Accompanying Todd was Charles Glidden, the pilot, and Mabel Loomis Todd, the astronomer’s extraordinary wife. A historian describes her as follows:
Cheerful, talented, sociable, popular enough to arouse jealous gossip, she was capable of sustaining affectionate and amorous ties; effective as a writer, lecturer, and editor, as wife, hostess, and lover, she constructed for herself, more than most, an enviably robust and resilient character.*
It was Mabel Todd, the daughter as well as the wife of an astronomer, who saw the comet first. The view, her husband said, was much better “than through the big 18-inch telescope at Amherst Observatory,” at the observatory he had himself designed and built. The head of the comet was near the horizon, and Todd made four sketches of the tail. This seems to have been the first successful astronomical observation from an artificial platform above the Earth’s surface.
“An astronomical observatory in the upper air: taking observations of Halley’s Comet from a balloon.” Drawing by Henri Lanos, published in the magazine Graphic, May 28, 1910. Courtesy Ruth S. Freitag, Library of Congress.
Fifteen years later, Todd went on to obtain the first photograph of the solar corona from an airplane. But afterwards, his behavior became increasingly erratic, and Todd was intermittently confined to mental institutions until his death at age eighty-four, in 1939. Mabel Todd predeceased him, mourned by their only child, who remembered especially “her indefatigable energy and her infectious gaiety.”
Today, new balloons are rising. 1985/1986 represented a historic moment in the study of comets. Previously, the comets came to us. Now, for the first time, we went to them. The occasion was the apparition of Comet Halley between fall 1985 and spring 1986. Apart from its historic significance (Chapters 3 and 4), Comet Halley is the only vigorously active comet with an orbit well enough known to permit detailed scientific planning years in advance. Following David Todd’s lead, aircraft flew above most of the Earth’s atmosphere and rockets briefly entered space to glimpse the visitor. The unmanned Solar Maximum Observatory, repaired by Shuttle astronauts in April 1984, examined Comet Halley; special instruments were flown on the Space Shuttle to scrutinize the comet; and the attention of the U.S. Pioneer-Venus Orbiter was redirected from Venus to the comet as it passed by. Compared to the usual levels of effort, this would already represent an extraordinarily concerted study. But, in fact, all this represents something of a sideshow—because a flotilla of five spacecraft flew by Halley’s Comet in March 1986.
A comet probe would certainly solve most of the cometary problems without ambiguity.
—POL SWINGS, UNIVERSITY OF LIÈGE, BELGIUM, AUGUST 1962
The close-up examination of comets from space is the fulfillment of an astronomer’s dream, because we have been painfully ignorant about the most fundamental aspects of comets. We live in the inner solar system, so that cometary nuclei approaching the Earth tend to outgas and cover themselves with coma. But if we could fly close to a comet, we might for the first time clearly view a cometary nucleus. What does it look like? What is its shape, appearance, color? Are there patches of ice, dark organics, rock outcroppings? Is it surrounded by a swarm of small boulders? Is there a lag deposit? Craters? What about signs of past surface melting? Hills? Any hint of layering as in the Earth’s sedimentary column? You might be able to tell a great deal about the nature and evolution of comets if you could photograph a nucleus close up.
Then there is spectroscopy. If we are restricted to the surface of the Earth, we can examine comets only in the visible spectrum, as Huggins did, and at a few “windows” at infrared and radio frequencies. If we wish to examine comets at other frequencies, we must get above the Earth’s absorbing blanket of air. But what can be lifted even into near orbit must be much smaller than major astronomical facilities on the surface of the Earth; the intrinsic capability of instruments in Earth orbit tends, therefore, to be inferior to their groundbased counterparts. Nevertheless, instrumentation in Earth orbit can make important new findings; the discovery of the hydrogen coma around comets (Chapter 7) is an example. But very few bright comets have come close to the Earth since the advent of astronomy from orbit. What you really need is to take the spectrometers close to the comet. Then you might find yourself discovering how various molecules are distributed through the nucleus, coma, and tail.
