rAt 20:18 PDT on Monday, July 4th (03:18 UT, Tuesday, July 5th) a spacecraft called Juno will fire its UK-built Leros-1b engine to commence a 35-minute burn designed to allow the spacecraft enter an initial orbit around the largest planet in the solar system, ready to begin a comprehensive science campaign.
As I write this, the craft is already inside the orbit of Callisto, the furthest of Jupiter’s four massive Galilean satellites, which orbits the planet at a distance of roughly 1.88 million kilometres. During the early hours of July 4th, (PDT), the vehicle will cross the orbits of the remaining three Galilean satellites, Ganyemede, Europa and Io, prior to commencing its orbital insertion burn.
In the run-up to the burn, Juno will complete a series of manoeuvres designed to correctly orient itself to fire the Leros-1b, which will be the third of four planned uses of the engine in order to get the craft into its final science orbit. Two previous burns of the engine – which NASA regards as one of the most reliable deep space probe motors they can obtain – in 2012 ensured the craft was on the correct trajectory from this phase of the mission.
Getting into orbit around Jupiter isn’t particularly easy. The planet has a huge gravity well – 2.5 times greater than Earth’s. This means that an approaching spacecraft is effectively running “downhill” as it approaches the planet, accelerating all the way. In Juno’s case, this means that as the vehicle passes north-to-south around Jupiter for the first time, it will reach a velocity of nigh-on 250,000 kph (156,000 mph), making it one of the fastest human-made objects ever.
Slowing the vehicle directly into a science orbit from these kinds of velocities would take an inordinate amount of fuel, so the July 4th manoeuvre isn’t intended to do this. Instead, it is designed to hold the vehicle’s peak accelerate at a point where although it will be thrown around Jupiter and back into space, it will be going “uphill” against Jupiter’s gravity well, decelerating all the time. So much so, that at around 8 million kilometres (5 million miles) away from Jupiter, and travelling at just 1,933 kph (1,208 mph), Juno will start to “fall back” towards Jupiter, once more accelerating under gravity, to loop around the planet a second time on August 27th, coming to within (4,200 km (2,600 mi) of Jupiter’s cloud tops, before looping back out into space.
On October 19th, Juno will complete the second of these highly elliptical orbits, coming to within 4,185 km (2,620 mi) of the Jovian cloud tops as it completes a final 22-minute burn of the Leros-1b motor. This will be sufficient for Jupiter’s gravity to swing Juno into an elliptical 14-orbit around the planet, passing just 4,185 km from Jupiter at its closest approach before flying out to 3.2 million kilometres (2 million miles) at it’s furthest from the planet.
The July 4th insertion burn is also significant in that it marks the end of a 5-year interplanetary journey for Juno, which has seen the vehicle cover a distance of 2.8 billion km (1.74 billion miles).
It’s a voyage which began on August 5th, 2011, atop a United Launch Alliance (ULA) Atlas V, launched from Cape Canaveral Air Force Station, Florida.
As powerful as it is, the Atlas isn’t powerful enough to send a payload like Juno directly to Jupiter. Instead, the craft flew out beyond the orbit of Mars before dropping back to Earth, passing us again in October 2013 and using Earth’s gravity to both accelerate and to slingshot itself into a Jupiter transfer orbit.
While, at 35 minutes, the engine burn for orbital insertion is a long time, the distance from Juno to Earth means that confirmation that the burn has started will not be received until 13 minutes after the manoeuvre has actually completed. That’s how long is takes for a radio signal to travel from the vehicle back to Earth (and obviously, for instructions to be passed from Earth to Juno. Thus, the manoeuvre is carried out entirely automatically by the vehicle
Juno is not the first mission to Jupiter, but it is only the second orbital mission to the giant of the solar system.
