The Week in Space and Physics: The Music of the Planets
The Week in Space and Physics: The Music of the Planets
On resonances and planets, Dragonfly, salt glaciers on Mercury and the Ariane 6
Venus, the ancients observed, traces a regular pattern in the sky. Over a period of eight years, the planet sketches out the shape of a five petalled flower; dancing from one side of the Sun to the other as it does. As soon as the cycle is complete, it starts again - each cycle beginning from the same position and on the same day as the last one.
This pattern is sometimes known as the pentagram of Venus. It owes its origins to a curious relationship between the orbits of the Earth and of Venus. For every eight times the Earth circles the Sun, Venus will do so thirteen times - forming a regular cycle of relative motion between the two worlds.
The ancients, lacking a knowledge of orbital dynamics, viewed this arrangement with some suspicion. The Mayans saw the death and resurrection of the sky god in the movements of Venus; a cycle of such importance they made it a foundation of their calendar. Scrolls from Ancient Babylonia tell of efforts to divine the future from those same cycles: a study which informed the course of empires and lives on today in the name of astrology.
Today we call this relationship between Earth and Venus a resonance. Such patterns are common - the moons of Jupiter follow one, for example, as do the orbits of Neptune and Pluto. Often they form as two bodies exert small gravitational tugs on each other, slowly pulling each other into synchronised motion.
Not all resonances are stable. Sometimes the constant tugs can cause chaos, sending planets ricocheting off into new orbits. Where they are stable, however, they can last for billions of years. Indeed, observations of other star systems have thrown up many cases of planets moving in perfect synchrony.
Recently, for example, a group of researchers announced the discovery of six planets moving in resonance around a nearby star. Each of the planets is large. All are bigger than the Earth but smaller than Neptune, and all orbit extremely close to their star. Indeed, the closest planet takes just nine days to complete a single revolution of its star.
Initial data from the star system revealed the presence of two planets. Calculations showed they were in a resonance, so that for every three orbits of the innermost planet, the outer one would complete two orbits. Using this, researchers then predicted the orbits of other planets in the system, assuming they also followed a resonance. When they returned to observe the star again, many of the predicted planets turned up.
In most star systems external forces - the passing of a nearby star, for example - will eventually upset the resonance. Unusually, however, the planets in this system seem to have kept up their dance for billions of years, making it the most stable example ever found.
Dragonfly: Not Quite Ready to Fly
Of all the planets and moons in the solar system, Titan is one of the most fascinating. Not only is it the largest moon of Saturn - and the second largest in the whole system - but is also home to a deep and rich atmosphere. Across its surface, as the Cassini spacecraft found in 2004, rolls a landscape of rivers and hills interspersed by lakes and seas.
Titan is too cold for liquid water to flow on its surface. Instead the seas and lakes seen by Cassini are filled with methane, a substance that exists as a gas on Earth. Clouds of methane seem to float through Titan’s atmosphere, some times unleashing torrential rains, much like water does on Earth. In many ways, indeed, Titan probably resembles the early Earth in the period before life began.
Scientists are thus keen to explore the moon further. Over the past few years NASA has been working on Dragonfly, a spacecraft that could reach Titan by the mid-2030s. Like the earlier Huygens probe, Dragonfly will descend to the surface of Titan, recording what it finds there. Unlike Huygens - which operated for only ninety minutes on the surface - Dragonfly will then take flight, darting through Titan’s thick atmosphere.
Since little sunlight penetrates to the surface of Titan, Dragonfly would not be able to rely on solar panels. Instead engineers have designed a nuclear power system for the probe, based on plutonium batteries. In principle that would allow Dragonfly to spend more than three years flying around Titan.
Before all this, however, NASA needs to sign off on the project. Last week they approved a key technical milestone for Dragonfly, paving the way to start actually building the spacecraft. At the same time, however, the agency postponed fully committing to the project, and delayed its launch date from 2027 to 2028.
At blame seems to be the rising cost of the project, which rumors suggest is now approaching one billion dollars. Such a sum is hard for NASA to justify, especially at a time when the agency is facing strict spending caps and has many other major projects ongoing. Things may look better next year, provided congress can agree on a spending package.
Glaciers of Salt
Millions of years ago, a sea stretched over much of modern Iran. At some point that sea dried up, leaving behind a layer of salt measuring hundreds of metres deep. Today much of that salt lies far underground, buried beneath miles of rock and dirt. In some places, however, geological forces have returned it to the surface, sending it erupting out in “glaciers” of salt.
Salt glaciers like these are rare, and can only survive in arid environments like Iran’s Zagros Mountains. Just a handful of other examples exist on Earth, mostly in the dry deserts of central Asia. Beyond our planet, however, researchers recently spotted them in an unlikely location: amid the arid craters of Mercury.
A recent analysis of data from Messenger, a probe that visited Mercury in the early 2010s, found hints of their presence around the planet’s north pole. The glaciers seem to coincide with a region of chaotic terrain, in which different layers of rock are mixed together.
The researchers behind the discovery think the salts could be the remains of Mercury’s early atmosphere. Soon after the planet formed, massive asteroid and comet impacts could have formed an atmosphere around the planet. As it later cooled, the gases in that air should have condensed, forming a deep layer of salt across the planet.
In 2025 a European spacecraft, BepiColombo, will reach Mercury and begin exploring it from orbit. The data it sends back could reveal much more about these glaciers - including whether any water was ever present in them. Mercury may be the Solar System’s hottest planet - it is, certainly, about to get a lot more attention.
Lift-off at last for the Ariane 6?
After a successful test firing in late November, the European Space Agency has set a target date for the first launch of the Ariane 6 rocket. If all goes to plan the launcher should lift-off sometime between June 15 and July 31 next year.
The Ariane 6 is designed to be an updated version of Europe’s successful Ariane 5 - the rocket which lofted the James Webb Space Telescope in 2020. Its design has been criticised, however, for failing to take advantage of advances in reusable rockets pioneered by SpaceX. It is also expected to cost far more than originally hoped - leaving it commercially uncompetitive with SpaceX’s Falcon 9.
More embarrassing for Europe, however, are the long delays the project has faced. Under original plans the Ariane 6 should have started flying in 2020. Instead it is now running more than four years late - a delay that has left Europe without a home-grown launch option for the past few months.
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