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Come with us aboard the Galileo Orbiter and leap into the future.
It is Arrival Day, December 7, and the view from here is out of this world!
After 2240 days en route, the Galileo Orbiter and the Probe it released 147 days before have arrived in the Jovian system. The preliminary phase, Jupiter Approach, began 2 months ago, when the Orbiter took a global image of Jupiter (with the probe entry site in view), followed by a series of optical navigation frames. As the day approached, the Orbiter became increasingly active, probing for the Jovian magnetosphere's bow shock, scanning the Io plasma torus in extreme ultraviolet, and examining dust in the Jovian neighborhood. The previous 24 hours were especially busy with the ultraviolet spectrometer (UVS), the solid-state imaging (SSI) camera, the near-infrared mapping spectrometer (NIMS), and the photopolarimeter radiometer (PPR) looking at Jupiter, the Galilean satellite Io, and the minor satellites Thebe and Adrastea.
The 24 hours of Arrival Day--from flybys of the satellites Europa and Io, to probe relay, to Jupiter orbit insertion (JOI), and finally to a radio occultation of Jupiter--will be the busiest by far in the whole mission. On Arrival Day, the Orbiter will swing right through the heart of the Jovian system. While the opportunity for unique science will never be greater, the Orbiter must perform two critical activities (probe relay and JOI) in the most hostile radiation it will ever face. But, thanks to a truly tremendous effort by its flight team, Galileo has never been more ready to fulfill its mission (see figure, Arrival Day).
Galileo flies by Europa some 32,500 km above its icy surface. At this distance, Jupiter looks about the size of a cantaloupe held at arm's length and Europa, about the size of a tennis ball. Even though objects in this part of the solar system get only 4 percent of the sunlight we get on Earth, Europa still looks very bright, like a half-lit, gleaming white snowball. The surface is remarkably smooth, with no discernible craters or mountains, just an intricate, frozen webwork of long, brownish cracks or streaks. The Orbiter will visit Europa later on several much closer, targeted passes, but this will be the only one at such high southern latitudes for an unobstructed view of its pole. All the science instruments are busy collecting data, but we won't be sending any back to Earth for some months. On Arrival Day, all the science data are stored in the 900 megabits of the Orbiter's recorder. Three of its four tracks store science data intermittently from five days out through about a half hour beyond Io; then, the remaining track records all the probe data as well as the engineering telemetry and low-rate science to an hour and a half beyond JOI.
Io has been described as looking like a pizza, or a diseased orange. The surface is covered with white, yellow, orange, red, and black features-most likely allotropes and compounds of sulfur spewed out by the constant volcanic eruptions. We swoop within 1000 km of the tortured surface, so close the disk fills our field of view and the SSI shows objects as small as 40 m across. We look at several specially targeted areas like Colchis Regio and the volcanos Volund and Prometheus. Our closest approach is just over the terminator (where the night and day sides meet), so the shadows are long and the relief shows well.
The Io flyby is also critical for JOI: passing in front of Io in its orbital path delivers a gravity-assisted brake on Galileo's trajectory. The reduction in the Orbiter's speed (its Delta V) by 175 m/s accounts for about a fifth of the total change needed for the capture orbit, the first in the orbital tour.
After its closest approach to Io (spacecraft event time 9:46 a.m. PST), Galileo has only a half hour for a backward glance. As we depart, the sunshade over the scan platform begins to obstruct our view. A turn maneuver for a better look is out of the question with the probe relay coming up; still, the NIMS looks for Loki's plume on the limb, and the PPR scans the dark side in several thermal and polarized bands.
The fields and particles instruments--the magnetometer, dust detector, plasma detector, plasma wave sensors, energetic particles detector, and heavy ion counter (see story, Meet the Fields and Particles Science Group)--have been busy collecting data on the ring plane and the inner magnetosphere, especially the Io plasma torus. This plasma-filled donut, fed by constant volcanic eruptions, encloses Io's orbit with a lethal fog of high-energy, charged particles. It is one of the more bizarre objects in the solar system and has been called by Lou Frank (who chairs the Magnetosphere Working Group) the "beating heart of the Jovian magnetosphere." Also, the Orbiter's dust detector may find some debris even though Jupiter's known ring lies some 150,000 km below us. Because data from these instruments are recorded at the relatively low rate of 7.68 kb/s, the instruments can collect data from Europa until more than an hour beyond JOI, almost 14 hours for a good section of the torus and the ring plane.