The Jovian system was first briefly visited by Pioneer 10 in 1973, followed by Pioneer 11 a year later. Both of these were deep space missions (which are still continuing today), destined to continue outward through the solar system and into interstellar space beyond. They were followed by the Voyager 1 and Voyager 2 missions in January and July 1979 respectively, again en route for interstellar space by way of the outer solar system.
In 1992 the Ulysses solar mission used Jupiter as a “slingshot” to curve itself up into a polar orbit around the Sun. Then in 2000, the Cassini mission used Jupiter’s immense gravity to accelerate and “bend” itself towards Saturn, its intended destination. New Horizons similarly used Jupiter for a “gravity assist” push in 2007, while en route to Pluto / Charon and the Kuiper Belt beyond.
It was in 1995 that the first orbital mission reached Jupiter and its moons. The nuclear RTG-powered Galileo was intended to study Jupiter for just 24 months. However, it remained largely operational until late 2002 before the intense radiation fields around the planet took their final toll on the vehicle’s systems. Already blind, and with fuel supplies dwindling, Galileo was ordered to crash into the upper limits of Jupiter’s atmosphere in 2003, where it burned up.
In the eight years it operated around Jupiter, Galileo complete changed our perspective on the planet. Juno has a 20-month primary mission, and it is hoped its impact on our understanding of Jupiter will be greater than Galileo’s. However, it is unlikely the mission will be extended.
Unlike all of NASA’s previous missions beyond the orbit of Mars, which have used RTG power units, Juno is entirely solar-powered, making it the farthest solar-powered trip in the history of space exploration. However, the three 8.9 metre (29 ft) long, 2.7 metre (8.9 ft) wide solar panels are particularly vulnerable to the ravages of radiation around Jupiter, and it is anticipated that by February 2018, their performance will have degraded to a point where they can no longer generate the levels of electrical energy required to keep the craft functioning – if indeed, its science instruments and electronics haven’t also been damaged beyond use by radiation. This being the case, Juno will be commanded to fly into Jupiter’s upper atmosphere and burn up.
Juno’s science mission is improve our understanding of Jupiter’s formation and evolution. The spacecraft will investigate the planet’s origins, interior structure, deep atmosphere and magnetosphere. Juno’s study of Jupiter will help us to understand the history of our own solar system and provide new insight into how planetary systems form and develop in our galaxy and beyond. It will also, for the first time, allow us to “see” below Jupiter’s dense clouds.
Hence the name of the mission: while the Roman god Jupiter attempted to hide his mischief from his wife, Juno, by weaving a mantle of cloud around himself, so was see able to look through those clouds and see Jupiter’s true self.
In order to achieve its goal, the Juno science mission has a number of key objectives:
- Atmospheric studies:
- Determine the ratio of oxygen to hydrogen, effectively measuring the abundance of water in Jupiter’s atmosphere, which will help distinguish among prevailing theories linking Jupiter’s formation to the Solar System
- Map the variation in atmospheric composition, temperature, structure, cloud opacity and dynamics to pressures far greater than 100 bars (10 MPa; 1450 pound/sq inch) at all latitudes
- Structure and fields:
- Obtain a better estimate of Jupiter’s core mass, which will also help distinguish among prevailing theories linking Jupiter’s formation to the Solar System.
- Precisely map Jupiter’s gravitational field to assess the distribution of mass in Jupiter’s interior, including properties of its structure and dynamics.
- Precisely map Jupiter’s magnetic field to assess the origin and structure of the field and how deep in Jupiter the magnetic field is created. This experiment will also help scientists understand the fundamental physics of dynamo theory
- Characterise and explore the three-dimensional structure of Jupiter’s polar magnetosphere and its auroras.
- Measure the orbital frame-dragging, known also as Lense–Thirring precession caused by the angular momentum of Jupiter, and possibly a new test of general relativity effects connected with the Jovian rotation.
