This Week on Galileo: The Final Day
20 Sep 2003
(Source: Jet Propulsion Laboratory)
Well, after twelve years of pre-launch development and planning, six years of interplanetary cruise, and nearly eight years in orbit, our exciting, quarter-century odyssey has finally come down to this: the final 19 hours of existence for the Galileo spacecraft. It began life in October 1977 as the Jupiter Orbiter Probe mission, was launched in October 1989, and arrived at Jupiter in December 1995. After circling the solar system's largest planet 35 times, it is about to plunge into the atmosphere of Jupiter, becoming only the second man-made object to do so, following the smaller Galileo atmospheric probe that accompanied the Orbiter to Jupiter. From launch to impact, the stalwart spacecraft has travelled 4,631,778,000 kilometers (2,878,053,500 miles) on 925 kilograms of propellant (246 gallons), not counting the fuel for the shuttle. In all that time, and across all those miles, Galileo has returned over 30 gigabytes of data, including 14,000 pictures.
One chapter of the volumes of scientific data produced by Galileo over the years includes the discovery of likely sub-surface water oceans on the icy satellite Europa. This has fueled speculation about the possibility of life existing in that environment, and is prompting plans for future spacecraft to return to Europa to search for life. Since the Galileo spacecraft was never designed to specifically search for life, it was never subjected to the rigorous sterilization procedures such as those mandated for craft going to Mars. To prevent any possible future biological contamination of Europa, the decision was made to provide a final resting place for the Galileo Orbiter that guarantees the spacecraft will never collide with any of the Jovian moons. That resting place is Jupiter itself.
The final day for Galileo begins on Saturday evening at 5:52 p.m. PDT [see Note 1], when the spacecraft is just over 18 Jupiter radii (1.32 million kilometers or 822,000 miles) from the center of the planet, and closing fast. At that time, the instruments that measure the magnetic and electric fields and the particle environment stop collecting data for a while. These instruments are the Dust Detector Subsystem (DDS), the Energetic Particle Detector (EPD), the Heavy Ion Counter (HIC), the Magnetometer (MAG), and the Plasma and Plasma Wave Subsystems (PLS and PWS). They have been collecting data continuously for the past six months, storing these data in a computer memory buffer for later transmission to Earth. By stopping this collection for 7 hours, this buffer is allowed to drain, and subsequent data collected can be transmitted to Earth almost immediately.
At 10:52 p.m. the spacecraft attitude control system is told to base its orientation calculations on the observations of a single star. Normally, three stars are used, but in the intense radiation environment near Jupiter, noise in the star sensor circuits overwhelms the signals from fainter stars, and a single, bright star is selected which can rise above the noise and still be detected. Today that star is Vega (Alpha Lyrae), the fourth brightest star in the sky.
At 12:52 a.m. Sunday, the 70-meter-diameter (230 feet) Deep Space Network tracking station antenna near Madrid, Spain, is listening to the spacecraft. The science instruments are configured properly, and begin again to send their data in real-time to Earth. Galileo has closed the distance to 13.5 Jupiter radii (965,000 kilometers or 600,000 miles).
At 5:55 a.m., the tracking antenna at Goldstone in the Southern California desert joins the Madrid station, and for the next three hours both stations are collecting the faint signals sent from a half a billion miles out in the solar system.
At 5:07 a.m., the distance has closed to 10 Jupiter radii (715,000 kilometers or 444,000 miles) and MAG changes the sensitivity of its measurements in anticipation of the stronger magnetic fields to come. At 6:24 a.m., all instruments except MAG stop collecting data for just over an hour. During this time the data collection rate on the spacecraft is greater than the rate the ground stations can reliably receive. If the collection continued, the data would accumulate in the storage buffer to such an extent that the buffer would not have a chance to empty before the spacecraft is lost from view. This brief pause allows the most valuable data collected nearest to Jupiter to be sent without buffering.
At 7:22 a.m., the radio signal sent from the spacecraft is changed to provide more power to the underlying carrier signal. This will help the ground stations keep track of the signal as Jupiter's increasing gravitational pull speeds the spacecraft up and alters the apparent transmission frequency, due to the familiar Doppler effect.
At 9:05 a.m., Galileo crosses the volcanic satellite Io's orbit at a distance of 6 Jupiter radii (422,000 kilometers or 262,000 miles). The spacecraft has spent most of its 8-year travels around Jupiter outside of this distance, to keep the received radiation dose down. It has ventured significantly inside this distance only twice. Once, in December 1995, as Galileo first entered Jupiter orbit, when we reached 4 Jupiter radii (3 radii over the clouds), and again in November 2002, when a flyby of the small inner moon Amalthea took the craft down to 2 Jupiter radii (1 Jupiter radius, 71,500 kilometers or 44,400 miles over the cloud tops). This time, though, it's a one-way trip. The distance will only get shorter.
By 9:42 a.m., the intensity of the radiation noise has reached a point where even a bright star like Vega can no longer reliably be seen by the attitude control star scanner. The software is now told to expect to see no more stars, ever.
