Magellan, see Venus without the clouds again




The “cousin of Galileo” marked the exploration of Venus, by revealing the surface of the planet with a precision never before seen. While it is slow to find a successor, Magellan has produced data that is still being studied 25 years later.

The probe was the first interplanetary mission to take off from the hold of a shuttle.



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Magellan, a discount mission?




Even if we know that the planet Venus has several characteristics in common with the Earth, it is interesting to study it ... If only to understand how it became a real hell, with its improbable cloudy atmosphere of 90 km thick, its rains of sulfuric acid, its incredible pressures and its devastating winds. In a few decades of the Cold War, the Soviets rather pushed their missions to study its atmosphere, but also to land on the ground: in this sense the Venera missions are successes. NASA, after a successful adventure in 1978 (Pioneer Venus), seeks to obtain an overview of the surface of our neighbor. For this, it plans to embark an emerging technology, which is barely appearing on satellites around the Earth: a synthetic aperture radar.

Magellan, originally, was called SEE (Venus Orbiting Imaging Radar): the mission was to map the planet using its radar but also to study its atmosphere, take on board various instruments and ... There is no point in listing the rest, since the project was canceled when NASA saw its budget drastically reduced in 1982-1983.

However, the initial study was not without effect. So as not to be purely and simply put back in the closet, the mission will ask the teams in charge of deploying treasures of ingenuity: a single instrument, elements recovered from the duplicates of other missions and a budget practically divided by three.

That which will be called a time VRM (Venus Radar Mapper) is finally approved and it is Marietta, who will later become Lockheed-Martin, who works on the probe to see it take off in 1988.



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An extended trip because of priorities ...

Obviously, Magellan must leave Earth in the hold of an STS shuttle. But the tragic Challenger accident in January 1986 shook up all agendas and budgets. The mission to Venus is not a priority, it is notably no match for its “cousin”, the impressive Galileo probe. Both projects suffer from the stoppage of the development of a more powerful engine to leave Earth (Centaur-G) ... And NASA finds itself in a dilemma: for its journey to Jupiter, Galileo must also fly over Venus.

Here is therefore a single orbital 'window' towards the planet-hell, for two probes to be sent with shuttles, at a time when frequent flights are not authorized. The decision is finally made to send Magellan first, on an extended path, which will not be optimized. The probe took off on May 4, 1989 in the hold of Atlantis, and was ejected before firing its engine a few hours later.





The journey to Venus would take over a year, until August 10, 1990, but no incidents to report. As expected, once released, Magellan ignites its engine and places itself in an elliptical orbit of 8,543 x 294 km altitude, inclined at 86 °. When it begins its mission, the probe weighs only a ton! Its only instrument is the radar, but since the antenna is shared with the high gain communication system, it cannot communicate with Earth and take measurements at the same time.

The orbits, which last 3h08 minutes, are therefore divided into two phases: the observation of the ground during the passage as close as possible to Venus (perigee), and the communication of data to the ground in the second phase.

Great fright six days after arriving in orbit, the probe goes into a safe position and hangs for 17 long hours ...


Venus cards are a game changer

Each 'phase' of Magellan's mission is intended to collect as much data as possible to map Venus, with a resolution of about 100 meters, depending on latitude. While that may sound like a lot today, these measures are quite a feat for a single vehicle with technology from the late 1970s.

The first phase lasts 243 days (a Venusian year) until May 15, 1991, and in that time frame the vehicle will record 150 GB of data, more than all other NASA robotic missions combined, at that time ( whose total data is 112 GB).

On September 13, 1992, the surface of Venus was mapped at 98% thanks to the radar, which was alternately pointed towards the forward trajectory or the rear trajectory of the probe, which made it possible to improve the content of the data when they were cross-checked by scientists on Earth.

For the first time, planetologists are discovering the surface of Venus in detail.





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Magellan will also be used to map the distribution of masses on Venus, thanks to a very detailed study of its variations in gravity, as and when its position in the orbits (measurements which make sense over several thousand orbits); the probe will also test a complex but crucial method for the future: airbraking, which consists of bringing the point closest to orbit somewhere in the highest layers of the atmosphere, to take advantage of a short orbital braking zone without generating too much heat and damaging the vehicle.

For Magellan, this will translate into an orbit that is ultimately circular, just a few hundred kilometers from the surface of Venus ... And leading to a final radar data campaign, which will further improve existing maps. Magellan re-entered the Venusian atmosphere (one last time this time) at the end of his mission, on October 13, 1994, to disintegrate there.


No other key to understanding Venus!

The problem with Venus's tens of kilometers thick clouds is that there is no other way (other than a site visit) to understand its surface, other than with a radar. Even though several Venera missions had already returned good results, Magellan's charts are still an incredible treasure trove of data today. They are still being studied, and for certain details, continue to produce scientific results, 30 years later!

The volcanic past of Venus was notably proven thanks to this mission, which revealed many volcanoes, some of which we hesitate to qualify as extinct ... Magellan also showed another unexpected result: there is no trace of large-scale erosion, caused by winds, despite their violence.




However, the planet has only been mapped by radar with a resolution between 50 and 100 meters, outside the poles. At such a resolution, extraterrestrials observing Earth could show that it is inhabited, but have absolutely no idea what or what our society looks like.

To know in more detail what is happening on Venus, we need better radars. And that's good, because in 35 years, synthetic aperture radar technology has radically evolved! It remains to decide to finance a mission ... However, in view of all the other more welcoming objectives in the solar system, such a project has so far not been successful. This is only a postponement ... There is still much to discover on Venus!



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