GRAND IDEAS: BEFORE VOYAGER
Extending our reach If our knowledge of Jupiter and Saturn was sketchy, the two outermost giants were a virtual blank before the Voyager probes. Uranus, discovered telescopically by William Herschel in 1781 – but seen and mistaken for a star, many times before that – exhibits a peculiar rotational
U A perfect circle projected over this Hubble shot of Jupiter reveals the giant planet’s oblateness, a trait spotted by Giovanni Cassini
tilt of 98°, while Neptune was found in 1846, through a strangerWKDQ Ć…FWLRQ PL[ RI inexplicable orbital mechanics, clever mathematics and direct observation. Uranus’s plain aquamarine façade was interpreted as being a methanerich atmosphere, and far-infrared spectroscopy showed that it radiated as much heat as it received from the Sun. In 1977, during a stellar RFFXOWDWLRQ Ć…YH QDUURZ ULQJV were detected by astronomers DERDUG WKH .XLSHU $LUERUQH 2EVHUYDWRU\ %\ WKLV WLPH evidence for rings had also been seen at Jupiter, intensifying similar searches at Neptune. Tantalisingly, observations from Chile in the early 1980s pointed to ‘ring-arcs’, extending part of the way around the planet.
%XW WKH P\VWHULHV GLGQĹ?W HQG WKHUH $URXQG DOO IRXU JLDQWV ZHUH large retinues of moons, ranging from Jupiter’s four ‘Galilean’ satellites to two-toned Iapetus and planet-sized Titan at Saturn. In 1944, spectroscopy revealed 7LWDQ DV WKH Ć…UVW QDWXUDO VDWHOOLWH known to possess a substantial DWPRVSKHUH $ERXW WKH UHVW IDU less was known. Neptune’s large moon Triton didn’t even have DQ RIĆ…FLDO QDPH XQWLO WKH ODVW century and, despite evidence of nitrogen ice on its surface, its exact mass was uncertain. Much of this uncertainty vanished overnight when the Voyager probes arrived. Their whistle-stop tours of the giants RSHQHG WKH ƆRRGJDWHV RI KXPDQ knowledge, unleashing an unprecedented torrent of data about these far-off worlds. Our entire understanding of Jupiter, Saturn, Uranus and Neptune would be revolutionised, though the probes would raise as many questions as they’d answer. V
Z It soon became evident to observers that families of moons orbited both Jupiter and Saturn. In this telescopic image the four Gallilean moons of Jupiter are easily seen
Jupiter’s colossal magnetic ďŹ eld is the largest (relative to the planet’s size) and strongest in the Solar System
www.skyatnightmagazine.com 11
THE STORY OF VOYAGER
revealed ammonia and methane, while the work of Harold Jeffreys, Rupert Wildt and others led to theories of a rocky core, an icy mantle and a deep gaseous atmosphere. $ PDJQHWLF Ć…HOG UHPDLQHG hypothetical until ZKHQ WKH $PHULFDQ DVWURQRPHUV %HUQDUG %XUNH DQG .HQQHWK )UDQNOLQ VHUHQGLSLWRXVO\ discovered radio bursts at 22.2 MHz, matching Jupiter’s 10-hour rotation rate. This allowed the Ć…HOGĹ?V H[LVWHQFH WR EH LQIHUUHG IRU WKH Ć…UVW WLPH $OO IRXU JLDQWV KDYH PDJQHWLF Ć…HOGV EXW Jupiter’s is by far the strongest and the only one that can be measured in the radio range. Efforts to observe emissions from Saturn were inconclusive until 1974, when bursts at 1 MHz were recorded.