Donald Mount Hunten 1925-2010

One of the leading planetary scientists of the Space Age, Prof. Donald M. Hunten of the Lunar and Planetary Laboratory of the University of Arizona, has died of a stroke at age 85.




I have known Don since 1967, when I was still a graduate student in La Jolla. Don, a refugee from Canadian winters, was already well established in the field as a professor at the University of Arizona. He had arrived there in time to join the fledgling Lunar and Planetary Laboratory at its inception, in 1964. His work to that point had centered on the chemistry and physics of Earth’s auroras.



By the time I met him, planetary exploration by spacecraft was under way, and Don had broadened the scope of his research to include the upper atmospheres of the planets. He focused on the interaction of the Sun with the outer fringes of planetary atmospheres; I worked on the chemistry of atmosphere-surface interactions at MIT. Our overlapping interests led to us serving together on a number of NASA advisory committees and on advisory panels of the National Academy of Science, spanning the exploration of Venus and the giant planets, including the potential role of entry probes on the planets and Titan.



Don was a no-nonsense researcher with a keen critical sense; he was always a font of ideas and a hard worker, a researcher whose work merited membership in the prestigious National Academy of Science. Those who got to know him well found a deeper and richer persona: a bassoonist in various groups in Tucson with a deep love of classical music; someone endowed with a sense of humor that merged British and American sensibilities; a respected and successful mentor of generations of outstanding graduate students. I look back on our 43 years of professional interaction and personal friendship, 30 of them as colleagues at LPL, with gratitude for having known him.

I extend my sympathies and condolences to his wife Ann Sprague and his family. They may take some solace from knowing that his professional legacy and his “second family” of students will carry on the work he began and loved.

The Solar System is no longer the limit

Voyager 1 was launched in 1977 on a mission to fly by Jupiter and Saturn. Barring damaging encounters with cosmic dust or Saturn’s ring debris, Voyager was expected to last a few years, after which the slow decay of its radioisotope power supply would make its radio transmissions inaudibly faint. That was then.




This is now: radio detection technology has advanced more rapidly than Voyager’s transmissions have faded. Now, 33 years after launch, Voyager 1 is still on line. It is now operating 17,400,000,000 kilometers from the Sun, nearly three times as far away as Pluto, receding from us at a rate of 17 kilometers per second. Its signals take 16 hours at the speed of light to reach us. Its instruments have monitored the outward rush of the Solar Wind since launch, but now it is so far from the Sun that the interstellar medium is getting in the way. The Solar Wind sweeps out an immense “bubble” in the interstellar plasma, within which the flow is steadily outward from the Sun. Next comes a relatively thin region of interaction of the Solar Wind with interstellar plasma, a turbulent shock front. (This turbulent sheath streams out behind the Sun as it travels through the interstellar plasma at 20 kilometers per second like the tail of a comet.) Next comes the outer edge of the bow shock, beyond which no trace of the Sun’s influence survives.



Voyager 1 is now in the turbulent shock region. The plasma flow it is now measuring is at right angles to the flow from the Sun. It is leaving the heliosphere, the region in which the Sun is dominant. It is already in the disputed realm where the Sun is losing its struggle against the interstellar medium. A vast, sparse cloud of Kuiper Belt Objects lies around it, and the Oort cloud of frozen, inactive comets lies ahead, but they are so widely scattered and so faint that Voyager will not see them. To its sensors, it is leaving the Solar System behind. In 4 or 5 years it will be beyond the shock front, cruising the void between the stars. This is interstellar space, the stuff of science fiction.

But space is vast. If Voyager had been aimed at the nearest star (it wasn’t), it would be less than 0.1% of the way there. On its present course it will be as far away as the nearest star in about 50,000 years. Will we still be listening then?

What Everybody Knows about Newton

Isaac Newton, during his exile in the English countryside during the plague year of 1666, conceived the idea that the force that caused things to fall to the ground might be the very same force that held the Moon in orbit around Earth.  He seems to have arrived at this hypothesis without reference to apples.

