Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/~tghosh/a1724/jon40 Дата изменения: Sun Apr 25 18:30:02 2004 Дата индексирования: Sun Apr 10 02:09:07 2016 Кодировка: Поисковые слова: р п р п р п р п р п р п р п

thanks!
--JF
--
On Apr 19, 2004, at 5:42 PM, Alice Hine wrote:

> 40th Anniversary Workshop
> Planetary Radar
> by Alice Hine
>
> Gordon Pettengill
> Don Campbell
> John Harmon
> Steve Ostro
> Don Campbell
>
> GP, who was an early site director, talked about the early development
> of
> the Observatory from a tobacco farm in 1960 to the discoveries in the
> late 1960s
> in planetary radar. It was Gordon who chose digital over analog
> equipment for
> pointing the telescope, giving 1 arcmin accuracy (now a few arcsecs).
> Arecibo improved
> the accuracy of the Astronomical Unit from a few thousand km to about
> 1 km, measured
> the rotation of Venus, discovered that Mercury rotation/revolution
> were not synchronous
> but tidally locked in 3:2 ratio to the orbital revolution.
> The telescope was also used to measure the retardation of the radar
> signal
> as it skimmed the sun on its way to a planet beyond, thus confirming
> General Relativity.
> Gordon told how one of the first maps of Venus was made by DC using a
> second dish to get interferometric fringes which resolved the
> north/south ambiguity
> of the radar echoes. Radar-bright features were seen on the surface
> which
> are covered by clouds in the optical view.
>
> DC, also a past site director, continued the story with the 1973
> upgrade which
> replaced the surface of the dish and introduced the 2.4 GHz radar with
> 420 kW
> power for planetary work. The second upgrade doubled the power and
> again improved
> the surface. He showed a rising sensitivity over the years of
> 32 dB, about 1 dB/year. The improvements came from the Love feed, the
> upgrades
> and the recent surface tune-up. Don gave credit for this to the many
> scientists,
> students, engineers, and Observatory employees who supported this
> project.
> The scientific achievments of this period were, for Mars: warning the
> Viking
> lander team of surface roughness; for Venus: mapping the surface
> through the
> clouds before the Magellan spacecraft arrived with 10 times the
> resolution, dating
> the Venus surface by crater counts, and investigating surface
> polarization by
> receiving the echo at the GBT; for the Galilean sattelites of Jupiter:
> finding
> icey surfaces by the strange polarization ettects of the echoes; for
> Mercury and
> the Moon: finding ice in permantly shadowed craters on Mercury and
> not on the
> Moon. Comets, asteroids, and the particles of Saturn's rings have
> been detected
> and their sizes and dynamics illuminated. Saturn's Titan recently
> showed indications
> of surface lakes, and Saturn's Iapetus with its optically bright/dark
> hemis-
> pheres showed uniform radar echoes, which imply that the optical
> effect is a surface
> deposit and not structural.
>
> JH, recently assistant site director, showed the spectacular radar
> maps which
> extend our knowledge of the surface beyond the images taken by the
> (only) Mercury
> mission, Mariner 10 in 1975. The polar ice patches are probably water
> ice,
> and have helped in refining the pole position. Mars has not been
> visible since
> the upgrade (it will be in 2005) but in the past we learned a great
> deal about
> the surface using the non-repeating long code technique to overcome
> the frequency
> folding due to Mars's rapid rotation. Small-scale roughness is seen
> as are
> lava flows. Since Comet Encke in 1980, 9 comets have been detected,
> some with
> both nucleus and coma (cm-sized grains). Density and mass loss are
> the goals,
> but comets are not often close enough to be radar targets becouse of
> radar's
> 4th-power dependence on distance.
>
> SO, who was at Arecibo as a graduate student, completed the story by
> summing up
> the asteroid work, now the most frequent use of the radar, usually on
> Near Earth
> Asteroids but also some Main Belt Asteroids. Many give strong enough
> echoes to
> be imaged in delay-Doppler and to give a good polarization ratio.
> Bulk density,
> proportion of rock/regolith on the surface, shape, spin state, average
> slopes, and
> metallic/rocky composition can be determined for some objects. The
> radar detec-
> tion improves our knowledge of the orbit, which we need for dynamical
> models and
> for impact prediction. Recently the long-term effect of radiation on
> a rotating
> body (Yarkovsky effect) was measured for the first time at Arecibo.
>
> In the part of the Workshop dealing with Future Instrumentation, DC
> posed the
> question: Where is the next increase in sensitivity to come from? He
> discussed u
> using the SKA for planetary radar; adding a transmitter station could
> give a factor
> of 20 in sensitivity but would cost about $100 million to build.
> Other ways are
> to change the Arecibo transmitter from S-band to X-band frequency, for
> $4 million,
> giving a factor of 2 or 3, or to use the Canadian LAR telescope, or
> transmit with
> Arecibo and receive with the VLA or VLBA. The science drivers are
> solar system
> dynamics, small bodies, location of water, and solar system formation.
>
>