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Planetary News: Cassini-Huygens (2004)Cassini-Huygens to Make First Rendezvous Within Saturn System: Mysterious Outer Moon PhoebeBy Emily Lakdawalla
Cassini-Huygens closes in on its first Saturn target: the outer moon Phoebe. On Thursday, June 11, 2004, Cassini-Huygens will pass within 2,000 kilometers (1,200 miles) of the surface of Phoebe at 10:33 a.m., Pacific Time (19:33 p.m., Universal Time). The spacecraft will not yet be in Saturn's orbit, but will speed by Phoebe at about 6 kilometers per second (13,500 miles per hour) on its way in to its rendezvous with the giant ringed planet, which will take place nearly three weeks later. Phoebe is so far from Saturn -- 13 million kilometers (almost 9 million miles) -- that after this flyby, Cassini will never again travel far enough from the planet to meet it. But strange, dark, distant Phoebe may hold the key to several important questions about the Saturn system and beyond. The most important question is: where did Phoebe come from? Scientists suspect that Phoebe originally formed within the Kuiper Belt. If true, then Cassini will give us our first look at these primeval remnants of the birth of the solar system. Phoebe quite likely did not form within the Saturn system: its distant, highly inclined, retrograde orbit and unusually dark surface suggest strongly that Phoebe formed elsewhere, was disturbed from its original orbit, and then captured into orbit around Saturn. "If it is a captured object, it was captured from somewhere," says Robert Brown, the Principal Investigator on Cassini's Visible and Infrared Mapping Spectrometer (VIMS). And that somewhere can only have been within the asteroid belt or the Kuiper Belt. Brown thinks that the measurements performed by his instrument will be able to determine the origin of Phoebe -- if he is lucky. The dark materials on the surface of Phoebe are the key. "If [Phoebe is] nice to us and tells us something about its composition we'll be able to tell the differences between ice and rocks and the composition of dark materials -- if they are organics, and what the size of carbon molecule chains are." Although asteroids contain some organic materials, complex molecules containing carbon, the most organic-rich object yet observed outside Earth was Halley's Comet, which originated in the outer solar system. The VIMS instrument can even determine the relative abundances of different isotopes of some chemical elements, like carbon, oxygen, and hydrogen; these isotope abundances could give clues to the location of Phoebe within the solar system when it formed, and how geologically active it has been since it formed. Not all scientists are as sanguine as Brown about the possibility of the Cassini encounter settling the question of Phoebe's origin. "We probably won't be able to tell the difference [between an asteroid or Kuiper Belt origin] because we don't actually know what Kuiper Belt objects look like up close," says researcher Jonathan Lunine, who will co-chair a scientific workshop on the results of the Phoebe encounter on June 21. "The primary difference might be that Kuiper Belt objects will still have water ice." There has already been some evidence for water ice on Phoebe, but "that evidence is weak." VIMS should be able to confirm the presence of water ice, but "whether water ice indicates that Phoebe originated in the Kuiper Belt, or whether it's a C-type asteroid with water ice on it, will probably be argued about for a while," Lunine noted. It would be a tremendous discovery if the Cassini observations do reveal Phoebe to be a captured Kuiper Belt object. No spacecraft has yet visited the Kuiper Belt, nor will there be such a visit for at least another decade; the New Horizons mission to Pluto, Charon, and the Kuiper Belt cannot arrive there before 2015. So studying Phoebe could give us our first and only glimpse of a Kuiper Belt object for a long time to come. Whatever the origin of Phoebe, the small, distant moon has a noticeable effect on other places within the Saturn system. The closest large moons to Phoebe, Iapetus and Hyperion, both appear to be contaminated with some dark material on their surface. One hypothesis is that this dark material came from Phoebe -- that dust spiraling in from Phoebe's orbit was swept up by Saturn's outer moons. This process has caused Iapetus to be simultaneously one of the darkest and one of the brightest moons in the solar system. Because it is tidally locked with Saturn and (like our Moon) keeps the same side pointed to the planet at all times, one hemisphere of Iapetus always faces forward along its orbit. This forward-facing or "leading" hemisphere is as black as asphalt. If the black smudge on Iapetus is Phoebe's fault, then spectral properties of the surface of Phoebe, and the dust in the space around Phoebe, should match characteristics of the darkened surface of Iapetus. To address these scientific questions, the mission's science planning team has developed a jam-packed observation plan for the Phoebe encounter. According to Amanda Hendrix of Cassini's Satellite Orbiter Science Team, science observations will begin at 04:40 on June 11 and continue for nearly 32 hours, until 12:10 on June 12. (All of these times are according to the spacecraft's clock, which matched Universal Time when it was launched; it now takes about an hour and a half for the spacecraft's signals to reach Earth.) The science campaign at Phoebe will begin with observations by all four optical remote sensing instruments -- including VIMS -- which will examine Phoebe at wavelengths from the ultraviolet to the infrared. All four instruments are fixed to the spacecraft with the same "boresight" -- that is, they look in the same direction -- so when one is targeting Phoebe, the other three may take measurements at the same time. But that also means that when the spacecraft is pointed at one spot for one instrument to take measurements, then all of the other instruments must point at the same spot for the same amount of time. As a result, scientists usually plan the observations with one instrument being "prime," controlling the spacecraft's pointing, with the other three instrument teams' approval. During the Phoebe encounter, the Imaging Science System (the spacecraft's main camera) and the Composite Infrared Spectrometer will be trading priority every 15 or 20 minutes, with the other two instruments "riding along." Five and a half hours before closest approach, the spacecraft will rotate to point the radar antenna at the surface, which means that the optical instruments will no longer be able to see Phoebe. The radar observations are critical because the radio waves should be able to penetrate 2 to 20 centimeters below Phoebe's surface, sampling what's beneath the dark outer coating. The spacecraft will rotate to point the optical instruments at Phoebe again an hour and a half later, and keep observing Phoebe until 21:33 on June 11, two hours after closest approach. During that time, the spacecraft will fly past Phoebe at 20 degrees north latitude, passing over what appears to be an unusually bright spot area on Phoebe's otherwise dark surface. This bright spot could be the site of an impact crater, exposing fresher material from underground, or a region containing an unusual concentration of water ice. At closest approach, the images captured by Cassini's Imaging Science System will have 1270 times better resolution than Voyager 2 achieved during the only other spacecraft encounter with Phoebe, which took place on September 4, 1981. Cassini's best resolution images will be at 15 meters per pixel. With images at that resolution, you can spot buildings and roads on the surface of the Earth; who knows what we'll see on Phoebe? At the same time, VIMS will capture the first ever spectra of the surface of Phoebe that measure an area smaller than Phoebe itself, producing a spectral map of the moon at a resolution as high as 1 kilometer per pixel. Near closest approach, the spacecraft must rotate again -- while keeping the optical instruments pointed at the surface -- in order to prevent the radiator element for the infrared instrument from pointing at the Sun or Saturn. (Infrared instruments usually must be kept quite cold, because heat radiation occurs at infrared wavelengths, so a warm instrument will have "fuzz" in the data from the spacecraft's own heat.) Two hours after closest approach, the spacecraft will turn again and perform four hours and fifteen minutes' worth of radar observations. A final turn will bring the optical instruments back to bear. All the while, the in-situ instruments will be gathering data on the magnetic fields, ions, and dust in Phoebe's environment. At closest approach, one instrument -- the Cosmic Dust Analyzer -- will even be able to sample Phoebian dust particles floating in the space above the moon. As exciting as the Phoebe encounter is, it's just a teaser for the rest of the mission. Once the encounter is over, Cassini-Huygens will be on final approach to Saturn. Saturn Orbit Insertion will take place on June 30 Pacific Time (July 1 UTC), when the spacecraft will perform a daring set of maneuvers. Approaching Saturn from below, Cassini-Huygens will pass through the gap between the faint F and G rings, and then start a 96-minute burn of the main engines to slow down the spacecraft enough to be inserted into Saturn orbit. If this burn does not take place successfully, Cassini-Huygens will fly by Saturn, never to return. Orbit insertion is a critical moment in the mission. Hundreds of scientists from 14 European countries and 32 of the United States have been working on 18 instruments and 9 interdisciplinary science investigations for up to 15 years to bring this mission together. How do they feel about their final plunge into the Saturn system? "Apprehensive," Lunine says. "A lot of things have to work right. Saturn Orbit Insertion will be a definite challenge, but as we get beyond that I'm ready to see what Titan is like." VIMS scientist Robert Brown is "scared and excited. We've spent a lot of time and energy putting the infrastructure together here so that we can get the observations planned and the data down and analyzed. When it gets this close, you always worry that there are things you might have forgotten. After 14 years of meetings and presentations and calibrations, it'll be nice to get a chance to do a little science." And as for the first act of Cassini's Saturn campaign being at Phoebe, "I'm quite excited. Since my interest is in icy satellites, this is a dream that has been a long time in coming." Scientists often don't agree on the interpretation of the data returned by spacecraft, but there does seem to be consensus about one thing: Cassini-Huygens will completely change our view of the Saturn system. "The objective of Cassini-Huygens is simple," says Charles Elachi, director of the Jet Propulsion Laboratory and Principal Investigator on Cassini's radar experiment," to allow us to completely rewrite the story of the Lord of the Rings." |
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