earth information of a sort never before obtained for
the solar system's largest planet.

To understand the fate of the probe requires that we consider its design.
The entry of the Galileo probe into Jupiter's atmosphere is the most
difficult ever attempted at a planet, in terms of the velocity of entry
and subsequent heating of the probe structure. The scientific instruments
are housed in what is termed the descent module, which is protected during
the high-speed entry by the fore and aft heat shields. Once the probe has
slowed sufficiently, the heat shields separate from the descent module,
which drops through the atmosphere on a parachute, taking scientific
measurements.

The orbiter is in position as a receiving station for 75 minutes.
The batteries on the probe are sized to last at most a few minutes beyond
that time. Bigger batteries would yield little more data, since as the
probe sinks deeper into the atmosphere radio signals become more and
more attenuated by the gases and clouds overhead. The probe, bereft of
an orbiter link and its batteries depleted, will continue to sink deeper
into Jupiter's interior.

One fate we know the probe won't suffer is to hit a surface, because
Jupiter is almost entirely gas.  Furthermore, the density of the probe
is greater than the density of Jupiter's gaseous interior, and the probe
continues to sink after transmissions cease. The probe is vented, so the
overall structure will remain intact, except for some sealed boxes (which
will be crushed as the pressure increases). Thus, what ultimately will
destroy the probe is the increasing temperature with depth, which will
cause the probe materials to melt and vaporize.

To find out where this happens, we borrowed a model of the
temperature-pressure-density structure of Jupiter's interior from
Didier Saumon and Tristan Guillot of the University of Arizona. We
simplified the lengthy list of probe materials to just three: Dacron for
the parachute, aluminum for the inner equipment shelf and titanium for
the outer aeroshell.

The first event is the melting of the Dacron parachute, at about
260 degrees Celsius (500 degrees Fahrenheit), about one half hour
after the latest possible probe mission end. Parachute failure causes
the probe to drop more rapidly. Roughly 40 minutes later, at a
temperature of about 660 degrees Celsius (1,200 degrees Fahrenheit)
and 280 bars pressure, the aluminum portions melt. (One bar equals
Earth's atmospheric pressure at sea level.) Although some interesting
chemistry could take place between the molten aluminum and the titanium
housing, more likely the aluminum separates from the probe in blobs or
droplets, which continue to descend on their own. The titanium itself
melts at a much higher temperature--about 1,680 degrees Celsius
(3,100 degrees Fahrenheit)--which the probe reaches at a pressure of
2,000 bars, more than nine hours after its entry into the atmosphere.
The probe housing becomes a molten mass, probably breaking up into
droplets of titanium that rain down through the dense jovian gas.

The fate of these molten materials is vaporization (or evaporation,
equivalently), as temperatures get so high that the liquid turns to
vapor. Where this occurs turns out to be a complicated issue, depending
on how much natural titanium and aluminum are in Jupiter's gaseous
interior, and what chemical form they're in.

To simplify the analysis, we note that evaporation is very slow at
low temperatures.  However, as the probe continues to fall and the
temperature goes up, the evaporation rate increases dramatically.
When sufficiently high temperatures are reached--for aluminum, around
1,170 degrees Celsius or 2,150 degrees Fahrenheit; for titanium, around
1,850 degrees Celsius or 3,360 degrees Fahrenheit--the metals will
evaporate within minutes.  By 10 hours into the mission, the probe has
been transformed into separate atoms and molecules.

So, after telling humans about the nature of Jupiter's atmosphere, the
atoms of the Galileo probe will become a part of that giant planet.
This exchange of material between one world and another is nothing new;
it has been happening across the solar system from the beginning
(witness Shoemaker-Levy 9). The atoms out of which the probe was
fashioned were originally produced as part of the debris of
nucleosynthesis in massive stars, spewed out with the rest of the
stellar material to form Earth, the solar system and life.

The epitaph "ashes to ashes" might be appropriate to describe the
origin and fate of this product of human ingenuity.

Jonathan Lunine is a full professor at the University of Arizona,
and Rich Young is a Galileo probe scientist at NASA Ames Research Center.

COPYRIGHT 1995 The Planetary Society