'Standing on a comet': Rosetta mission will contribute to space weather research

A comet-bound spacecraft that's been in sleep mode for more than two years is scheduled to wake up on the morning of Jan. 20 -- beginning the home stretch of its decade-long journey to a mile-wide ball of rock, dust and ice.

If all goes as planned, Rosetta -- a European Space Agency-led mission that involves University of Michigan engineers and scientists -- will be the first craft to actually land on a comet as well as track it for an extended period of time.

The Philae lander will latch on to the core of comet 67P/Churyumov-Gerasimenko in November and the orbiter will operate until the end of 2015. No mission has ever attempted such an in-depth look at one of these relics of the earliest days of our solar system.

Engineers at U-M's Space Physics Research Lab built electronic components for an onboard instrument that's believed to be the most sensitive of its kind ever flown in space. And a team of researchers will be involved in the mission science as well.

While most of the big questions Rosetta aims to answer deal with the origin and evolution of the solar system, U-M scientists will make a unique contribution that could provide very practical insights into how the sun and planets interface today.

They'll analyze measurements taken at the comet to study solar wind interactions that can lead to solar storms. The solar wind is a stream of charged particles emanating from the sun. Solar storms are bursts of activity that can threaten astronauts and damage Earth's satellites and electric grid.

"How the solar wind operates is one of the biggest outstanding questions about the solar system today. By studying how it interacts with cometary gases, we can learn a lot about the composition of the solar wind," said Tamas Gombosi, the Rollin M. Gerstacker Professor of Engineering in the Department of Atmospheric, Oceanic and Space Sciences.

Gombosi and his research group are leaders in the field of space weather. A model they developed was recently adopted by the national Space Weather Prediction Center.

At the sun's equator, the wind travels rather slowly, Gombosi said. It moves faster at high latitudes. Interactions between the two varieties can lead to magnetospheric storms. Earth orbits near the equator, so it's hard to study the fast wind from our vantage point.

"But comets pass through all of it. With their help, we can study the fast solar wind," Gombosi said.

Gombosi and other U-M researchers will be involved in additional Rosetta goals. They'll study and simulate how quickly the comet outgases material from its nucleus to its tail as it rings around the sun. They'll be involved in examining what elements are in the comet's tail, atmosphere and ionosphere, as well as how fast the electrified particles in the ionosphere are traveling.

Michael Combi, the Freeman Devold Miller Collegiate Research Professor in the Department of Atmospheric, Oceanic and Space Sciences, is a co-investigator on several instruments. He'll look into the rate at which the comet's core is sublimating, or turning from a solid into a gas, and he'll also work on a team that's analyzing those gases. They'll explore the levels of carbon monoxide and carbon dioxide, for example. They can't detect carbon dioxide from Earth.

"It's very difficult to observe some of the chemical species when they're far away and faint. Carbon dioxide is probably the second most abundant species at most comets, but it's not been observed in the thousands we've looked at from Earth," said Combi, who has studied comets for more than 30 years.

Comets -- small rock and ice bodies -- were present in the nebula that spawned the solar system and have been orbiting ever since in far away, cold belts either just past the orbit of Neptune or a quarter of the distance to the nearest star. For scientists, they're archeological artifacts that help them understand how the solar system formed and evolved. They're believed to have delivered Earth's oceans and perhaps the seeds of life in organic materials.

"People use the analogy that it's been in the freezer for the past 4.5 million years and brought in for convenient study. So we're looking as much as we can at the way the way the solar system was 4.5 billion years ago," Combi said.

Comet 67P/Churyumov-Gerasimenko is one of the smallest bodies humans have ever tried to land on. Its gravity is about 1,000 times less than that of Earth.

"On the lander, there's a camera that can look straight down like you're standing up and looking at the ground. Then there's a panoramic camera that can look out and see a picture of the horizon. It'll be fun to see what this landscape looks like," Combi said. "It'll be like standing on a comet."

