During the course of over half a century, we have sent spacecraft to encounter every planet known in the Solar System. Having grown up in the 1970s and an enthusiastic observer of planetary missions in the decades afterwards, I always watched in wonder as a new world was seen closeup for the first time often revealing many unexpected discoveries. I often wondered what it would be like to explore Earth for the first time using a highly instrumented planetary spacecraft and what it would find.
Since the launching of the first planetary spacecraft in 1961 (see “Venera 1: The First Venus Mission Attempt”), departing probes have observed the Earth and its environment as they began their journeys to worlds beyond. Some of these probes have even taken images of the Earth and Moon the first being NASA’s Mariner 10 after its launch in November 1973. Four years later, Voyager 1 took one of the more famous images from a departing planetary spacecraft showing the Earth and Moon (see “First Pictures: Voyager 1 Portrait of the Earth & Moon – September 18, 1977”).
Here is a color image showing the Earth and Moon together for the first time as seen from a departing interplanetary spacecraft. Voyager 1 took this color image on September 18, 1977 from a range of 11.66 million kilometers during its outbound trip to Jupiter. (NASA/JPL)
But what about a spacecraft approaching the Earth from interplanetary space for a close flyby before heading out once again as happens with encounters with other worlds? The first opportunity for such a full encounter with the Earth did not happen until December 8, 1990 when NASA’s Jupiter-bound Galileo spacecraft first flew by the Earth.
The Galileo Mission
Approved in 1977 (when it was called JOP for “Jupiter Orbiter Probe”), the objective of NASA’s Galileo mission was to drop a probe for a parachute-assisted descent through the atmosphere of Jupiter followed by entering into orbit to study the giant planet, its environment and its varied collection of planet-sized moons. In addition to its 340-kilogram entry probe, the 2,560-kilogram Galileo spacecraft carried 118 kilograms of instrument including a Solid State Imager (SSI) incorporating a 800×800 pixel CCD array – the first NASA planetary spacecraft to employ a solid state camera instead of the vidicon-based technology flown on earlier missions.
Diagram showing the major components of NASA’s Galileo spacecraft. Click on image to enlarge. (NASA)
Galileo used a unique dual-spin design with the upper part of the spacecraft spinning at a rate of 3 RPM while the lower part remained fixed in inertial space. The spinning portion, in addition to providing stability, allowed the particles and fields instruments to scan their surroundings more effectively. The fixed portion supported most of the optical instruments, including the SSI, mounted on a scan platform to provide stable pointing for this hardware.
An artist conception of Galileo and its Centaur G-Prime upper stage being deployed from the cargo bay of the Space Shuttle. (NASA)
Because of a 1975 NASA policy decision, Galileo was meant from the start to be launched using the Space Shuttle. In order to send the spacecraft on its way to Jupiter, Galileo would use the new Centaur G-Prime stage whose dimensions and mass were optimized for deployment from the Space Shuttle cargo bay compared to the earlier Centaur variants flown. But because of the longer than planned development of the Space Shuttle (and, to a lesser extent, delays in the development of the Centaur G and Galileo spacecraft), the launch of Galileo had been pushed out years beyond its initially planned 1982 launch date. Eventually, Galileo was added to the launch manifest for the STS-61G mission onboard the Space Shuttle Atlantis scheduled for liftoff on May 6, 1986 with Galileo arriving at Jupiter 30 months later. The tragic Challenger accident on January 26, 1986 put any further Shuttle missions on hold adding further delays to Galileo’s launch
This diagram shows the VEEGA (Venus-Earth-Earth Gravity Assist) trajectory Galileo used to reach Jupiter employing the less capable IUS upper stage. Click on image to enlarge. (NASA)
In the wake of the investigation of the Challenger accident and a reevaluation of the risks inherent in Space Shuttle launches, the decision was made on June 19, 1986 to cancel the development of the Shuttle-Centaur due to the risks associated with carrying a huge load of liquid hydrogen and LOX in the cargo bay. With only the less capable, all-solid Interim Upper Stage (IUS) available to launch Galileo, NASA engineers and scientists came up with an innovative VEEGA (Venus-Earth-Earth gravity assist) trajectory that used multiple planetary flybys to increase the velocity of the spacecraft so it could eventually reach Jupiter even with the IUS. While the new, less direct trajectory would double the transit time to Jupiter to five years, it did open the door to making scientific observations of Venus and Earth as well as flyby opportunities for the main belt asteroids, 951 Gaspra and 243 Ida.
