Thursday, July 7, 2016
Juno Mission Set To Make Astronomical History
Observing Jupiter and its Galilean moons from an altitude of 10,500 feet last night, Janice and I marveled that at the same time the Juno spacecraft had begun its mission to finally pierce the mysteries shrouding Jupiter's structure and magnetic field.
For those who may have been living under rocks, Juno began its sojourn 5 years ago, on August 5, 2011. Since then - launched on an Atlas V rocket - it has traversed 1.74 billion miles along a circuitous route. Its initial trajectory took it beyond the orbit of Mars where the spacecraft's engine fired twice in the summer of 2012 to redirect it toward a flyby of Earth on October 9, 2013. In a classic 'slingshot effect';, the craft used some of Earth's orbital energy to gain 8,800 mph en route to Jupiter.
On July 4 it arrived to the jubilation of the Juno team at NASA. At that point, Juno entered the Jovan system at 165,000 mph, a record for the fastest human -constructed object. The craft then had to brake, literally threading the needle between the planet and its lethal radiation belts. The end result will be to position Juno in a 107 -day capture orbit split into two halves, or 53.5 days each. This will enable the Juno science team to test the probe's instruments during a close pass before the main mission begins.'
Subsequently this capture orbit will be adjusted and refined so by October Juno will be in a highly elliptical orbit swinging out as far as 1.17 million miles to the outermost moon Cellist, and as close as 3,100 miles to Jupiter's cloud tops. Thus the half 53.5 day orbit will be adjusted to a 14 day orbit. This will allow Juno to effectively render a global mapping of Jupiter's magnetic and gravitational field by its 8th orbit.
This orbit should prolong the craft's time at Jupiter from 15 to 20 months and increase its total orbits to 37. This extended time frame has advantages, in particular allowing the Juno team to react to unexpected events, conditions.
Probably the most serious arise from the planet's extraordinarily powerful magnetic field and radiation pervading its magnetosphere. For reference, Jupiter's magnetic field is thousands of times stronger than Earth's mean field - which is roughly 0.1 gauss. That means Jupiter's field rivals that detected in the strongest sunspots. It's also so strong it literally carves a cavity into the solar wind flowing past it.
Humans got their first hints of its lethality when the Pioneer 10 spacecraft flew past Jupiter at a distance of 5 million miles in December, 1973. The radiation levels were so intense that the electronics of Pioneer 10 sustained multiple circuit failures with an equivalent absorption 1,000 times the lethal dosage for a human.
The magnitude of the radiation effects were later confirmed when Voyager 1 and 2 flew past in 1979, and showed Jupiter's magnetotail extended out at least to Saturn's orbit 400 million miles further out.
For this reason, Juno has literally been designed as an orbiting "armored tank" , i.e. encased in a protective radiation shield or vault. Without such protection our information from the spacecraft would likely terminate in minutes, if not sooner.
Tied up with the powerful magnetic and radiation field effects is the origin for the planet's magnetosphere. Typically planets like Earth get their magnetic fields and magnetospheres on the basis of rotating iron cores - which generate the fields. However, in the case of Jupiter there is no reason to suspect such a similar core exists. After all, we are talking about conditions of extreme pressures of up to 4 million bars capable of stripping electrons from any molecular hydrogen in the atmosphere (e.g. H2) forcing it into en electrically conductive state we call "metallic hydrogen".
This is why many planetary astronomers are convinced the only core Jupiter can possibly sustain must be of metallic hydrogen, not iron. But this is one of the areas of conjecture we hope Juno can resolve.
Juno can and almost certainly also will peel back more of the structural layers of the planet, earlier exposed by the Casting spacecraft which mapped Jupiter from its north and out poles in December, 2000. These polar mappings showed a layered structure and with the famous 'Great Red Spot' actually situated two thirds of the way from the pole to the equator. This as opposed to being at the very top of the surface cloud layer.
Much of Juno's power will derive from a trio of 8.9m solar arrays, the first ever such set used this far from the Sun. Together they will generate nearly 500 watts or a half kilowatt. This is an incredible feat given that at Jupiter's distance the Sun is barely 4 percent of the brightness it has from Earth.
Jupiter's aurora, which I have posted about before - including early papers published by astronomers at the University of South Florida, will also come under scrutiny. Of particular interest is the Jovian Aurora Distribution Experiment. This will examine the angular distributions, energies, and compositions of ions and electrons at lower energies. Complementing it will be the Jupiter Energetic Particle Detector Instrument which will examine distributions and energies for hydrogen, helium, oxygen and sulfur particles at higher energies.
Then there is the Jovian Infrared Auroral Mapper which will investigate the actual dynamics of Jupiter's aurora as well as identify -assay water and ammonia abundance at depths of 30 to 45 miles (50 to 70 km) below its cloud tops. Bear in mind that unlike Earth, Jupiter's auroras are triggered via volcanic activity erupting at its moon, Io, which then interacts with the powerful magnetosphere of the planet.
We can be fairly certain of one thing: by the end of Juno's mission astronomical history will have been made, at least rivaling that when Galileo first spied the four moons (Galilean) circling the planet - standing most Church dogmas at the time on their head. And also firmly paving the way for acceptance of the heliocentric theory.