Generally, spectroscopy performed from Earth reveals the presence of molecular fragments, not the parent molecules of the cometary nucleus from which they come. For example, the nature of the organic molecule from which C3 derives is entirely unknown. There is an elaborate machinery of outgassing, dissociation by ultraviolet light, and further chemistry that occurs before we detect the daughter fragment. Many of these chemical mysteries could be resolved if we could fly close to the nucleus, into the cloud of parent molecules and there measure directly, before they are dissociated by sunlight, the molecules, organic and inorganic, that have just been evaporated off the comet.
To detect some sign of the material in the tail of Halley’s Comet in 1910, many studies were made at ground level. In France, a large volume of air was brought to very low temperatures, the oxygen and nitrogen liquefied, and the residuum examined for exotic constituents. Nothing was found. Metal plates coated with glycerin were attached to the struts of a tower at the Mount Wilson Observatory in California, but no cometary dust motes were reported. The experiment is the forerunner of modern studies in which similar glycerin-coated plates are attached to aircraft and flown to stratospheric altitudes—where cometary debris is much easier to come by (Chapter 13). But now it is possible to fly spacecraft with mass spectrometers aboard that can measure directly the parent molecules of the comet.
We think the magnetic field in the solar wind drapes itself over the cometary nucleus, while solar flares produce gusts in the solar wind that generate elegant patterns in the ion tails. But our understanding of comets would be far superior if we flew instruments very near the comet, and measured charged particles and magnetic fields directly.
Visual observers, straining at the limit of the resolution of their telescopes, have detected great fountains of gas and dust shooting into space from the cometary nucleus. We think much of the dust released by comets to space arises in this way. We need to sidle up to a comet, photograph the jets—obtaining a motion picture if possible—and fly through the dust cloud, counting and measuring the fine particles. All this and more has now been accomplished.
At the Japanese Space Center at Uchinoura in Kagoshima Prefecture, Kyushu, two new launch vehicles of the Mu class lift off. The first is called Sakigake (Japanese for “pioneer”); the second bears the simple designation, Suisei (Japanese for “comet”). They are the first interplanetary vehicles ever launched by Japan.
On the equatorial island of Kourou, off French Guiana in South America, an Ariane rocket is launched. In its nose cone is the first interplanetary spacecraft of the European Space Agency, a consortium of Belgium, Denmark, France, the Federal Republic of Germany, Ireland, Italy, the Netherlands, Spain, Sweden, Switzerland, and the United Kingdom. It is named Giotto—after the Florentine painter who incorporated his own observation of Comet Halley, during its apparition in the year 1301, into his Adoration, a celebrated fresco on the Arena Chapel in Padua (see plate xv).
In Tyuratam in the Kazakh Soviet Socialist Republic, two Proton launchers rise to the heavens. Their ambitious mission: to fly to Venus—depositing two spacecraft to perform a night landing there, and dropping off two balloon stations to examine the meteorology of the middle atmosphere* —and then continue on to encounter Halley’s Comet eight months later. The spacecraft are called Vega—Ve from Venera, the Russian word for Venus, and Ga from Galley, the Russian language having an aspirated G, but no H. There are twelve separate scientific instruments aboard Vega’s cometary payload. In addition to Soviet instrumentation, the Vega missions carried equipment from Austria, Bulgaria, Czechoslovakia, the German Democratic Republic, the German Federal Republic, Hungary, Poland … and
the United States.
The United States of America has played a key role in spacecraft exploration of the solar system—examining for the first time every planet from Mercury to Uranus, and dozens of moons. But the United States did not have a spacecraft to Comet Halley. Several innovative mission designs were proposed by American scientists and engineers; they would have procured a range of essential data that Giotto, Vega, and the Japanese missions would not. But the proposals were turned down by Democratic and Republican administrations both. There was not enough money. The United States had more important things to do. The cost of a major mission to Comet Halley was roughly that of a single B-1 bomber. The United States was committed to 100 B-1 bombers. We couldn’t have gotten by with 99. Perhaps in the year 2061, when Halley’s Comet next returns, there will be a different American response. But while the United States had in this respect opted out of cometary exploration, the flotilla of five spacecraft from twenty nations represents a stirring response by the human species to this emissary from the depths of space and from the early history of the solar system.