Newton, being a scientist, was not content to make a qualitative generalization.  Instead, he constructed a quantitative description of his idea, an equation intended to predict how the force of gravitation depends on the mass of the attracting body and its distance.  Then Newton, being a scientist, solved the equation for the two cases of interest.  

The first case was that of the acceleration of a falling body near the ground (at a distance of one Earth radius from Earth’s center).  For this calculation he used the best available measurement of Earth’s radius.  The second case, the motion of the Moon, specifically the acceleration required to bend the Moon’s trajectory into a closed orbit around Earth, required using the best available measurement of the Moon’s distance from Earth.  Both calculations also depended on the exact mass of Earth, since both accelerations were, by his hypothesis, proportional to Earth’s mass.  Newton realized that  the ratio of these two accelerations was therefore independent of Earth’s mass, which was fortunate because Earth’s mass was not well known.  Indeed, in both calculations the predicted  acceleration was proportional to the product GM, where G was a very poorly known constant called the Universal Gravitational Constant, and M was the equally poorly known mass of Earth.  But the numerical value of the product of G times M could be calculated with good precision from measuring the acceleration of falling objects in the laboratory-- and in the ratio of the two accelerations, the product GM cancelled out perfectly.

Newton did the calculation with the best available data and found to his chagrin that there was a small but significant discrepancy.  In effect, the product GM estimated from the Moon’s motion and the value of GM deduced from laboratory measurements were not exactly the same.  Being a scientist, Newton concluded that his hypothesis was in error, and tucked it away in a drawer.  

Several years later an astronomer made new observations of the parallax of the Moon (the apparent displacement of its position seen when two observers in different places simultaneously measured the Moon’s position precisely against the background of distant stars), from which the Moon’s distance can easily be calculated.   He published his more accurate determination of the Moon’s distance.  Newton read the article and remembered his old hypothesis, gathering dust in a drawer.  He pulled it out, inserted the new measurement of the Moon’s distance, and found that it worked!  GM was the same for the Moon and for cannonballs in the laboratory!  Encouraged by this success, Newton published the Theory of Universal Gravitation, and immediately the motions of Solar System bodies became predictable!

A contemporary of Newton, Edmund Halley, was fascinated by the apparently unpredictable motions of comets.  All contemporary wisdom held that comets were not even physical objects; they were signs from God, not subject to any law understood or even understandable by mere humans.  But Halley found in ancient records reports of a series of comet appearances that very nearly fit the same orbit, and very nearly the same orbital period.  He suspected that they were all the same comet.  Halley found by laborious calculations that the comet’s orbit changed whenever it passed close to Jupiter.  Using Newton Law of Universal Gravitation, he showed that the gravitational influence of Jupiter accounted for the changes in the comet’s orbit.  Halley then predicted the next appearance of the comet.  When the comet reappeared in accordance with his prediction, years after Halley’s death, the comet was named after him.  Comets ceased to be regarded as “signs” and became Solar System bodies with predictable orbits.  This was an astonishing verification of Universal Gravitation.

Back in his day and using this law, Newton showed that a body sent from Earth with a high enough velocity, such as a projectile from a giant cannon fired from a mountain top, could enter orbit around Earth just above the atmosphere.  That critical speed was called “circular orbital velocity”  With twice as much energy (a velocity greater by a factor of the square root of 2) the projectile would coast out to infinite distance from Earth.  Any speed equal to or above this “escape velocty” would guarantee that the projectile would never return to Earth.

Everybody knows that Newton said “what goes up must come down”.  But, like almost everything that “everybody knows”, this is errant nonsense.  In the history of the human race, the first person to prove that what goes up does not have to come down was Newton.  In doing so, he paved the way for launching Earth satellites, interplanetary probes, and the Pioneer and Voyager missions into interstellar space.

For more about comets and their interaction with Earth, see Rain of Iron and Ice and Physics and Chemistry of the Solar System.