Category Dreams of Other Worlds

The Cassini orbiter has twelve scientific instruments and the Huy­gens probe had six. Tools on this “Swiss army knife” fall into three categories: remote sensing using visible light, remote sensing using microwaves, and studies of the environment near the spacecraft (figure 5.4). Despite using 13,000 electronic components and 10 miles of cable, each of the instruments has worked as planned. Many scientific studies use data from more than one instrument. The optical remote sensing instruments are all mounted on a pal­ette so that they face the same way. The main camera on Cassini is the Imaging Science Subsystem and it has been most people’s en­tree into the exotic world of Saturn and its moons. Carolyn Porco, the principal investigator of the instrument team, maintains a web­site loaded with amazing images and evocative descriptions.27 As noted earlier, she uses a Captain’s Log motif with affectionate al­lusions to Star Trek, and its clear she’s having the time of her life as Cassini images new worlds with unprecedented levels of detail. She’s compared a leadership role in a big space mission with child rearing—a journey of thrills and heartache, excitement and oc­casional disappointments, lasting twenty years or more. Even the most jaded cynic about big science would be entranced by the best pictures from the main imager.

Two mapping spectrometers look at the properties of Saturn, its moons, and its rings at optical and infrared wavelengths. This is the key to determining their composition and temperature. An ul­traviolet spectrometer does the same thing at shorter wavelengths than the eye can see, as another guide to chemical composition. Cassini can’t capture samples of the atmospheres or rings, so re­mote sensing with spectrometers is the best guide to chemistry. The

CASSINI SPACECRAFT

Low-Gain
Antenna (1 of 2)

445 N Engine (1 of 2) –

Figure 5.4. Cassini is a large and complex spacecraft about the size of a bus, with twelve scientific instruments, plus six on the Huygens lander. The instruments include cameras and spectrometers and others to measure magnetic fields, high energy particles, radio waves, microwaves. The power source is radioactive plutonium (NASA/Jet Propulsion Laboratory).

spacecraft’s microwave remote sensing instruments function a bit differently. Optical remote sensing just uses available light or radi­ation reflected from the planet, moon, or ring particle. Cassini has to generate its own radio waves and microwaves, send them to the target with a 4-meter high gain antenna at one end of the space­craft, then “listen” for a weak echo signal. The radar instrument can penetrate Titan’s atmosphere and so make topographic and compositional maps. It can also see deeper into the atmosphere of Saturn than any other instrument. The radio instrument looks for fine structure. It also makes Doppler measurements that allow the masses of Jupiter’s moons to be precisely calculated.28

A set of onboard instruments measure energetic particles, ions, and magnetic field strengths at the position of the spacecraft at any time. Here the complex orbital gymnastics are essential; to make a map of these properties the onboard instruments need to sample data from as many locations within the system as possible.

Several of the six instruments are devoted to understanding Sat­urn’s magnetic field via the charged particles that it often accel­erates. To avoid overspecialization, NASA selected six teams of interdisciplinary scientists. Their job is to use Cassini’s instruments in concert to maximize the learning and perhaps answer questions that hadn’t been anticipated. All this instrumentation is power – hungry and like other probes to the outer Solar System, there isn’t enough sunlight for solar panels, so Cassini uses 72 pounds of Plutonium.

Astrometry may be the “Cinderella” of modern astronomy, but astronomers in all fields are continually reminded that everything starts with mapping brightness and position. Hipparcos leveraged historical measurements by providing the most accurate reference frame.46 With the invention of the photographic plate in the mid­nineteenth century, comparing photographs of star positions from different eras could in principle reveal star motions, but it’s usually unclear which set of plates has the largest errors. With Hipparcos as the rock-steady “gold standard,” astronomers gleaned new in­sights from century-old data.

The hundred or more observations that the satellite made of each star allowed it to detect variability. Over 12,000 variable stars were found in the database, about 10 percent of all the stars studied, two-thirds of which were previously unknown.47 The observation of the variables was coordinated with a network of amateur astronomers, who filled in with data when a star was tem­porarily out of the satellite’s viewing zone. Hipparcos was also able to resolve or distinguish over 24,000 double and multiple star systems.48 Binary stars were actually a headache for the science team, because they mimicked problems in the photometry and a faint companion could throw off the position of the brighter star in a pair if the two images were not well separated.

A sampling of projects will give a sense of the dizzying range of investigations enabled by the Hipparcos data. Galactic archaeol­ogy is a good example. Hipparcos data showed that some of the stars in the neighborhood of the Sun are part of a disk that’s ten times thicker than the disk where most of the Milky Way’s star formation takes place.49 Differences in the heavy element abun­dance in the two components are consistent with a model where the Milky Way was assembled from smaller galaxies over billions of years.50 Ten percent of the stars in the spherical halo as well as some in the “thick” disk seem to come from a single “invader” gal­axy that was disrupted soon after the Milky Way formed. Also, the fine view of stellar motions provided by Hipparcos allows astrono­mers to turn back the clock and trace the Sun’s passage around the galaxy and in and out of the galactic disk over the past 500 million years. During that time the Sun has passed through spiral arms four times, each corresponding to an extended cold spell in the climate history of the Earth. It is speculated that exposure to high cosmic ray flux in the spiral arms leads to more cloud cover and longer Ice Ages.51

Hipparcos data were used to show that the dim companions in some stellar systems are brown dwarfs. These elusive objects are gas balls less than 8 percent of the mass of the Sun; too cool to shine by nuclear fusion, they emit a feeble infrared glow and slowly contract as they leak their energy into space. In 1991, a star being observed by Hipparcos dimmed slightly on five occasions due to the shadow of a giant planet passing in front of it. This was four years before Mayor and Quleoz stunned the world with their discovery of the first planet beyond the Solar System. But nobody was looking for such signals in the Hipparcos data, so the eclipses remained undetected until 1999.52 Since then additional exoplan­ets have been dug out of the database.

Hipparcos also produced a beautiful confirmation of general relativity (we earlier described a test of relativity by Cassini). Ein­stein’s theory states that mass bends light, and it was first con­firmed in 1919 by observations of the deflection of starlight as it grazed the limb of the Sun while observed during an eclipse. Gen­eral relativistic bending is 1.7 arc seconds at the limb of the Sun, and it declines with the projected distance from the Sun’s gravity but is still a detectable 0.004 arc seconds at right angles to the sight line toward the Sun.53 This subtle measurement shows that the paradigm of curved space applies everywhere. The curvature is so slight that it doesn’t negate the use of Euclidean triangles to measure distances. To test general relativity, while firming up mea­surements of the size and expansion rate of the universe, is quite an achievement for a small and often-overlooked space mission.