At 11:31 a.m., Galileo is two Jupiter radii above the clouds (143,000 kilometers or 89,000 miles) and the Magnetometer instrument has taken its final data for the mission. At this distance from Jupiter, the magnetic field is so strong that the instrument, even in its most robust configuration, would produce a signal that would be completely saturated, and of no further scientific value.
Seventeen minutes later, at 11:48 a.m., the spacecraft passes the orbit of the tiny satellite Amalthea, and at about 12:17 p.m., passes the orbits of the innermost moons, Adrastea and Metis. Galileo is now just 57,500 kilometers (35,700 miles) above the clouds, closing fast, and picking up speed. As the spacecraft passes Amalthea a special measurement will be taken using the star scanner. During our previous flyby of this small body on November 5, 2002, flashes of light were seen by the star scanner that might indicate the presence of rocky debris circling Jupiter in the vicinity of the satellite. Though on this final pass, Galileo will not be near Amalthea, the measurement may help confirm or constrain the extent of this hypothesized orbital debris.
At 12:26 p.m. the Galileo Orbiter joins the Galileo Probe in going closer to Jupiter than any other man-made object, passing the 1973 mark that Pioneer 11 set on its swing through the Jupiter system. At 43,000 kilometers altitude (26,725 miles), the spacecraft is now at a distance that is 1/9th of the span between Earth and its own Moon. This is also the approximate altitude that geosynchronous communications satellites orbit above the Earth's surface. What seems like a vast expanse when viewed in terms of the Earth seems like such a small step away when viewed in terms of Jupiter, which has a diameter that is 11 times that of Earth.
At 12:42 p.m. with 7 minutes and 10 seconds to go, Galileo moves from day to night as it passes into Jupiter's shadow, and, one minute later, passes behind the limb of the giant planet as seen from Earth. Only 9,283 kilometers (5,768 miles) above the clouds, the path of the spacecraft now takes it out of sight of ground controllers, never to be seen again. The last data ever to be received from the Galileo spacecraft has now been sent. The remaining few minutes of the craft will be spent in darkness, and alone...
Finally, at 12:49:36 p.m., Sunday, September 21, 2003, Galileo reaches the end of its nearly 14 year odyssey through space with a final, glorious meeting with the king of the solar system's planetary entourage. This event, though described as an impact, is actually the gradual, but very rapid, immersion in the gas giant's vast atmosphere. The time stated is when the spacecraft reaches that point in the atmosphere where the pressure reaches one bar, the equivalent of Earth's atmospheric pressure at sea level. For reference, this point is 71,492 kilometers (44,423 miles) from the center of the planet, at the point where Galileo enters.
The entry point is approximately 1/4 degree south of Jupiter's equator. If there were observers floating along at the cloud tops, they could see Galileo streaming in from a point about 22 degrees above the local horizon. Streaming in could also be described as screaming in, as the speed of the craft relative to those observers would be 48.26 kilometers per second (nearly 108,000 miles per hour!). That is like travelling from Los Angeles to New York City in 82 seconds. In comparison, the Galileo atmospheric Probe, aerodynamically designed to slow down when entering, and parachute gently through the clouds, first reached the atmosphere at a slightly more modest 47.6 kilometers per second (106,500 miles per hour).
The Galileo Orbiter, with the aerodynamic qualities of a brick, and no parachute, will not endure nearly as controlled and graceful a fate. It will rapidly burn up through friction with the atmosphere, returning to its constituent atoms as it makes its unnoticeable impact on the vast weather systems of Jupiter. In the life of the giant planet, Galileo will look like another speck of cosmic debris. It would not be nearly as observable as the fragments of Comet Shoemaker-Levy 9, which crashed into Jupiter's atmosphere in July 1994 and was observed by Galileo. In the life of the planet Earth, with its inhabitants, Galileo will live on in the memories of those who worked on the project, as well as those of new generations who study the astronomy textbooks, rewritten with the reams of data returned over the years.
Nearly 393 years ago, Galileo Galilei first turned the newly developed telescope on Jupiter, and spied the four satellites that bear his name; lonely, silent sentinels that began to show proof that Earth was not the center of the universe. Mankind has since dreamed, and schemed, and developed robotic messengers to visit and study our ever more complex surroundings. What more fitting end to the messenger that is also Galileo's namesake, than to find final repose in Jupiter, while overhead circle those four sentinels, no longer lonely, no longer silent. For we were there. We listened to what they had to say, and they spoke volumes. They have become part of our family, our circle of familiar friends. As with all good friendships, they have inspired us to do more, go farther, look closer and deeper. To learn.
The future lies before us. Thank you for sharing our journey so far ...
Note 1. Pacific Daylight Time (PDT) is 7 hours behind Greenwich Mean Time (GMT). The time when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at which radio signals reach Earth indicating that an event has occurred is known as Earth Received Time (ERT). On the day of impact, it takes Galileo's radio signals 52 minutes 18 seconds to travel between the spacecraft and Earth. All times quoted above are in Earth Received Time at JPL in Pasadena.