Through a bit of orbital legerdermain, the United States was nevertheless the first nation to examine a comet close up by spacecraft. It is in its instrumentation a rudimentary spacecraft, unable to obtain either pictures, spectra, or much compositional information, but mainly data on charged particles and magnetic fields. (See the box on this page.) However, without a doubt, it was first.
There was also an American experiment aboard Vega—there entirely through the individual initiative of John Simpson, a professor of physics at the University of Chicago and a veteran of dozens of U.S. unmanned space missions. In a time when the United States had allowed its agreement on cooperation in space science with the Soviet Union to lapse out of displeasure with Soviet foreign policy, Simpson designed a novel analyzer of cometary dust. He discussed his device at an ESA meeting in the Netherlands, hoping to have it included in Giotto; less than a month later, Simpson was informed by Roald Sagdeev, director of the Institute for Cosmic Research of the Soviet Academy of Sciences, that the instrument had been accepted for the Vega mission. Simpson had never even proposed his instrument for the Vega spacecraft. But after obtaining permission from appropriate American authorities, Simpson put together a device that employed technology at least a decade old; he did not wish to violate U.S. strictures on “technology transfer.” When the time came to integrate pay-loads, Soviet engineers asked Simpson why his instrument did not have a computer microprocessor as all of theirs did. Simpson smiled.
International Cometary Explorer
In 1978, a satellite called International Sun-Earth Explorer 3 [ISEE 3], designed to study how the solar wind interacts with the magnetic field of the Earth, was placed in an orbit between the Earth and the Sun. After its prime mission was over, ISEE 3 was redeployed to a quite different task. Robert Farquhar of NASA’s Goddard Spaceflight Center designed an ingenious orbital maneuver by which the spacecraft would encounter an extremely interesting object called Giacobini-Zinner, an active short-period comet which some astronomers suggest is shaped like a rapidly rotating pancake—with its equatorial radius eight times its polar radius. The comet is also the source of the fluffy particles that make up the Draconid or Giacobinid meteor stream. It would be a joy to see close-up pictures of Giacobini-Zinner, but that was not in the cards. ISEE 3 did not have a camera.
To get to Giacobini-Zinner, the spacecraft’s thrusters altered its orbit to take it on two leisurely transsections of that part of the Earth’s magnetic field that points, like a comet’s tail, away from the Sun; and then on five consecutive close approaches to the Moon, the last of which, in late 1983, took it to 120 kilometers from the lunar surface. A minor malfunction in the spacecraft’s small rocket engine could have crashed it into the Moon. But instead, the cumulative effect of successive passes by the Moon’s gravity flung the spacecraft (like a comet from the Oort Cloud passing close by Jupiter) into a very different trajectory which successfully passed through the tail of Comet Giacobini-Zinner on September 11, 1985.
This was almost six months before the spacecraft flotilla reached the vicinity of Halley’s Comet. So you might think that the exercise was mainly political—like the unsuccessful attempt, in July 1969, by the unmanned Soviet Luna 15 spacecraft to return a sample from the Moon a few hours before the American manned Apollo 11 could do so. But ISEE 3—now renamed International Cometary Explorer, with the agreeable acronym ICE—obtained much more useful information than if it had merely continued to languish in the vicinity of the Earth. It discovered the interplanetary magnetic field draped over the cometary nucleus, and unexpected energetic particle and field events in the tail. ICE is a testament to the remarkable abilities to tool around the inner solar system that are now at hand—if you know how to use a small rocket motor, the masses of the Moon and planets, and Newton’s laws of motion.
All spacefaring organizations involved in the Halley encounter pledged to make the results available to scientists worldwide. The missions have been jointly organized—not only so that the various scientific instruments complement each other and the data are exchanged rapidly, but also so that the results of one mission will help assure the success of the others.