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Raindrop research dials in satellite forecasting accuracy

Dialing in the accuracy of satellite weather forecasting is the goal behind basic research into raindrop size and shape being done at The University of Alabama in Huntsville by a UAH doctoral student who is also an atmospheric scientist in the NASA Pathways Intern Employment Program.

Patrick Gatlin says his work measuring the height and width of raindrops using ground instruments provides an accuracy baseline that is then scaled up to ground radar and then to satellite measurements. He is co-author of a paper on the topic.

"That's really the whole purpose of measuring raindrops, is for remote sensing purposes," Gatlin says. Scaling up accuracy from a small sensor on the ground to large sections of the Earth being observed from space requires very accurately calibrated instruments. "Our ability to correctly depict rainfall using a sensor in space is closely tied to knowing how precipitation varies, right down to the individual raindrop and snowflake size."

Perfecting ground-level instrument observations, enlarging those to encompass ground-based radar and then going even larger to develop accurate satellite measuring instruments is the best way to reduce error as the area under observation increases. "Before we invest in all this satellite instrumentation," Gatlin says, "let's make sure we've got it right."

A coming big step in scaling up precipitation forecasting is NASA's planned launch of its Global Precipitation Measurement (GPM) satellite toward the end of February. UAH is a mission contractor, headed at the university by Dr. Larry Carey, an associate professor of atmospheric science, and involving UAH Earth System Science Center research scientist Matt Wingo, who is working with NASA at their flight facility in Wallops Island, Va.

"UAH designed the platform for some of the ground-based instruments that will validate the information from the satellite," says Gatlin.

Carrying an advanced radar/radiometer system to measure precipitation from space, the GPM is the core of what will be a global network of measuring satellites that will provide next-generation global observations of rain and snow. It will serve as a reference standard to unify precipitation measurements from a constellation of research and operational satellites.

Through improved measurements of precipitation globally, the GPM mission will help to advance understanding of Earth's water and energy cycle, improve forecasting of extreme events that cause natural hazards and disasters, and extend current capabilities in using accurate and timely precipitation information.

In his own research, Gatlin has ranged from Iowa and Oklahoma to Canada, Finland, Italy and France. Rather than raindrops, the Canadian research was designed to collect snowflake images in order to improve the accuracy of measuring devices for snowfall.

In each locale, an integrated network of ground-level measuring devices have been deployed, like the Parsivel2, a disdrometer that measures the particle size and velocity of raindrops falling through a laser. Also in use are two-dimensional video disdrometers, which use two video angles to create 2-D pictures that enable determination of raindrop shapes. A video disdrometer on loan from frequent research collaborator Colorado State University is located on the UAH campus behind Cramer Hall.

During a field study, the instruments on the ground take measurements while a plane flies through the clouds to collect actual raindrop information and another flies high above the clouds with remote sensing equipment to mimic satellite radar detection. Results from all the measurement methods are compared.

Enhanced satellite-based precipitation measurements will improve both rainfall and snowfall predictions on a global scale, Gatlin says. "We'll be measuring rain and snow in some areas where we've never measured it before." The ability to better measure raindrop size also can have impact on severe weather forecasting, as small raindrops lead to higher evaporation rates that have been correlated with larger and more forceful microbursts by UAH's Dr. Kevin Knupp and others.

Gatlin is about to finish up a global study focusing just on very large raindrops 5 millimeters in size and larger. These drops are difficult to capture in the small measuring area afforded by measuring instruments, and so their observation is rare. Gatlin says out of 224 million drops he has researched, only 8,000 have been 5 mm or larger.

"Even though large raindrops can have the greatest impact on radar measurements, we don't have a good idea of their concentration," he says. "What I've been doing is bringing together all the raindrop data bases that have collected various types of rainfall data using the same techniques."

Interestingly, while Sumatra holds the honor of having the greatest number of large drops overall, the largest drop collected in his study fell through a measuring device at the UAH campus. It measured 9.1 mm and was formed in a hailstorm when a falling piece of hail melted before landing.

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