SP-474 Voyager 1 and 2 Atlas of Six Saturnian Satellites

 

Appendix A

The Voyager Mission

 

 

[143] The Voyager 2 and 1 spacecraft were launched on August 20, 1977, and September 5, 1977, respectively, to explore the planets and satellites in the outer solar system. After extremely successful traverses through the Jovian system in March and July of 1979, the two Voyagers continued on to Saturn, where they arrived in November of 1980 and August of 1981. Figure 3 in the section entitled "Introduction" diagrams the trajectories of the Voyagers through the Saturnian system. At the time of Voyager 1's closest encounter with Saturn, it was 1524 million km (947 million miles) from Earth. The radio signals from the spacecraft, traveling at the speed of light, required 1 hr and 25 min to reach tracking stations on Earth. Voyager 2 was 1556 million km (967 million miles) from Earth during the Saturnian encounter, and its readio signals required 1 hr and 27 min to reach Earth.

Many remote sensing instruments are carried on each spacecraft, including two television cameras with telephoto lenses. Figure A-1 is a diagram showing the locations of various instruments on the Voyager spacecraft.

Each Voyager spacecraft carries a wide-angle camera, with a field of view of about 3°, and a narrow-angle camera with a field of view of less than 1/2°. The optical configuration of the Voyager cameras is diagrammed in figure A-2. Tables A-1 and A-2 show selected specifications of the cameras. Objects as small as 1 km in diameter (about 0.6 mile) can be detected on the surface from a distance of 55000 km (about 34000 miles). Figure A-3 is a telescopic picture of the Moon that has been digitized and modified to illustrate what the Voyager narrow-angle camera could have resolved from....

 


Figure A-1. The Voyager spacecraft, showing locations of the various instruments (JPL photo P1881 AC).

Figure A-1. The Voyager spacecraft, showing locations of the various instruments (JPL photo P1881 AC).


Figure A-2. Optical configuration of the Voyager cameras.

Figure A-2. Optical configuration of the Voyager cameras. (a) The narrow-angle cameras have nominal focal lengths of 1500 mm and use catadioptric mirror telescopes for imaging. (b) The wide-angle cameras have 200-mm focal lengths and use conventional transmission-type lenses.

 

[144] Table A-1. Specifications of the Voyager Cameras.

Parameter

Wide-angle camera

Narrow-angle camera

.

Nominal focal length, mm

200

1500

Focal ratio

f/3.5

f/8.5

Angular field of view:

.

.

Milliradians

56 x 56

7.4 x 7.4

Degrees

3.2 x 3.2

0.42 x 0.42

Nominal shutter speeds

Milliseconds

5, 12.5, 25, 50, and 97.5

5, 12.5, 25, 50, and 97.5

Seconds

1/200, 1/80, 1/40, 1/20, and 1/10

1/200, 1/80, 1/40, 1/20, and 1/10

Active target raster (image size on vidicon tube), mm

11.14 x 11.14

11.14 x 11.14

Active scan lines per frame

800

800

Active pixels per line

800

800

Field of view of a single pixel:

Microradians

70

9.25

Degrees

4.0 x 10-3

5.3 x 10-4

Bits per picture element

8

8

Number of filters

8

8

Source: Benesh and Jepsen (1978).

 

Table A-2. Color Filters on the Voyager Cameras.

Type of camera

Filter wheel position

Filter

Wavelength, nm

.

Narrow-angle

0

Clear

-

1

Violet

50 to 400

2

Blue

50 to 480

3

Orange

570

4

Clear

-

5

Green

530

6

Green

530

7

Ultraviolet

45 to 325

Wide-angle

0

Methane

5 to 169

1

Blue

50 to 480

2

Clear

-

3

Violet

50 to 400

4

Sodium D

589

5

Green

>530

6

Methane

5 to 541

7

Orange

>590
Source: Benesh and Jepsen (1978).

 

.....a range comparable to that between Earth and the Moon. The pixels are approximately 3.5 km square on the Moon, and the complete lunar disk will not quite fit in one frame. For comparison, the best Earth-based telescopes can resolve features with dimensions as small as about 1 km on the Moon. Figure A-4 is a simulated Voyager wide-angle picture of the Moon from Earth.

Voyager images are identified by a picture number, or "picno." This number contains information identifying the spacecraft that took the picture, planetary system being traversed at the time the picture was taken (Jovian or Saturnian), and the location (in time) of the spacecraft with respect to the major planet when the picture was taken. The cameras are capable of taking 1800 pictures in one Earth day; each picture is, therefore, coded first by a number between 1 and 1800, denoting one of the 1800 opportunities to take a picture during a given day. Not all opportunities were taken, so many gaps exist in the numeric sequence. This number is followed by a "J" or an "S," depending upon whether the spacecraft was nearer to the Jovian or Saturnian system when the picture was taken, and a "1" or a "2," depending upon whether the picture was taken by Voyager 1 or 2. The final number contains three digits, including leading zeros, and indicates the number of days from closest approach to the primary planet of the system. This number is separated from the preceding ones by a "+" or a "-," depending on whether the picture was taken before or after closest approach to the planet. Thus, picno "0272S1+000" was taken on the 272d opportunity, by Voyager 1, on the day the spacecraft made its closest approach to Saturn.

 


[
145]

Figure A-3. Simulated Voyager narrow-angle picture of the Moon that approximates the resolution and field of view (0.42° X 0.42°) of a narrow-angle Voyager picture taken from the distance of Earth. the film image, taken through an Earth-based telescope, was scanned and digitally encoded at 8-bit format (256 gray shades) with pixel dimensions of 3.5 x 3.5 km on the Moon. (Lick Observatory full-Moon photo taken on January 16, 1946, by J. H. Moore and J. F. Chapel).

Figure A-3. Simulated Voyager narrow-angle picture of the Moon that approximates the resolution and field of view (0.42° x 0.42°) of a narrow-angle Voyager picture taken from the distance of Earth. the film image, taken through an Earth-based telescope, was scanned and digitally encoded at 8-bit format (256 gray shades) with pixel dimensions of 3.5 x 3.5 km on the Moon. (Lick Observatory full-Moon photo taken on January 16, 1946, by J. H. Moore and J. F. Chapel).

 

The Voyager cameras can be used to take black and white pictures through a variety of color filters (table A-2). The primary purpose of the filters was to permit analysis of target reflectance in different color bands, not the construction of true color pictures. Nevertheless, a reasonable approximation of true color could often be made in the photolab by matching spacecraft filters as closely as possible with the primary darkroom color filters and printing the composite photographically (fig. A-5).

More details on the Voyager missions are available in two special issues of Space Science Reviews (1977), Science (1981, 1982), and Morrison (1982).

 


[
146]

Figure A-4. Simulated Voyager wide-angle picture of the Moon.

Figure A-4. Simulated Voyager wide-angle picture of the Moon. This digital treatment of the photograph used for figure A-3 approximates the resolution and field of view (3.2° x 3.2°) of a Voyager wide-angle picture taken from the distance of Earth. The pixel dimensions are 27 x 27 km on the Moon.


[
147]

Figure A-5. A Color picture of Dione, made with three black and white Voyager pictures taken through different filters.

Figure A-5. A Color picture of Dione, made with three black and white Voyager pictures taken through different filters. (a) Picno 0274S1+000, taken through the violet filter, printed in red. (b) Picno 0276S1+000, taken through the green filter. (c) Picno 0272S1+000, taken through the clear filter, which actually has a bluish cast and is printed in blue. (d) Composite full-color picture of frames (a), (b), and (c).


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