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« ...
An Italian in the space adventure. His is the “EDT (Electro - Dynamic
Tether) Project” - NASA-Aeritalia ... » |
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z Short Biography |
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z
In 1955, he
became Full Professor of Applied Mechanics at the faculty of Engineering in
Padua. During this time, he lectured on Celestial Mechanics, Spatial Geodesy,
Vibration Mechanics and on Space Vehicles and Carriers. In addition
to his University commitments, he also became involved in research work at
the Harvard Smithsonian Centre for Astrophysics and at the California Institute
of Technology (Caltech) and at the Jet Propulsion Laboratory (JPL). Additionally,
he became a consultant for many important space centres in the USA and a member of
different advisory committees of national and international academies. He was
awarded a gold medal by the NASA for his outstanding scientific achievements.
In 1971 he won the Feltrinelli Award and many other
prizes. He sturdily
promoted the space research at the Agenzia Spaziale Italiana (the geodesy space
centre in Matera was named after him) and collaborated with Universities (Padua,
Pisa, Turin) and with a number of aerospace industries ... » |
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z Mariner 10 - The robotic space-probe explores
Mercury ... |
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z
In 1965 some doppler-radar observations of Mercury’s features allowed to allocate Mercury’s
period of rotation within 59±5 days approx. Previously,
numerous optical observations made believe that the period of rotation was: 88
days, thus
meaning Mercury had a 1/1 spin-orbit resonance, same as the Moon’s with respect to Earth. This belief was
quite strong among astronomers, because it was easy to assume that tidal
frictions had a role similar to that of the Moon’s. Another
clue also contradicted this common opinion : temperature of Mercury’s hidden
side, at its maximum elongation, was measured and found too high to be capable of
coexistence with a continuous persistency in shade. This is the
context in which Giuseppe (Bepi) Colombo operated. He understood that the 59 days
rotation value [a measurement he trusted] was about two-thirds of Mercury’s
orbital period, therefore locked into a 3:2 spin orbit resonance. Trusting
his ideas, he decided to re-analyze the optical observations of Mercury. Taking into
special consideration the formations those features, the more recent ones, he found proof that
the period of rotation could be allocated around 70 days. Therefore, the estimated 88 days
were largely a result of prejudice. Colombo
believes, more and more, in his convincement and proceeds, as usual [this proceeding was typical
of Colombo],
to
outline a scenario that allowed Mercury to evolve its orbit into a stable 3:2 resonance. Two, the
ingredients of the recipe and both simple: the first one is the tidal torque exerted on Mercury
by the Sun, slowing down its motion; the second one is an asymmetry in the
equatorial distribution of Mercury’s low-mass [similar to that of the Moon’s], that creates an
additional torque [the comparison is similar to two small masses, symmetric with
respect to the centre of gravity] that must countervail the other torque to
gain stability.
This allows
Mercury to be steadily captured into that resonance. It is
important to underline that the (Colombo and Shapiro) article was published in 1965, well before the Mariner 10 mission, launched instead
on 3 Nov.
1973. Colombo was convinced and
so much sure of his ideas that, when he was definitively assigned to the Mariner 10 mission, he studied
an orbit for the space probe that, using the “fly-by technique”, after the meeting with
Venus would be in a 1:2 orbit-orbit resonance with Mercury. Mariner 10 encounters Mercury three
times, exactly after two revolutions, in the identical spatial and light
configurations. Photographs
prove Giuseppe
(Bepi) Colombo was right since 1965. |
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z The strange retrograde irregular moons of the gas giants ... |
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z
In the last
decade of the 20th century, discoveries of irregular moons
proceeded quite rapidly. It even was discovered that retrograde irregular
moons outnumbered the prograde ones [Colombo will also
provide an answer for this puzzle]. It was
clear those satellites could not be part of the nebula that, whirling giddily
anti-clockwise, gave shape to Jupiter and to the regular prograde
satellites. Retrograde satellites must have been captured from outside. It was also
quite certain that the four gas giants had approx. the same number of
irregular moons. Explanations
were attempted on how these bodies had been captured in a stable orbit. On this subject, according
to Kepler, a satellite is in
orbit, else, it will never be in orbit, unless dissipating factors, that may
change the values of the semi-major axis and of the eccentricity, are taken
into consideration, allowing the capture. Unfortunately,
because of the distance from the body of reference [one of the gas giants], and
of the limited size of the satellites, the tidal effect is reduced so
much that it is not possible to believe such phenomenon may be useful for a
capturing operation. Planetary
bulging scenarios, likely to incorporate the orbits of some of these bodies,
were imagined. Friction with such weak atmospheres could lead to a capture situation.