A view of Galileo and its IUS upper stage in the cargo bay of the Space Shuttle Atlantis prior to its deployment at 23:15 UT on October 18, 1989. (NASA/JPL/KSC)
Galileo, mounted atop of a two-stage IUS, was finally sent on its way on October 18, 1989 by the Space Shuttle Atlantis during the first day of the STS-34 mission. The IUS successfully sent Galileo on its way to its first target, Venus, which was reached on February 10, 1990. The flyby, at a range of 16,106 kilometers, added 2.2 kilometers per second to the spacecraft’s heliocentric velocity changing Galileo’s solar orbit from 0.67 by 1.00 AU to a larger 0.70 by 1.29 AU orbit. Since Galileo’s umbrella-like high gain antenna was not designed to withstand the punishing thermal environment this close to the Sun, it remained furled behind a sunshade during this stage of the mission. This forced Galileo to delay the transmission of its recorded Venus observations until it was closer to Earth when the data could be transmitted home using a low gain antenna – a task which was completed by mid-November 1990.
This picture of Venus was taken by the Galileo spacecrafts Solid State Imager (SSI) on February 14, 1990 at a range of almost 2.7 million km from the planet. A highpass spatial filter has been applied in order to emphasize the smaller scale cloud features, and the rendition has been colorized to a bluish hue in order to emphasize the subtle contrasts in the cloud markings and to indicate that it was taken through a violet filter. (NASA/JPL)
The First Encounter with Earth
Diagram showing Galileo’s trajectory through the Earth-Moon system during its encounter on December 8, 1990. Click on image to enlarge. (NASA)
With the 952-kilometer flyby of the Earth scheduled for December 8, 1990, the Galileo science team took advantage of the opportunity to observe the Earth and Moon. This was not only to checkout the performance of Galileo’s instruments, but to gather new data for this pair of worlds. With Galileo approaching the Earth from almost the center of its dark side (preventing any imagery or other observations requiring solar illumination), it was in a unique position to observe the Earth’s magnetotail. With nearly continuous contact between Galileo and NASA’s Deep Space Network established, the spacecraft started gathering particles and fields data 30 days before its flyby. At a range of 560,000 kilometers, Galileo’s instruments entered Earth’s magnetotail. As Galileo continued to close in of the Earth, its Plasma Wave experiment detected radio emissions known as “whistlers” created by lighting in the atmosphere as well as from phenomena related to auroras.
This image of the crescent Moon was obtained by the Galileo Solid State Imager (SSI) on December 8, 1990 at about 13:00 UT as the Galileo spacecraft neared the Earth. The image was taken through a green filter and shows the western part of the lunar nearside. The smallest features visible are about 8 kilometers in size. Major features visible include the dark plains of Mare Imbrium in the upper part of the image, the bright crater Copernicus in the central part, and the heavily cratered lunar highlands in the bottom of the image. The landing sides of the Apollo 12, 14 and 15 missions lie within the central part of the image. (NASA/JPL)
During its final day before closest approach, Galileo began making multispectral observations of the Moon including the first images about 11 hours out from Earth. The Ultraviolet Spectrometer also started observations of the Earth’s tenuous geotail looking for emissions of hydrogen and other gases. Radio tracking during this time was used to help refine the mass of the Earth as well.
This diagram shows the detail of Galileo’s closest approach past the Earth on December 8, 1990. Click on image to enlarge. (NASA)
Galileo made its closest approach to Earth at 20:34:34 UT on December 8, 1990 as it passed the evening terminator at an altitude of 960 kilometers – within a half a second of the predicted time and only 8 kilometers off target. As Galileo sped away from its close encounter, it was afforded a nearly completely illuminated view of the Earth allowing it to make continuous global observations. In addition to a series of spectacular color images from the SSI, the spacecraft’s spectrometers made observations of mesospheric water vapor and ozone concentrations to study Earth “ozone hole” and the mechanism responsible for it.
