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Cassini & Polar Join Forces to Study Earth

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Auroral camera will help interpret data
from Cassini flyby

Ultraviolet Imager supports planetary science

Aug. 17, 1999: A bit of interplanetary luck will let two separate space missions take advantage of each other's instruments Tuesday evening and produce measurements in a configuration that otherwise would not be possible.

Right: A May 4, 1998, view of the north polar region as seen by the Ultraviolet Imager aboard Polar from an altitude of about 37,672 km (23,356 mi). Credit: NASA/Marshall Space Flight Center

As the Cassini mission to Saturn flies past Earth, its magnetometer will be deployed to measure the Earth's magnetic field. At the same time, the Polar spacecraft will take a quick series of pictures of the aurora borealis.

"The Cassini project asked us to operate the Ultraviolet Imager at the highest time resolution as they fly by," said Dr. James Spann, a co-investigator for the UVI aboard Polar. "Since they are cutting through the Earth's magnetosphere, we may as well take advantage of their readings, too."

Polar is one of several satellites orbiting the Earth to study the space environment around our world. As the name suggests, Polar orbits over the Earth's poles. As it slowly arcs as high as 9 Earth radii (about 57,600 km) above the North Pole, its instruments can make long-term measurements of conditions where the magnetic field leaves the Earth open to space.

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Cassini is the most capable and ambitious planetary mission ever launched. It comprises a spacecraft to orbit Saturn and to drop the Huygens probe into the atmosphere of Titan. The mass of the spacecraft was too great for a direct flight to Saturn, so scientists devised a series of "gravity slingshots" - two flybys of Venus and one of Earth, plus one of Jupiter in December 2000 - to pump the orbit outward to Saturn.

In addition to using the Earth as a booster, Cassini's science team will do a little research. They will use the Visible and Infrared Mapping Spectrometer (VIMS), the magnetometer, and others to collect data that will help in calibrating their instruments when they take measurements at Saturn.

By chance, Polar will be high above the North Pole as Cassini zips past the Earth.

Left: A diagram of Cassini's flight past the Earth. Credit: Jet Propulsion Laboratory

"We'll be hanging over the pole, at about 9 Earth radii, with a pretty good view of the polar regions," Spann said. The spacecraft won't literally hang. It's always in motion but, like a basketball player leaping high for a basket, it gives the impression of being motionless.

During this "hang time," the UVI will take a picture every 37 seconds. By not cycling the filters, each image can be compared to the next.

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"For Cassini, we'll stay at one filter, the LBH-Long filter" Spann explained. "That way we can measure the changes in the aurora and measure the total energy deposited in the ionosphere."

LBH-Long refers to the Lyman-Birge-Hoffman band of ultraviolet light centered around 170 nm wavelength (the human eye can't see anything shorter than about 400 nm). The special filter, developed by the University of Alabama in Huntsville and NASA/Marshall, blocks most of the sunlight reflected by Earth and sky while just admitting ultraviolet generated as electrons slam into the upper atmosphere. This lets the UVI record the aurora even in broad daylight.

Web Links
How the Polar Ultraviolet Imager works and what it tells us about the ionosphere. Information about the filters also is available. Polar is part of the International Solar-Terrestrial Physics (ISTP) program
Where is Polar? Ask J-Track (caution: it's computationally intensive)
The Cassini misson home page carries moree extensive information on the spacecraft and its mission.

The intensity of that light is a direct measure of activity in the magnetosphere, the region of space defined by electrons and ions trapped by the Earth's magnetic field. As the solar wind squeezes the magnetosphere in and out, the aurora in effect acts as a footprint of what is happening downstream.

So, although Cassini will not fly through the open polar region, the Polar spacecraft's measurements will reflect conditions upstream in the magnetosphere and down the magnetotail as Cassini flies by.

Right: a computer plot shows a typical orbital track for Polar as viewed from the east, and from the direction of the sun. Credit: NASA/Goddard Space Flight Center

Having images from UVI will help the Cassini magnetometer team interpret variations during Cassini's 12 hours inside the magnetosphere. Measurements will also be taken by Wind and IMP-8 (an aging yet useful interplanetary monitoring platform) outside the magnetosphere, and by ground-based radar that sound the ionosphere from the surface of the Earth.

Cassini was to cross the bow shock (where the solar wind encounters the magnetosphere) at 9 p.m. EDT, Aug. 17 (~0200 UT 18 Aug), pass closest to Earth at 11:28 p.m. (0428 UT), cross the magnetopause at 7 a.m., Aug. 18 (~1200 UT Aug.l 18) about 75 Earth radii distant, and finally cross the shock wave back into interplanetary space at around 8 p.m. Aug. 18 to 6 a.m. Aug. 19 (0200 to 1200 UT 19 Aug) between 175 and 275 Earth radii. A final set of measurements will be made during Sept. 9-19 when the Earth's orbit around the sun sweeps the magnetosphere's distant wake across Cassini.

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Author: Dave Dooling
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