Radar images (from December 1992) of the near-Earth asteroid Toutatis
![[Images of Toutatis]](gif/toutatis.gif)
For the1996 December Radar Images ofToutatis
Mercury is the planet closest to the Sun. In 1974 Mariner-10 flew by
and returned data including pictures showing that the Mercury surface resembled
that of the Moon - heavily cratered with an abundance of features dating
from the heavy bombardment of the early Solar System The usual picture presented
is one of an impossibly hot (430 C) world at the height of the long day
and inconsolably cold (-180 C) at night, like our Moon, dry, barren, arrid,
devoid of heat retaining atmosphere and totally inhospitable. No moisture
or evidence of would remain at the surface after the billions of years of
Solar System existence and the relentless heat of the Sun. The Mariner fly-bys
imaged only 45% of the surface and only part of the polar regions. Thus
much remains to be seen of the planet. Since we can easily investigate the
surface of a nearby planet with radar, Mercury is a natural target for us,
and ranging keeps the orbital knowledge up-to-date for any future spacecraft
mission. In 1991 we observed Mercury in the full disk imaging mode possible
with the VLA ... to discover Ice(?)
at the Poles!!
One of the premises for the calculation of the relativistic "perihelion precession" is the nature of the gravitational field of the Sun. If this is not "uniform", as from a uniform sphere, because the Sun itself is not "internally symmetric", or the Sun is oblate to some degree (squashed at the poles) then such imbalances will make themselves apparent over time by perturbing the orbital behavior away from that currently predicted.
Since the General Relativity change in Mercury's orbit is so small (43 arc second per century) we have to be either very patient , or very precise in our measurement, in order to find the exact value.
So this is where the Radar comes in. It is, with care of course, very precise. Mercury is typically 180 million kilometer distant for an observation. We can measure this easily to much better than a kilometer (after removal of systematic effects we can get to about 20 meter! This is 1 part in 9 billion). Repeated measurements, over several years, allow us to characterize, to define and refine, the orbit of the planet. As we build this picture we can compare the calculation with our actual measurement, and then identify and track any divergence.
The next step is to explain the results. How oblate is the Sun? Does the gravitational "constant" G change? in space? in time? If so how?
We have been making very precise radar range measurements to Mercury
for 6 years and the program recently received a high rating in the peer
review of the Astrophysics Program.
Mass (kg) ......................................... 3.303e+23
Mass (Earth = 1) ................................. 5.5271e-02
Equatorial radius (km) .............................. 2,439.7
Equatorial radius (Earth = 1) .................... 3.8252e-01
Mean density (gm/cm^3) ................................. 5.42
Mean distance from the Sun (km) .................. 57,910,000
Mean distance from the Sun (Earth = 1) ............... 0.3871
Rotational period (days) ............................ 58.6462
Orbital period (days) ................................ 87.969
Mean orbital velocity (km/sec) ........................ 47.88
Orbital eccentricity ................................. 0.2056
Tilt of axis ........................................... 0.00°
Orbital inclination ................................... 7.004°
Equatorial surface gravity (m/sec^2) ................... 2.78
Equatorial escape velocity (km/sec) .................... 4.25
Visual geometric albedo ................................ 0.10
Magnitude (Vo) ......................................... -1.9
Mean surface temperature .............................. 179°C
Maximum surface temperature ........................... 427°C
Minimum surface temperature .......................... -173°C
Atmospheric composition
Helium .............................................. 42%
Sodium .............................................. 42%
Oxygen .............................................. 15%
Other ................................................ 1%
Mass (kg)............................................3.3 x 10^23
Diameter (km)........................................4878
Mean density (kg/m^3) ...............................5420
Escape velocity (m/sec)..............................4300
Average distance from Sun (AU).......................0.387
Rotation period (length of day) (in Earth days)......58.65
Revolution period (length of year) (in Earth days)...87.97
Obliquity (tilt of axis) (degrees)...................0
Orbit inclination (degrees)..........................7
Orbit eccentricity...................................0.206
Mean surface temperature (K).........................452
Maximum surface temperature (K)......................700
Minimum surface temperature (K)......................100
Visual geometric albedo..............................0.12
Largest known surface feature........................Caloris Basin
(1350 km diameter)
Atmospheric components...............................trace amounts of
hydrogen and
helium
Surface materials....................................basaltic and
anorthositic rocks
and regolith
For general information about Mercury
Venus was the first planetary target seen by radar. In 1961, Richard Goldstein working from Goldstone with a mere 10 kilowatt of transmitter power, detected definite echoes from the planet. These measurements, along with the confirming successors, were important for many reasons. Most significantly they yielded an improved value for the Astronomical Unit (the mean distance from the Sun to the Earth), which is the scale size of the Solar System. Knowledge of this is key to successful navigation of spacecraft for interplanetary exploration.
Secondly they showed that the rotation of Venus was unusual. It was retrograde (i.e opposite to that of the Earth and most other bodies known) and was very slow, taking about 243 terrestrial days to rotate once, while its year lasts 225 days.
Thirdly they revealed that the surface of the planet, swathed permanently in cloud and hidden from eye view, abounded in craters.
Since Venus is cloud obscured we cannot have any visual reference points on the surface. Radar measurements alone allow us to define a coordinate grid on the planet. The mission of the Magellan spacecraft was to image the surface. Complete coverage requires the spacecraft to be in a polar orbit, moving over each pole in turn as the planet rotates slowly underneath it. The question naturally arises: "where exactly are the poles?". Radar beams to the rescue! The distillation of many measurements made at Arecibo and Goldstone yielded a refined estimate of the position of the Venusian axis. This directed the final navigation of Magellan, so that it plopped neatly into the desired orbit. Later analysis from the Magellan images showed that the discrepancy between the actual and calculated positions was about 0.5 kilometer. "Not bad for Government work" at a range of 250 million kilometer or more at the time of orbit insertion :-)
For the history of Venus Orbiting Image Radar consult Magellan
For general information about Venus
Following the surprise finding of the radar signature of ice in permanently
shadowed craters near the poles of Mercury, it has been proposed (again)
that craters near the lunar poles might also have collected ice from meteorites,
comets, etc.
Earlier days:
VLA
Poles
Stealth
The next major spacecraft mission to Mars is Pathfinder
For general information about Mars
Jupiter itself is not a target for radar observation. This is because
it is a "gas giant" - the bulk of the planet is gas, cold or frozen,
surrounding Galilean moons
The current spacecraft mission to Jupiter is Galileo
For general information about the Jupiter
system
Rings
Titan - the largest moon in the Solar System
Images of Titan
from the Hubble Space Telescope
Info about Titan images
The current mission to Saturn is Cassini
For general information about Saturn
Main Belt
Earth Crossing
Small Bodies
For general information about Small Bodies
Meteorites