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Pegasus/AIM preview

An air-launched Pegasus rocket will loft NASA's AIM satellite into orbit to study mysterious clouds at the edge of space. On the eve of launch, officials held these briefings from Vandenberg Air Force Base, California.

 Mission | Science

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AIM mission overview
Posted: April 24, 2007

Artist's concept of AIM. Credit: NASA

What is AIM?

AIM is an acronym for Aeronomy of Ice in the Mesosphere. Aeronomy is a science that deals with the physics and chemistry of the upper atmosphere of planets.

What is NASA's AIM Mission?

AIM is a two-year mission to study Polar Mesospheric Clouds (PMC's), the highest clouds in our atmosphere, that form an icy layer 50 miles above the Earth's surface at the edge of space. Over the course of its two-year mission, AIM will collect the data needed to address the fundamental question of why these clouds form and vary. The mission will document for the first time, the entire complex life cycle of these clouds. With this information, scientists will be able to resolve many of the mysteries about how these clouds form and be better able to predict how they will change in the future.

What are the mission science goals?

The goal of the AIM mission is to determine why PMCs form and vary.

What is unique about this mission?

AIM is the first satellite mission dedicated to the study of Polar Mesospheric Clouds (PMCs). These clouds are important because it is believed that they may be indicators of climate change in the upper atmosphere. They usually exist at high latitudes in the summer near 83 km (50 miles) above the Earth surface. They are special because they are visible in the night sky after sunset. Prior to 1885, these clouds had never been observed. Today they are a common phenomenon in many locations and they are changing; they are becoming more frequent, they are brighter and they are being observed at lower latitudes than ever before. We need to understand what is causing these changes and allowing these clouds to become more commonplace. In order to do that we need to understand why these clouds form in the first place. There are many mysteries about PMC formation. The atmosphere at the formation altitude is 100,000 times dryer than the Sahara desert. It is a hostile environment for clouds. AIM will measure the relevant environmental conditions of the upper atmosphere as well as microphysical properties of the clouds to answer why PMCs form and why they vary.

What are the primary objectives of the mission?

PMC Microphysics: What is the global morphology of PMC particle size, occurrence frequency and dependence upon H2O and temperature?

Gravity Wave Effects: Do gravity waves (GWs) enhance PMC formation by perturbing the required temperature for condensation and nucleation?

Temperature Variability: How does dynamical variability control the length of the cold summer mesopause season, its latitudinal extent and possible interhemispheric asymmetry?

Hydrogen Chemistry: What are the relative roles of gas phase chemistry, surface chemistry, condensation/sublimation and dynamics in determining the variability of water vapor in the polar mesosphere?

PMC Nucleation Environment: Is PMC formation controlled solely by changes in the frost point or do extraterrestrial forcings such as cosmic dust influx or ionization sources play a role?

Long-Term Mesospheric Change: What is needed to establish a physical basis for the study of mesospheric climate change and its relationship to global change?

What are Noctilucent Clouds?

Noctilucent clouds are the highest clouds in our atmosphere, occrring 50 miles above the surface, mostly in polar regions in the spring and summer of each hemisphere. In recent years, they have been observed as low as 40 degrees North which is the approximate latitude of Utah and Colorado. They are referred to as 'night shining' since they are visible from the ground when illuminated by sunlight from below the horizon while lower layers of the atmosphere and Earth surface are in the Earth's shadow.

What are some theories that AIM will address concerning Polar Mesospheric Clouds?

There are several theories about PMCs that will be addressed by AIM. For example a leading idea is that cosmic dust is key to PMC formation. AIM will address this theory with cosmic dust measurements made by a dust impact measuring devise on the top of the satellite and by cosmic dust observations made in the altitude region where the clouds exist. Another theory is that cold temperatures and water vapor are the driving forces for cloud formation but the relative importance of these variables is unknown. Still another theory is that gravity waves must play a major role in PMC existence because these waves appear to be reflected in the characteristics of the clouds, but the mechanisms and importance of these waves relative to other formation causes is unknown.

What are the instruments on this satellite and how do they contribute to the overall mission?

The AIM satellite carries three state-of-the-art instruments: Cloud Imaging and Particle Size (CIPS), Solar Occultation For Ice Experiment (SOFIE) and the Cosmic Dust Experiment (CDE). Each will take precise measurements of NLCs and related parameters in the Earth's upper atmosphere.

CIPS has four cameras positioned at different angles, allowing scientists a 2-D look at the clouds as the satellite passes and looks forward, back and to the sides at them. Multiple views of the clouds from different angles allows a determination to be made of the sizes of the ice particles that make up the cloud. The cameras will provide panoramic PMC images of the polar cap daily.

SOFIE will use solar occultation to measure cloud particles, temperature and atmospheric gases involved in forming the clouds. The instrument will reveal the recipe of chemicals believed to be involved in PMCs' formation. It will provide the most accurate and comprehensive look to date of ice particles and chemicals within the clouds as well as of the environment in which the clouds form.

CDE records the amount of space dust that enters the atmosphere from the cosmos. It will allow scientists to assess the role the particles have in PMC formation.

By observing the PMCs, chemicals and small dust particles for at least two years, the AIM mission is designed to answer the most important questions about the origin of these mysterious clouds.

Do researchers believe that there is a connection between PMC's and Global Change?

Scientists have pointed out a possible connection with global change because the clouds are becoming brighter, occurring more frequently with time and they are being observed at lower latitudes than ever before. One plausible explanation is that temperatures where the clouds form have become colder with time due to the build up in the lower atmosphere of greenhouse gases from human activities. High in the atmosphere, the greenhouse-gas buildup results in cooling.