To achieve these objectives, the vehicle caries a suite of nine science instruments, of which – and purely from a public outreach perspective – the JunoCam will be of the greatest interest. A visible-light camera/telescope, JunoCam uses a uses a Kodak image sensor capable of capturing colour images at 1600 x 1200 pixels, almost twice that of the primary imaging system used by Galileo, and which is expected to return the most detailed images of Jupiter, with a resolution of 15 km per pixel.
Again, due to the intense radiation levels the craft will experience while in orbit around Jupiter, JunoCam is only expected to operate for the first 7 or 8 orbits of Juno’s primary mission.
As well as the science payload, Juno is carrying four special items. The first is a plaque dedicated to Galileo Galilei. Supplied by the Italian Space Agency, the plaque depicts a portrait of Galileo and a text in Galileo’s own hand, penned in January 1610, while observing what would later be known to be the Galilean moons. The text reads:
On the 11th it was in this formation, and the star closest to Jupiter was half the size than the other and very close to the other so that during the previous nights all of the three observed stars looked of the same dimension and among them equally afar; so that it is evident that around Jupiter there are three moving stars invisible till this time to everyone.
Also travelling aboard the vehicle are three unique Lego figures, depicting Galileo with his telescope and holding a model of Jupiter; the Roman god Jupiter, holding his lightning bolt; and Jupiter’s wife, Juno, who is holding a magnifying glass, a reference to Juno’s ability to pierce Jupiter’s veil of clouds. In order to withstand the rigours of the mission, all three figures have been cast from aluminium rather than constructed from plastic pieces.
I’ll have more on the Juno mission is future Space Sunday reports.
NASA Test Fires World’s Most Powerful SRB
On Tuesday, June 28th, NASA, in partnership with prime contractor Orbital ATK, completed the second static firing of the most powerful solid rocket motor (SRM) in the world, and which forms a critical component in NASA’s new gigantic Space Launch System (SLS), designed to spearhead US government missions to cis-lunar space, to the Moon, and potentially to Mars.
The Solid Rocket Motor – so-called because it uses a “solid” fuel which looks somewhat like wet cement, rather than traditional liquid fuels – forms the main part of NASA’s Solid Rocket Boosters. These were a core – if controversial – element of the space shuttle launch system, being (the two booster rockets mounted on the sides of the shuttle’s huge External Tank. Controversial because while SRBs provide a lot more thrust than liquid fuelled rockets, once ignited they cannot be shut down until all their fuel is expended.
In fact, the entire SRB assemblies which will be used by the SLS are essentially an improved, single-use version of the space shuttle Solid Rocket Boooster. At 45.46 m (177 ft) in length, the SLS SRBs are over 9 metres (almost 30 ft) longer than their space shuttle progenitors, the extra length being taken up but a fifth segment of the Solid Rocket Motor, which gives each SLS SRB 25% more propellant its space shuttle equivalent.
Like the shuttle SRBs, the boosters for the SLs will be used in pairs with each SLS launch, providing some 70% of the total thrust required to lift the rocket through the densest part of Earth’s atmosphere. Although they contain 25% more propellent, they will in fact operate for the same 126 seconds as the shuttle’s SRBs prior to them shutting down and being discarded. Instead, they’ll burn their fuel much faster than the old space shuttle SRBs, at a rate of almost 6 tonnes per second. This will allow then to generate 25% more thrust than the shuttle SRBs – 3.6 million pounds of thrust apiece compared to 2.8 million pounds.
The SRBs are not the only element of shuttle derived hardware being used in the SLS. Each rocket will use four of NASA’s veritable RD25 engines, once known as the Space Shuttle Main Engine (SSME) for its core stage propulsion. In addition, the first stage of the SLS booster is designed around a modified version of the shuttle’s External Tank.
The June 28th firing was a critical step along the road to the first SLS booster launch, currently scheduled for 2018, and stands alongside similar static tests which have been carried out with the modified RD25 engines. If all goes according to plan, the next time the SRB system is fired, it will be on that maiden SLS flight.