After its launch in 1977 and the flight over Jupiter, then Saturn, the
probe flew over Uranus at a distance of 81 000 km, and revealed a quite surprising planetary system :
* a complex system of rings and 27 satellites including a number of large ones (Oberon, Titania, Umbriel, Ariel, Miranda),
* fast axial rotation, with a period of 17,24 h,
* unique atmospheric chemistry and dynamics,
* no (or very small) internal energy source
* a magnetic envrionement unlike any other in the solar system.
We did expect that Uranus would spring surprises. Its rotation axis is aligned
towards the Sun, probably as a consequence of a collision in the distant past
with a giant body, so that its atmosphere is subject to extremely unequal
solar heating, one hemisphere being constantly heated. The orbital plane of
its satellites is perpendicular to the ecliptic. Moreover, wigth a magnetic axis
inclined at 60° - a record in the solar system ! - this asymmetic magnetosphere became
from the very start an extreme case in the very exclusive club of magnetic planets.
And ever since, interest in this intriguing planetary system has not waned.
And that is because the harvest of data obtained by Voyager 2 has left many
questions unanswered.
While it is true that regular ground based telescopic observations do enable us
tp monitor the activity of its clouds and its ring system, the spectral range
and sensitivity which can be achieved from the terrestrial orbit
(for an object more than 2,7 billion km away) bear no comparison with in-situ observations, some
of which can only be done in this way (such as corpuscular studies, magnetic field measurements , etc).
And so, after the numerous terrestrial space missions, after the orbital exploration of Jupiter
(Galileo, Juno at the end of this year, and et JUICE in 2025), of Saturn (Cassini-Huygens), of Mercury
(Messenger, and soon Bepi-Colombo) and even of comets (Rosetta/Philae), Uranus is the next key step in the
exploration of the solar system.
Together with Neptune, it is a member of the class of geant icy planets
(less massive than the gas giants, with a core made up of ices and rocks),
but since it is twice as close as that planet, it is much more accessible.
The rebirth of interest by the space agencies
A number of scenarios have been under study already for some time :
• Firstly, at NASA d’abord : such a mission has been awared priotity status in a number of recent reports
(decadal surveys in planetology and heliospheric physics), while a science definition team for such a mission
is currently being organized.
• On the ESA side : the project of the Uranus Pathfinder mission – which includes a very significant contribution
from the French community - has been the subject of a recent ESA call for tender, and has been well received
scientifically, even though it has so far not been selected.
A first technological challenge concerns the size of the solar panels needed
to furnish electricity at a distance of 19 AU (roughly the size of a football field)
which makes this option unfavourablecompared to that of using nuclear generators, which
ESA has not yet mastered.
The second challenge : the duration of the flight is 15 ans for much of which the probe will be in hibernation,
which is much longer than for the Rosetta mission. En 2011, the project as submitted to ESA specified a possible arrival
time in … 2037 !
For this kind of long duration mission, it is essential to make the selecttion as soon as possible, and to anticipate
the preparatory work. Note in this context that the CNES has decided to initiate in 2016 a feasibility study for a mission to
explore the environment of Uranus.
Reanalysis of the Voyager 2 data
In the meantime, the time is ripe for a reanalysis of the Voyager 2 data, in the light of our current understanding of planetary systems.
The "magnetospheres" group of the Space and Astrophysical Instrumentation Laboratory of the Paris Observatory begain thisummer to reconstitute the high resolution data of the PRA instrument (Planetary Radio Astronomy), originally stored on magnetic tape and now archived at the CNES in digital form.
At the same time, Uranus aurorae had been detected by the Hubble space telescope in2011, during a period when the Sun was particularly active. The results obtained are witness to a rich variety of magnetospheric processes which vary with the seasons, and so fuincite farther studies of Uranus, and that as soon as possible.