NASA’S JUNO MISSION IS TO BE CAPTURED BY JUPITER
AFTER traveling for five years and nearly 1.8 billion miles, NASA’s Juno spacecraft will announce its arrival at Jupiter with the simplest of radio signals: a three-second beep.
The long-awaited beep will also mark the end of a 35-minute engine burn to slow the spacecraft down and allow it to be captured by Jupiter’s gravity. NASA expects the beep to arrive at Earth at 10:53 p.m. Central time July 4.
“I can tell you when that completes, you’re going to see a lot of celebration,” said Rick Nybakken, Juno’s project manager, “because that means we’ll be in orbit around Jupiter, and that’ll be really cool.”
Juno’s mission is to explore the enigmas beneath the cloud tops of Jupiter. How far down does the Great Red Spot storm that has swirled for centuries extend? What is inside the solar system’s largest planet?
“We still have questions, and Juno is poised to begin answering them,” Diane Brown, Juno’s program executive, said during a news conference this month.
Juno will be the first craft to orbit Jupiter in more than a decade. NASA’s earlier robotic explorer, Galileo, spent eight years there and sent back astounding images of the planet and its many moons. It revealed features like a large ocean under the icy crust of the moon Europa, now considered one of the most promising places to look for life elsewhere in the solar system.
This time, the focus will be on Jupiter itself, and in particular what cannot be seen beneath its colorful cloud stripes.
“One of the primary goals of Juno is to learn the recipe for solar systems,” said Scott Bolton, a scientist at the Southwest Research Institute in San Antonio who is the principal investigator for the $1.1 billion mission. “How do you make the solar system? How do you make the planets in our solar system?”
Jupiter is the titan among planets called the “gas giants,” with more than three times the mass of Saturn, the next largest. But it is far more than a bland ball of hydrogen and helium.
What particularly piques scientists’ interest are the small amounts of heavier elements like lithium, carbon and nitrogen.
“Jupiter is enriched with these elements compared to the sun,” Bolton said. “We don’t know exactly how that happened. But we know it’s really important. And the reason it’s important is the stuff that Jupiter has more of is what we’re all made out of. It’s what the Earth is made out of. It is what life comes from.” On Monday, as the main engine on the spacecraft fires, in the control room at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., there will be nothing to control, and all anyone there will be able to do is wait and watch. If anything goes wrong, there is no way for anyone to intercede. The radio signals take 48 minutes to travel from Jupiter to Earth. By the time engineers receive word the engine firing has begun, the engine should have already switched off, with the spacecraft in orbit.
If the engine shuts off prematurely, Juno might still end up in orbit, albeit in the wrong orbit. If the engine fails, “we don’t end up in a very exciting spot,” Nybakken said.
In other words, Juno would zip right past Jupiter and end up in a useless orbit around the sun.
Through Monday evening, mission control will receive only a series of radio “tones” — three-second bursts at different frequencies — telling the sequence of operations the spacecraft is performing. To point the engine in the correct direction, the main antenna will not be pointed at Earth, preventing more detailed telemetry. The spacecraft will also not send back any photographs or data from the instruments, which will be shut down Wednesday, five days before its arrival, and will not be turned back on until a couple of days after its arrival.
Juno is to make a series of 37 highly elliptical orbits passing over Jupiter’s north and south poles over 20 months. At its farthest, it will be about 2 million miles from Jupiter. For each orbit, it will accelerate inward, passing within 3,100 miles of Jupiter’s cloud tops. The slight fluctuations in Jupiter’s gravitational pull, measured by shifts in the frequency of Juno’s radio signals, will tell the density of the planet’s interior and whether there is a rocky core within, where pressures might reach half a billion pounds per square inch. “We don’t really know if there is a core in the middle of Jupiter,” Bolton said. “If there is, it tells you sort of when and how and a little bit of where Jupiter must have formed.”
Juno’s science instruments include one to measure Jupiter’s powerful magnetic fields and an infrared camera to observe the glowing auroras around the poles. At depth, increasing pressures transform hydrogen from a gas into a liquid. At even greater depths, the hydrogen is squeezed so tightly that the electrons squirt out, changing it into a metal. It is probably the churning of liquid metallic hydrogen that generates the magnetic fields.
After the first two orbits, Juno is to fire its engine again to move into the orbit for making its scientific measurements.
Although the craft are very different in appearance, much of Juno’s electronics and programming are based on the design of Mars Reconnaissance Orbiter, an earlier spacecraft also built by Lockheed Martin.
Instead of developing electronic circuits that could operate in Jupiter’s intense radiation, Lockheed Martin used the same circuitry as Mars Reconnaissance Orbiter but shielded it within the half-inchthick walls of a 400-pound titanium vault measuring about 1 yard on each side.
Unlike earlier missions to the outer solar system, Juno is powered by sunlight, not plutonium. Three solar panels 30 feet long with a total of 18,698 solar cells gather the dim sunlight to produce about 500 watts.
Even if everything goes better than planned, the mission will not last much beyond the planned 20 months. Despite the titanium armoring, “we know the radiation is going to kill us,” said Guy Beutelschies, the director of interplanetary missions at Lockheed Martin.
Juno is expected to receive a radiation dose equivalent to more than 100 million dental X-rays.