Our next visits to Jupiter, Saturn, Uranus, & Neptune are incredibly important
They won’t be without their issues
But first, what are giant planets?
Unlike rocky planets, giant planets don’t have a surface to land on. Even in their lower atmospheres, they remain gaseous, reaching extremely high pressures that would crush any spacecraft well before it could land on anything solid.
There are two types of giant planets: gas giants and ice giants.
The larger Jupiter and Saturn are gas giants. These are mainly made of hydrogen and helium, with an outer gaseous layer and a partially liquid “metallic” layer below that. They’re also believed to have a small rocky core.
Uranus and Neptune have similar outer atmospheres and rocky cores, but their inner layer is made up of about 65% water and other so-called “ices” (although these technically remain liquid) such asmethane and ammonia.
Slingshots to the edge of the Solar System
Any giant planet mission is extremely difficult. Still, there have been some past missions sent to the gas giants.
NASA’s 1989 Galileo mission had to slingshot around Venus and Earth to give it enough momentum toget to Jupiter, which it orbited for eight years. The 2011Juno missionspent five years in transit, using a flyby around Earth to reach Jupiter (which it still orbits).
Similarly, the Cassini-Huygens mission run by NASA and the European Space Agency (ESA)took seven yearsto reach Saturn. The spacecraft spent 13 years exploring the planet and its surroundings and launched a probe to explore Saturn’s moon,Titan.
Flight times get even longer for the two ice giants, which are much further from the Sun. Neither has had a dedicated mission so far.
A complex journey
The last and only spacecraft to visit the ice giants wasVoyager 2, which flew by Uranus in 1986 and Neptune in 1989.
While momentum is building for a return, it won’t be simple. If we launch during the next convenientlaunch windowsof 2030–34 for Uranus and 2029–30 for Neptune, flight times would vary from 11 to 15 years.
A major issue is power. The Juno spacecraft is the most distant object from the Sun to haveused solar panels. It orbits Jupiter, which isfive times further awayfrom the Sun than Earth is. Yet, where Juno’s solar cells would generate 14 kilowatts of continuous power on Earth, they onlygenerate 0.5kW at Jupiter.
Meanwhile, Uranus and Neptune are20and30times further away, respectively, from the Sun than Earth is. Power for these missions would have to be generated from the radioactivedecay of plutonium(the power source for both the Galileo and Cassini missions).
This radioactive decay can damage and interfere with instruments. It is therefore reserved for spacecraft which really need it, such as missions operating far away from the Sun.
Fighting the heat
The massive scale of giant planets means orbit speeds for incoming spacecraft are incredibly fast. And these speeds greatly heat up the spacecraft.
The Galileo probe entered Jupiter’s atmosphere at47.5 kilometers per second, surviving the harshest entry conditions ever experienced by an entry probe. The shock layer which formed at the front of the spacecraft during entry reached a temperature of 16,000℃ – around three times the temperature of the Sun’s surface.
Even so, the distribution of theheat shield’smass was found to be inefficient – showing we still have a lot to learn about entering giant planets.
Proposed future probe missions to Uranus and Neptune would occur at slower entry speeds of22km/s and 26km/s, respectively.
For this, NASA has developed a tough but relatively lightweight material woven from carbon fiber, calledHEEET(Heatshield for Extreme Entry Environment Technology), designed specifically for surviving giant planet and Venusian entry.
While the material has been tested with afull-scale prototype, it has yet to fly on a mission.
The next steps
In 2024, NASA’s Europa Clipper missionwill launchto investigate Jupiter’s moon Europa, which is believed to house anocean of liquid waterbelow its icy surface, where signs of life may be found. TheDragonflymission, planned to launch in 2026, will similarly aim to search for signs of life on Saturn’s moon Titan.
There are plans for a jointNASA-ESA missionto visit one of the ice giants within the upcoming launch window. But while there has beenextensivepreparation, it’s undecided which ice giant will be visited.
A single mission to both planets is being considered. An entry probe is planned, too. But if the mission visits both planets, it’s undecided which planet’satmosphere the probe would explore.
If we want to meet the upcoming launch window, it’s expected mission concepts will need to be finalizedby 2025, at the latest. In other words, crunch time is coming.
Should a mission go forward, the two most importantgoalsfor NASA’s scientists will be to determine the interior makeup of ice giants (exactly what they are made of) and their composition (how they are formed).
Other objectives will include studying their magnetic fields, which arevery differentfrom gas giants and all other types of planets.
They’ll also want to study the heat released by both Uranus and Neptune, which both have average temperatures of around -200℃. All giant planets are meant to be very slowly cooling down, as they release energy gained during their formation.
This heat release can be detected for Jupiter, Saturn, and Neptune. Uranus, however, doesn’t seem to release heat – and scientists don’t know why.
Article byChris James, ARC DECRA Fellow, Centre for Hypersonics, School of Mechanical and Mining Engineering,The University of QueenslandandYu Liu, Honorary Fellow,The University of Queensland
This article is republished fromThe Conversationunder a Creative Commons license. Read theoriginal article.
Story byThe Conversation
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