But to
maintain it stable, a planetary contraction, able to avoid frictions, needed to
be taken into consideration. Furthermore,
this scenario could apply to the gas giants [Jupiter and Saturn], but not to the
ice giants [Uranus and Neptune]. Ideas, therefore, were confused and uncertain. Once again,
this is the context in which Giuseppe (Bepi)
Colombo operated. He studied a possible capturing mechanism that could
explain the numerousness of these satellites. Thanks to a
brilliant intuition, Colombo assumes that possible spatial collisions among bodies may cause a body to
exit from the system, while its opposite looses energy and becomes
captured by the gas giants. This is an advanced
idea, outside classical models. Anyway, statistically, collisions are not so
uncommon. Moreover,
modern improvements of the model, allow to say that a collision is not
strictly necessary. A close encounter [very near to a collision], during which remarkable
quantities of energy are exchanged, may be sufficient. Therefore, chances
become more lasting.
This model
allows capture within the region of the Hill sphere [the sphere into which the
planet’s gravitational influence prevails. Outside this sphere, a body would
be progressively perturbed by the tidal forces of the central (e.g. the Sun)
eventually ending up orbiting the latter]. Of course, the region of the Hill
sphere is proportional to the mass of the planet, but is also inversely
proportional to the distance of the Sun. This allows the giant planets to
have comparable Hill spheres notwithstanding the mass differences. This is a
situation that allows to understand why the four gas giants are surrounded by
so many moons. This was
the subject of an article (David Jewitt, Scott S.
Sheppard, Jan Kleyna - 2004). The capture assumption, imagined
by Giuseppe (Bepi) Colombo, was a winning idea! |
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z Artificial tethered satellites ... |
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In the
seventies (1970) Mario Grossi and Giuseppe Colombo carried out studies
on satellite systems with a conductive tether [EDT electro-dynamic tether]. The idea
connected together two masses with a conductive tether. From a mechanical
point of view, this is equivalent to a system having its centre of gravity
aligned on the tether.
As per Kepler’s laws, the lower mass tends towards a lower orbit,
while the upper one tends towards an upper orbit. So the tether is kept straight:
the system behaves as a sole body and its position of equilibrium is the
local vertical. An electric
difference of potential is generated, at both ends of the tether, by its
motion through the Earth’s magnetic field. When applied to a payload, it can
provide energy to the main spacecraft. Of course, the
orbit becomes slightly lower [nothing is created, nothing is destroyed]. A “tether” applied
when an abandoned rocket stage is expended, would quickly burn in the
atmosphere. Colombo greatly
advanced these ideas. Many missions, even those that took place after his death, technically confirmed his ideas. Construction
problems, such as [material
strength];
correct use [pendular motion instability]; practical materials:
etc. Were not fully successful. It is
useful to remark some incidents described by Franco Malerba in his book[“La vetta” – Franco Malerba - 1993], that allow to
understand how complex this technology is. Malerba remembers that the mission to which he participated aimed to deploy a
cable 20 km long to experiment tether technology. This is his description of what
happened: «... Now we can open the
pylon … But the joint does not come off … [with] a sudden burn of the shuttle
rockets … the satellite … [is] now only held by a cable … » « … But soon after the
fleeting, the satellite bent right; the cable was still visible, at an angle about
45° on the vertical, near to that limit value … beyond which instability
compromises the system … » « ... Cable blocks again
at 256 m. … voltage is only 60 volts and the current only 2 milliampère …» The cable
will jam itself in a definite way for an unforeseen mechanical failure. Astronauts,
however, will be able to recover the satellite for the next mission, which,
unfortunately, failed to be successful. |
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z Malerba remembers
Giuseppe (Bepi) Colombo
"the space mechanic"
... on board of Atlantis, 1992 |
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