AIM will test this hypothesis by providing a clearer understanding of why polar mesospheric clouds form and how they respond to short-term environmental changes. The comprehensive data from the mission will allow scientists to build computer simulations that reproduce the observed changes in these clouds. With these tools in hand, scientists will be able to predict future changes in the clouds and see to what extent they are an indicator of global climate change.

If one of the instruments were to fail could you still accomplish the mission?

The three AIM instruments were selected because of their ability to provide important data required to address the six science objectives. Some redundancy is included in the measurements both for cross checks between instruments and to provide additional measurement coverage should data from an instrument be lost. CIPS and SOFIE for example both measure PMC particle size while SOFIE and CDE both measure cosmic dust in different forms. The prime mission requires that data from SOFIE and CIPS be collected.

What new technologies are used on the spacecraft?

SOFIE uses a science enabling silicon carbide detector technology for ozone measurements that has high sensitivity at low ultraviolet wavelengths while at the same time has the ability to reject longer wavelength light that would otherwise render the measurement virtually impossible.

What will be going on with the spacecraft between orbital insertion and when the spacecraft begins its primary mission?

The spacecraft transmitter is turned on immediately after payload separation from the third stage of the Pegasus rocket and telemetry signals start immediately being sent to the NASA Tracking and Data Relay Satellite System (TDRSS). About a minute after separation primary and redundant commands are issued sequentially to deploy the solar panels to power the spacecraft. During this time, the magnetic torquer bars and spacecraft reaction wheels will null out any residual motion caused by rocket motions and the separation event. Approximately 82 minutes after separation, the attitude control system transitions to the sun pointing safe hold condition signaling start of a six day spacecraft commissioning activity to verify satisfactory performance of all spacecraft subsystems.

How long after launch will the spacecraft begin science operations?

SOFIE instrument turn-on and checkout will occur from L+7 to L+14 at which time it will begin science operations. Similar activities will occur for CIPS between L+11 to L+15 and for CDE from L+15 to L+17.

What is the orbit of the satellite and how is it relevant to collecting the appropriate science?

AIM will be placed in a 600km sun synchronous noon/midnight orbit. This orbit will concentrate the SOFIE measurements of atmospheric limb absorption of sunlight in the polar regions where most PMCs form. The orbit is also well suited for PMC data collection using the sub-satellite viewing CIPS high spatial resolution cameras.

What is the orbit of the satellite and how is it relevant to collecting the appropriate science?

AIM will be placed in a 600km sun synchronous noon/midnight orbit. This orbit will concentrate the SOFIE measurements of atmospheric limb absorption of sunlight in the polar regions where most PMCs form. The orbit is also well suited for PMC data collection using the sub-satellite viewing CIPS high spatial resolution cameras.

Who provided the spacecraft bus and the launch vehicle?

Orbital Sciences Corporation, Dulles Va., provided the AIM spacecraft bus, and performed instrument integration, satellite environmental testing and launch operations. AIM will be the fifth satellite built using Orbital's LeoStar-2 bus design. The launch vehicle is an Orbital Pegasus XL rocket which will be dropped from the Stargazer aircraft (Lockheed Tristar L-1011) at a pre-determined location off the California coast.

How will the AIM science benefit society?

It is clear that PMCs are changing; they are becoming more frequent, brighter and they are being seen at lower latitudes than ever before. This is science that the public can see and the changes are raising questions about human influences on our atmosphere. All these changes are signs that a distant and rarified part of our atmosphere is being altered and we do not understand how, why or what it means. These observations suggest a connection with global change in the lower atmosphere and could represent an early warning that our Earth environment is being altered.

Does the water vapor deposited into the atmosphere from Space Shuttle exhaust contribute to the formation of NLCs?

There is clear evidence that water vapor from shuttle exhaust can be transported to the polar regions and then lead to the formation of NLCs. What is unclear is the relative importance of Shuttle effects versus other causes of NLC formation. The effect cannot be dominant because the number of shuttle flights in northern summer are sparse over the history of the Shuttle program since its start in 98 and the variation in the number of flights does not correlate with the variation in PMC frequency of occurrence. AIM will measure the amount of ice present over the polar regions each day and from this, the enhancement of ice in response to shuttle launches can be determined. Thus AIM will quantify the role of shuttle exhaust in NLC formation.

Do noctilucent clouds form on other planets?

Noctilucent clouds have been observed several times in the atmosphere of Mars. The clouds have a somewhat similar appearance and are visible in the night sky because of their high altitude, just as on Earth. The primary difference is that on Earth the noctilucent clouds are composed of water ice. On Mars the clouds are composed of ice made from CO2 ice.

Who owns the AIM spacecraft? Is it best described as a Hampton University satellite or a NASA satellite?

The Explorers Program Office at Goddard Space Flight Center funded the AIM mission that is led by the Principal Investigator, James Russell of Hampton University. It is a NASA satellite but Hampton University is the prime contractor.

What are NASA's Strategic Goals for Heliophysics and how are they related to AIM science?

Understand Our Home in Space - Discover and understand the response of the Earth and near-Earth space to solar variability; Discover the origins of the Polar Mesospheric Clouds (PMCs); Determine the response of PMCs to climate change in the mesosphere. AIM bridges the disciplines of planetary, space and Earth sciences: Planetary sciences because high altitude noctilucent clouds have been observed in the Martian atmosphere and understanding their formation in Earth's atmosphere will help in understanding their existence in the Martian atmosphere; space science because PMCs occur literally on the edge of space in Earth's atmosphere and cosmic dust is a leading theory for the source of nucleation particles needed for their formation; and Earth science because the lower atmosphere is the primary source of water vapor needed for cloud formation in the high atmosphere and global changes in the lower atmosphere are likely connected with PMC changes observed in recent years.