About 4 hours after Io, the Orbiter reaches perijove, where the banded face of Jupiter bulks as large as a basketball (at arm's length). Ten minutes later (2:04 p.m. in Pasadena), the Probe hits the top of the Jovian atmosphere, about 450 km above the 1-bar level (pressure at sea level on Earth) at the comet-like speed of 47 km/s. DSN stations at both Goldstone and Canberra have been tracking the event.
The Probe enters at a shallow (8.6 degrees) angle in the equatorial zone of rising air and pale-colored clouds. From down inside Jupiter's atmosphere, the Probe is a spectacular fireball, streaking east out of the setting sun and towards the gathering night. In about a minute, deceleration in the thickening hydrogen-helium atmosphere pushes the tiny Probe's weight up nearly 230 times as the heat shield glows white hot at 15,000 K. Two minutes into entry, the Probe has slowed from Mach 75 to Mach 1 (local speed of sound), slow enough to deploy its parachutes--first a tiny drogue, then the main chute--and drop what's left of the heat shield (lighter now by more than half from ablation). Three minutes past entry, the Probe, now swaying beneath its main chute (and weighing 2-1/2 times what it weighed on Earth), establishes radio link with the Orbiter, more than 200,000 km overhead.
Pointing the Orbiter's antenna is of prime importance in establishing the radio link, and the Orbiter depends on its gyros to maintain inertial attitude. The primary backup is the star scanner, which normally uses a three-star set. But the high radiation environment at four Jovian radii may interfere with the usual attitude-control strategy. That's why the Orbiter now carries the new Attitude and Articulation Control System (AACS) flight software to enable it to establish its clock angle (measured around the spin axis) with the first-magnitude star Canopus. For secondary clock-angle backup, the Orbiter can also use sun pulses.
The parachute descends. When the Orbiter first picks up its signal, the probe should be 30 or 40 km above the 1-bar level, in the early Jovian evening, dropping through a frigid brown aerosol haze just above the tops of the white ammonia ice clouds. The 256- b/s signal from its six instruments reports the sunlight and heat input, pressure, temperature, cloud structure, lightning activity, and composition of the atmosphere. The Probe's ultrastable oscillator controls the signal's frequency, so measurements aboard the Orbiter of Doppler shifts in the Probe's signal give information on wind speed and direction as the Probe drifts down through the thin layer of ammonium sulfide clouds, and the thicker billows of blue-white water clouds. After some 30 minutes, it finally drops into the zone below the clouds at the 7- or 8-bar level, maybe 80 km below 1 bar, where the temperature should exceed 310 K (around 100 degrees F).
At this time, the Orbiter's probe relay antenna slews to compensate for the Probe's changing position below it. It will slew three more times at 10-minute intervals to maintain lock. As the Probe sinks even lower, it continues to send its data until the thickening gases block the signal or the battery gives out (after only 75 minutes of life). Here, 163 km down, the Probe's weakest components may already have succumbed to the scorching 465-K heat or the crushing 30-bar pressure. Whatever the Probe's fate, the Orbiter can listen for only 75 minutes; the critical JOI burn approaches, and the Orbiter must give that event its full attention.
The burn, which begins about an hour after the end of probe relay (4:27 p.m. PST), automatically cuts off (at 5:15 p.m. PST) less than an hour before the Orbiter recrosses the orbit of Io. JOI is being tracked by Canberra. The folks back in Pasadena will learn of this a bit after 6 p.m. The Orbiter continues to record fields and particles data for another hour and a half to complete the work on the near-Jovian environment.
This ends Galileo's longest day. If everything has gone according to schedule, the Orbiter is now on its 7-month first orbit in the Jovian system. It carries a load of data from the probe mission, the inner magnetosphere, and encounters with Jupiter and four satellites. It will begin to play the probe data back to Earth just after its first post-JOI orbit trim maneuver on December 9. All probe data will be in hand by mid-March 1996, and the rest of the recorded data will be returned by mid-July. With these last Arrival Day duties completed, Galileo can now look forward to the first 2 years (the prime mission) of a long, interesting, and history-making tour.
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"The Morning After" Press Conference