This color image of the Simpson Desert in Australia was obtained by the Galileo spacecraft at about 22:30 UT on December 8, 1990, (about 2 hours after closest approach) at a range of more than 56,000 km. The color composite was made from images taken through the red, green and violet filters. The area shown, about 450 km wide by about 550 km north-to-south, is southeast of Alice Springs. (NASA/JPL)
This multispectral map of Australia and surrounding seas was obtained by the Galileo spacecraft’s Near Infrared Mapping Spectrometer shortly after closest approach from a range of about 80,500 km. The image shows various ocean, land and atmospheric cloud features as they appear in three of the 408 infrared colors or wavelengths sensed by the instrument. The wavelength of 0.873 micron, represented as blue in the photo, shows regions of enhanced liquid water absorption. The 0.984 micron band, represented as red, shows areas of enhanced ground reflection as on the Australian continent. This wavelength is also sensitive to the reflectivity of relatively thick clouds. The 0.939- micron wavelength, shown as green, is a strong water vapor absorbing band and is used to accentuate clouds lying above the strongly absorbing lower atmosphere. (NASA/JPL)
This color picture of Antarctica is one part of a mosaic of pictures covering the entire Antarctic continent around 01:45 UT on December 9. The view shows the Ross Ice Shelf and an occasional mountain can be seen poking through the ice. (NASA/JPL)
As Galileo continued to pull away from the Earth, it acquired a series of distant, multispectral images of the Moon. Among the regions imaged was the western lunar hemisphere dominated by the Mare Orientale impact basin. These images supplemented older observations from the Apollo era about the variations in surface composition of the Moon as well as helped to refine the sequence of volcanic events in the region of Mare Orientale. These images also helped to fill in a small strip of the lunar far side near the southern polar region which had been unphotographed almost a quarter century earlier by NASA’s Lunar Orbiter missions (see “Lunar Orbiter 5: Filling the Gaps in the Maps”). Also confirmed by these global views of the Moon was the existence of the 2,500-kilometer South Pole Aitken basin – one of the largest impact features known in the Solar System.
These pictures of the Moon were taken by the Galileo spacecraft at (right photo) at 02:47 UT on December 9, 1990 from a distance of almost 354,000 km and at (left photo) 17:35 UT at a range of more than 563,000 km. The picture on the right shows the dark Oceanus Procellarum in the upper center, with Mare Imbrium above it and the smaller circular Mare Humorum below. The Orientale Basin, with a small mare in its center, is on the lower left near the limb or edge. The picture at left shows the globe of the Moon rotated, putting Mare Imbrium on the eastern limb and moving the Orientale Basin almost to the center. At lower left, near the limb, is the South Pole Aitken basin, similar to Orientale but very much older and some 2,500 km in diameter. (NASA/JPL)
These false color images correspond to those shown above. The color composites used images taken through the violet and two near infrared filters. The greenish-blue region at the upper right in the full disk and the upper part of the right hand picture is Oceanus Procellarum. The deeper blue mare regions here and elsewhere are relatively rich in titanium, while the greens, yellows and light oranges indicate basalts low in titanium but rich in iron and magnesium. The reds (deep orange in the right hand picture) are typically cratered highlands relatively poor in titanium, iron and magnesium. In the full disk picture on the left, the yellowish area to the south is part of the newly confirmed South Pole Aitken basin perhaps rich in iron and magnesium. Analysis of Apollo lunar samples provided the basis for calibration of this spectral map; Galileo data, in turn, permit broad extrapolation of the Apollo based composition information, reaching ultimately to the far side of the Moon. (NASA/JPL)
Starting at 14:10 UT on December 11 when Galileo had receded to 2.1 million kilometers from the Earth, the SSI began taking a series of images through its six color filters once each minute for 25 hours. The images were then assembled into a time lapse color video of the Earth as it rotated and its dynamic cloud systems moved. Eight days after its closest approach, Galileo wrapped up its observations of our home world as it headed back into interplanetary space.
