Galileo is plowing through the most intense interplanetary dust storm ever measured as it closes in on Jupiter after a six-year journey, scientists reported at the end of August. This is the latest and greatest of several large dust storms encountered since December 1994, when the spacecraft was still almost 110 million miles from Jupiter. The current storm has lasted more than three weeks. The spacecraft, launched in October 1989, is now about 39 million miles from Jupiter, which it will begin to orbit on December 7.
During the current dust storm, Galileo has counted up to 20,000 dust particles per day, compared to the normal interplanetary rate of about one particle every three days, said Dr. Eberhard Grun, principal investigator on the spacecraft's dust detector experiment.
The particles are apparently emanating from somewhere in the jovian system, perhaps from volcanos on Io or from Jupiter's faint two-ring system. The dust particles, probably no larger than those found in cigarette smoke, may also be leftover material from Comet Shoemaker-Levy 9, which crashed into Jupiter last year.
Scientists believe the particles are electrically charged and accelerated by Jupiter's powerful magnetic field. They have calculated that the dust is speeding through interplanetary space at 90,000 to 450,000 miles per hour, depending on particle size. Even at these speeds, the tiny particles pose no danger to the Galileo spacecraft, scientists say.
Galileo's dust detector is about the size of a large kitchen colander. It counts particle impacts and observes their direction and energy allowing estimates of particle size and speed. Grun, of the Max Planck Institute for Nuclear Physics, also has dust detectors aboard Ulysses, which flew by Jupiter in 1992 on its way to study the Sun. His team first discovered dust emanating from Jupiter in 1992 using the Ulysses instrument. The Galileo instrument first observed dust coming from Jupiter in June 1994. Although both Ulysses and Galileo show that the storms seem to come from Jupiter, the intensity and timing of the recent storms differ from those detected by Ulysses.
With the onset of the current storm, Galileo flight engineers commanded the spacecraft to collect and transmit dust data as often as three times a day instead of only twice a week, according to Dr. Carol Polanskey, team chief for the dust instrument subsystem at the Jet Propulsion Laboratory. "This puts us in an excellent position to view the dust phenomena as Galileo moves toward Jupiter," she said. "We're looking forward to determining the source of the dust storms once we get into the Jovian system."
Its flight across the remaining 51 million miles to Jupiter will end abruptly on December 7 when it slams into the gas giant's atmosphere. After hitting the top of Jupiter's atmosphere at the highest impact speed (106,000 mph) ever experienced by a man-made object, the rugged probe will unfurl its main parachute and float downward. Seven onboard instruments will make the first ever direct measurements of Jupiter's chemical make-up, winds, clouds, and lightning. The probe will radio its data to the Galileo spacecraft for up to 75 minutes.
The probe mission is likely to end when the main Galileo spacecraft passes beyond radio contact with the probe as the spacecraft enters Jupiter orbit. The ultimate fate of the probe may be determined by its battery lifetime, or it may first succumb to the immense pressure of Jupiter's atmosphere and be crushed. Galileo, meanwhile, will begin two years of close-up studies of Jupiter, its moons, rings, and powerful magnetic field.
Periodic oscillations originating from deep within the Sun's interior have been detected for the first time in interplanetary space by particle detectors onboard the Ulysses spacecraft. The discovery was reported in the July 13 issue of Nature by Drs. Louis J. Lanzerotti, Carol G. Maclennan, and David J. Thomson of Bell Laboratories, Murray Hill, New Jersey.
In addition to finding that these signals affect energetic particles far from the Sun, the scientists reported that their experiment was able to identify oscillations, or wave motions, that have long been sought, but never detected, by Earth-based observers. "This is a breakthrough for studies of the Sun, the interplanetary medium, and the detrimental effects of energetic particles on terrestrial systems," said Lanzerotti, principal investigator of the Ulysses particle detector experiment.
The Sun vibrates to produce a number of discrete waves simultaneously, which travel through the Sun and arrive at its surface, much like seismic waves caused by earthquakes, which propagate through the Earth to the planet's surface. At the surface of the Sun, these waves appear as weak inward and outward motions, said Dr. Edward J. Smith, Ulysses Project Scientist for the U.S. portion of the mission, Jet Propulsion Laboratory.
Much of scientists' knowledge of the Earth's interior comes from studying these waves (seismology). The discovery of solar oscillations about 20 years ago provided a way of probing the Sun's interior (helioseismology). Scientists believe that such wave motions must occur on other stars and are actively searching for stellar oscillations.
The Ulysses scientists analyzed energetic particle measurements in search of narrow bandwidth waves. Using a sophisticated method of analysis, they found a large number corresponding with observations using other techniques. They cluster around wave periods of about five minutes, Thomson said. Each five-minute period represents the time it takes for the Sun's motion to change from moving outward to moving inward and then back outward again. These waves are equivalent to normal sound waves traveling through the Earth's atmosphere, but the periods are too long for the human ear to hear.
Lanzerotti and his co-investigators said their signals are probably the result of the effects of the solar motions on the magnetic fields that originate in the Sun's interior and are stretched outward into space by the solar wind. "As the magnetic lines of force oscillate in response to the passage of the waves, their motion is communicated to the energetic particles traveling along them," Lanzerotti said. Confirming their particle results, the investigators also found corresponding waves in the magnetic field data.
