Weather May Disrupt Cassini Signals
|With antennas in Spain; near Canberra, Australia; and in California's Mojave Desert, the Deep Space network has the ability to provide radio communications with spacecraft at all times. The three sites are spaced approximately one-third of the way around Earth from each other so they cover spacecraft in any direction as the world turns.
"Everything still appears to be right on track." That was the word from Robert Mitchell, Cassini program manager, as he addressed reporters Wednesday morning at a briefing at the Jet Propulsion Laboratory (JPL) in Pasadena, California. Cassini, a $3 billion international mission to explore Saturn and its rings and moons, is scheduled to arrive at the ringed planet tonight. But Mitchell offered a caveat. Things are on track at the Saturn end of things, he said. Back on Earth, however, predicted high winds threaten to force engineers to stow a massive dish-shaped antenna at Canberra, Australia, to protect it from possible damage. If this antenna is stowed during Cassini's Saturn orbit insertion (SOI), flight controllers on Earth will not be able to receive a signal from the spacecraft during the majority of the manuever. This will not affect the SOI itself (the spacecraft will be under the automatic control of its onboard computers), but it will undoubtedly add to the drama in the JPL control room.
The Canberra antenna is one of three 70-meter (230-foot) dishes that comprise the Deep Space Network (DSN). Each of the three antennae is placed one-third of the way around Earth from the other two. In addition to the one at Canberra, there is an antenna in the Mojave Desert near Goldstone, California, and one near Madrid, Spain. The DSN is responsible for communicating with all interplanetary spacecraft. Their placement ensures that at least one of the three is always available to communicate with spacecraft anywhere in the solar sytem. Unless the weather decides not to co-operate.
|Cassini transmits using a large (Italian-built) antenna to the global DSN dishes.
Cassini has two antennae, a high-gain and a low-gain, for communicating with Earth. The bulk of the scientific data collected by the spacecraft will be sent back using the high-gain antenna, which is more powerful and can transmit at a higher data rate than the low-gain. But before the SOI begins, the high-gain antenna will be turned to point away from Earth, to serve as a protective shield against possible damage by dust particles as Casssini passes through the plane of Saturn's rings. The antenna is durable enough to withstand these collisions, but because the spacecraft will be travelling at speeds between 50,000 and 70,000 miles per hour, Cassini's sensitive scientific instruments could be irreparably damaged by slamming into a tiny particle of dust.
The signal that engineers will be listening for will be from the low-gain antenna: a simple carrier wave. By monitoring the frequency of this carrier signal, engineers will be able to determine whether the SOI does its job. This frequency will change as Cassini changes speed, and it is this change in frequency that will tell flight controllers whether or not the SOI has succeeded. Cassini has already begun speeding up under the tug of Saturn's gravity. By the time the craft passes through Saturn's ring plane for the first time, just before the SOI, it will be travelling at 22.5 kilometers per second (about 50,000 miles per hour). An hour and a half later, at the beginning of the SOI, its speed will have increased to 24 km/sec (53,700 mph). At the completion of the burn, it will be travelling at 30.44 km/sec (68,000 mph). Yes, that's faster than before the engines fire, but the engines won't fully counteract the speedup effect of Saturn's tremendous gravity; they merely reduce it. As the spacecraft changes speed throughout the SOI, the frequency of its carrier signal will continuously change. By monitoring this frequency, mission engineers will be able to track precisely the spacecraft's speed, and thus the success of the engine burn.
|Huygens parachutes onto Titan. "To give you an idea of how sensitive these antennas are, if we were to 'listen' to one spacecraft in the outer solar system by Jupiter or Saturn for 1 billion years and add up all the signal we collected, it would be enough power to set off the flash bulb on your camera once," said Peter Doms, manager of the Deep Space Network [DSN] systems program at JPL. Once a signal is received it begins its journey to the printers: the DSN data are relayed using microwave links, communications satellites, land lines, and submarine cables to their final destinations.
Once the burn is complete, while Cassini is still flying above Saturn's ring plane, the spacecraft will turn, pointing its instruments downward to take some snapshots of the rings. Scientists are excited about this chance to study these close-ups of Saturn's rings. What will these images look like? "We really don't know," says Mitchell. Their quality could range from "quite spectacular images with a lot of features" to "just kind of blurry and hazy." Interesting to scientists in any case, but " for somebody wanting to look at a gee-whiz kind of picture, we might not have that."
If the SOI completes successfully these images should begin to arrive back at Earth at about 5 a.m. PDT Thursday morning. Unless the weather intervenes again - this time, the rain in Spain. Cassini is scheduled to send image data back to Earth, some 4.5 hours after the beginning of the SOI. By then, Earth will have rotated one-sixth of the way around. Canberra will have passed out of contact with Cassini and the Madrid DSN antenna will be online. Rain is not in the current forecast for Thursday in Madrid, but summer storms are not uncommon there. Engineers aren't worried, though. If Cassini isn't able to send its images back through the Madrid antenna, it will simply wait a few hours until it can transmit them to Goldstone. And weather in the Mojave Desert is usually pretty good.
Related Web Pages
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Parkes Radio Telescope
DSN antennas in Spain; near Canberra, Australia; and in California's Mojave Desert
The Deep Space Network Summary
Projected Cassini orbit