Global images of Earth from Galileo using red, green and violet filter images from the SSI taken at six-hour intervals on December 11, 1990, at a range of between 2.1 and 2.7 million kilometers. In each frame, the continent of Antarctica is visible at the bottom of the globe. South America may be seen in the first frame (top left), the great Pacific Ocean in the second (bottom left), India at the top and Australia to the right in the third (top right), and Africa in the fourth (bottom right). (NAS/JPL)
Afterwards
During its encounter with the Earth, Galileo responded to over 7,000 commands and transmitted 58 billion bits of data including 2,675 images from the SSI. As anticipated, the close flyby increased the spacecraft’s heliocentric velocity by a whopping 5.2 kilometers per second placing it into a larger 0.90 by 2.27 AU orbit with a slightly higher inclination. This new orbit would now penetrate the Asteroid Belt allowing Galileo’s encounter with 951 Gaspra in October 1991 and the subsequent second flyby of the Earth in December 1992 which would give the spacecraft the final boost it needed to reach Jupiter in December 1995.
This diagram shows how Galileo’s trajectory was altered by its first encounter with the Earth. Click on image to enlarge. (NASA/JPL)
With a treasure trove of data of the Earth from a modern planetary spacecraft at their disposal, Carl Sagan (1934-1996) and some of his colleagues decided to examine the data for signs of life. They argued that this analysis would serve as a control experiment for the search of extraterrestrial life using modern spacecraft. The Ultraviolet Spectrometer clearly detected free oxygen in the atmosphere in a concentration of 19±5% (compared to the actual value of 21%) while spectra from the Near Infrared Mapping Spectrometer revealed variable amounts of water vapor and carbon dioxide at a concentration of 500±25 parts per million (close to the value of 350 ppm at the time). Also detected, in nonequilibrium amounts, were traces of methane and N2O which we recognize as being potential biosignatures.
Sample IR spectra from Galileo’s Near Infrared Mapping Spectrometer covering from 2.4 to 5.0 microns. Absorption features caused by various gases are indicated. Click on image to enlarge. (Sagan et al.)
Multispectral global imagery showed that the Earth’s landscape is covered in a surface pigment with a sharp absorption edge at wavelengths redder than about 700 nm. We know that this is the result of chlorophyll contained in the plants that cover much of the Earth’s land surface. While all these findings are suggestive of life, probably the most definitive proof of life on Earth came from the radio emissions detected by Galileo’s Plasma Wave experiment. It clearly showed near-constant, modulated emissions at frequencies in the 4 to 5+ MHz range. The properties of these emissions make it unlikely that they are natural in origin but are instead artificial VLF transmissions suggesting intelligent life on Earth – perhaps we will one day locate this intelligence 😉 .
Time series spectra of radio emissions from the Earth as observed by Galileo’s Plasma Wave experiment. The vertical axes are frequency while the horizontal axes are in units of time (UT), distance (in Earth radii) and local time (hours). Labelled in the upper panel are emissions from various sources including auroral kilometric radiation (AKR), Type III solar radio bursts and electrostatic oscillations in the plasma surrounding the Earth (fP). Time of closest approach is denoted by “C/A”. The lower panel provides a closeup view showing modulated VLF transmissions of artificial origin. Click on image to enlarge. (Sagan et al.)
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Related Reading
“First Pictures: Voyager 1 Portrait of the Earth & Moon – September 18, 1977”, Drew Ex Machina, September 18, 2020 [Post]
“The Future That Never Came: Planetary Missions of the 1980s – II”, Drew Ex Machina, December 1, 2014 [Post]
General References
Theodore C. Clarke, “Galileo – The Encounter with Earth”, Proceedings of the Second Annual NASA Science Internet User Working Group Conference [San Mateo, CA; February 11-14, 1991], NASA Conference Publication 3117, pp 233-256, February 13, 1991
Torrence Johnson, “The Project Science Group”, The Galileo Messenger, NASA/JPL, Issue 25, pp 5-7, September 1990
Carl Sagan et al., “A Search for life on Earth from the Galileo spacecraft”, Nature, Vol. 365, pp 715-721, 21 October, 1993
Paolo Ulivi with David M. Harland, Robotic Exploration of the Solar System Part 2: Hiatus and Renewal 1983-1996, Springer Praxis, 2009
“A Closer Look at the Earth and Moon”, The Galileo Messenger, NASA/JPL, Issue 27, pp 1-6, April 1991