An even more surprising result was discovery of oscillations with longer periods of about three hours. Waves with these periods have been sought since theorists first predicted them. They can be used to probe even more deeply into the solar interior. "Observations of these oscillations in the energetic particle data are truly astounding," Smith said.
In late October, Ulysses will complete its pass over the northern pole and begin to journey back out to the orbit of Jupiter, not returning to the Sun until September 2000.
New Hubble Telescope observations may have helped solve a two-decade-old cosmic mystery by showing that mysterious clouds of hydrogen in space may actually be vast halos of gas surrounding galaxies. "This conclusion runs contrary to the longstanding belief that these clouds occur in intergalactic space," says Ken Lanzetta of the State University of New York at Stony Brook. The existence of such vast halos, which extend 20 times farther than the diameter of a galaxy, might provide new insights into the evolution of galaxies and the nature of dark matter_an apparently invisible form of matter that surrounds galaxies.
The possibility of galaxy halos was first proposed in 1969 by John Bahcall and Lyman Spitzer of the Institute for Advanced Study at Princeton. Previous observations with groundbased telescopes, the International Ultraviolet Explorer satellite, and Hubble have suggested that these clouds might be galaxy halos. However, the latest results are the most definitive finding yet, says Lanzetta, because they come from a large sample of 46 galaxies.
For the past two decades, observations with groundbased telescopes have shown that the light from distant quasars is affected by intervening gas clouds. These clouds are invisible, but betray their presence by absorbing certain frequencies of a quasar's light. Across a spectrum, the missing wavelengths appear as a complex "thicket" of absorption features. Groundbased observations also showed that the number of these clouds rapidly rises out to greater distances. One possible explanation was that these were primordial clumps of gas that dissipated over time.
However, in 1991, independent observations made with Hubble's Faint Object Spectrograph and the Goddard High Resolution Spectrograph detected more than a dozen hydrogen clouds within less than a billion light-years of our galaxy. These clouds could not be detected previously because they are only visible in the ultraviolet, which cannot be seen through Earth's atmosphere. This gave astronomers a powerful opportunity to further test the halo theory by imaging nearby galaxies and attempting to match them with nearby clouds.
Lanzetta, David Bowen of the Space Telescope Science Institute, David Tyler of the University of California at San Diego, and John Webb of the University of New South Wales, Australia, attempted to match galaxies and clouds by first collecting Hubble archival data on six quasars. Next, using telescopes at the National Optical Astronomy Observatory, the Anglo Australian Observatory, the Lick Observatory, and the Isaac Newton Telescope, they identified galaxies near the clouds and measured distances. In the majority of cases they found galaxies within about 500,000 light-years of the clouds.
"These results are a surprise. We have never seen these halos in the local universe," said Bowen. The results explain why so many clouds are seen at greater distances: The light from distant quasars was more likely to pass through a galaxy's halo because the halo is so large. These results appeared in the April 1, 1995, issue of the Astrophysical Journal. The researchers plan to extend their research to a larger sample of galaxy/cloud pairs.
The spacecraft will continue speeding out into interstellar space toward the center of the Milky Way, taking an engraved gold plaque bearing a message about Earth to other civilizations. It will pass near the star Lambda Aquila in almost four million years.
"Pioneer 11 has had a spectacular life," said project manager Fred Wirth of Ames Research Center. "It was the second spacecraft to visit Jupiter, roaring through the heart of the planet's huge radiation belts at 107,373 mph, by far the fastest speed ever traveled by a human-made object."
Launched in April 1973, Pioneer 11 reached Jupiter in December 1974, following Pioneer 10, which flew by the planet a year earlier. It came in under the giant planet's south pole and skimmed within 26,600 miles of its cloud tops. The flyby was so close_the closest pass yet made_that the spacecraft received heavy bombardment from Jupiter's radiation belts, which are 40,000 times more intense than Earth's. Only Pioneer's speedy passage saved its electronics from severe damage.
The gravity assist from the close flyby threw the craft 100 million miles above the plane of the ecliptic and 1.5 billion miles across the inner solar system to Saturn. Pioneer flew high enough above the Sun's equatorial plane (17 degrees) to characterize the solar magnetic field for the first time.
In 1979, Pioneer 11 made the first flyby of Saturn, passing within 13,000 miles and discovering two new moonlets, a new ring, and charting the magnetosphere, magnetic field, and general structure of Saturn's interior.
In 1990 Pioneer 11 became the fourth spacecraft to journey beyond the orbit of Pluto, heading in the same direction that the Sun moves through interstellar space. Pioneer 10 is traveling in the opposite direction, and with the two Voyagers, will continue to return information about the Sun's influence deep in space.
Although running out of power, most systems onboard the remarkably durable Pioneer 11 spacecraft are still healthy. For many years the spacecraft has been sending back limited data on the solar wind, magnetic field, and cosmic rays, but it can no longer be maneuvered to point its antenna accurately at the Earth.
"Some time in late 1996, its transmitter will fall silent altogether, and Pioneer 11 will travel as a ghost ship in our galaxy," Wirth said. "We plan to listen to it once or twice a month to learn about the fade- down process. This will help us understand the future fate of its sister craft, Pioneer 10," he said. Pioneer 10 continues to return scientific data and may have enough power to last until 1999. At almost six billion miles, Pioneer 10 is the most distant object built by humans.