The most modest were the Japanese missions. Sakigake was considered mainly a test of the machinery for getting there, and did not pass closer than a million kilometers to Comet Halley. Even so, it measured the distant solar wind for comparison with measurements made closer in of the interaction of the comet with the solar wind. Suisei, on the other hand, approached to within about 200,000 kilometers, and included an ultraviolet television camera to photograph the hydrogen coma for a month or more before closest approach. Since the hydrogen is produced by dissociated cometary water ice, we have a record of the outgassing history of the main cometary volatile.
The orbit of Halley’s Comet is inclined 162 degrees to the plane of the zodiac or ecliptic, the plane that includes the Earth’s orbit. Thus because of limitations in the propulsion systems of modern interplanetary spacecraft, the comet can be intercepted only when its orbit and the plane of the Earth’s orbit intersect. For this reason, the trajectories of the Vega spacecraft took them by Comet Halley on March 6 and March 9, 1986. The closest approach of Vega 1 was designed to be about 10,000 kilometers, and perhaps a little less for Vega 2. Giotto was designed to encounter the nucleus of Halley’s Comet on March 13, 1986, and pass a few hundred kilometers above the sunlit side. Because of the enormous relative velocity, the encounter lasted only a few hours; and some of the key measurements only a few minutes.
But to pass a few hundred kilometers from the comet, you have to know where this rapidly moving iceberg is to within a few hundred kilometers, and nobody yet knew its orbit to that accuracy—you cannot see the nucleus, enveloped in all that coma, well enough from Earth to measure its precise position. Accordingly, a cooperative navigation scheme between the Soviet Union, the United States, and the European Space Agency was organized, called Pathfinder. Radio telescopes of the U.S. National Aeronautics and Space Administration listened, with Soviet cooperation, to the radio transmission from the Vega vehicles, against the background of much more distant quasars lying far beyond our Milky Way Galaxy.* The position and motion of the spacecraft relative to the Earth can thus be determined with high accuracy. From the first Vega pictures, Soviet space scientists knew to high accuracy the direction of the nucleus of Halley’s Comet. But if we know where Vega is relative to the Earth, and where Halley’s Comet is relative to Vega, we know where Halley’s Comet is relative to the Earth. This information was extracted from the observations very rapidly so Giotto was able to effect a most delicate maneuver: To optimize the fine detail that the Giotto cameras would see in the cometary nucleus, the spacecraft had to pass very close to the comet. But if it passed on the night side, there would be almost nothing to see. Without the cooperation by American radiotelescopes and Soviet spacecraft, it would have been impossible for the Giotto scientists
to plan their close fly-by, and the miss distance might have been 1,000 kilometers or more.
At a 500-kilometer range, the smallest detail visible was about 30 meters; if pictures are taken at closer ranges, the resolution will be better.
It was our pleasure to witness the spectacular encounters of Vega 1 and 2 and of Giotto from, respectively, the Moscow control center of the Institute for Cosmic Research and from the European Space Agency’s center at Darmstadt, Germany. While not all the instruments worked—both spacecraft were unexpectedly pummeled by debris orbiting along with the cometary nucleus and were temporarily put out of commission (Giotto attaining only a few hundred kilometers of the nucleus of Comet Halley)—many important discoveries were made, a few of which have already been alluded to. Perhaps the most significant is that Fred L. Whipple’s model of comets as dirty snowballs was essentially confirmed. This does not exclude the possibility that some comets consist of hundreds of small objects in comparable orbit, but at least some, and probably most, comets have a solid, icy, and organic nucleus.
Comet Halley in its 1986 apparition, taken with du Pont telescope at Los Campanos Observatory by Alan Dressler. Courtesy Alan Dressier.
The most stunning image was a composite of the nucleus taken by an ESA team headed by Dr. Uwe Keller, with the Halley Multicolour Camera on Giotto. It shows the nucleus as an irregular, lumpy, peanut- or potato-shaped object—very roughly 15 by 10 by 10 kilometers across, just in the size range predicted for the comet that ended the Cretaceous Period and began the age of the mammals. There are small hills gleaming in the morning sunlight, and perhaps a crater. (Plate XXIV)