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“Gateway to space” mission lives on

Artist's concept of Earth's atmosphere

Artist's concept of Earth's atmosphere from 0 to 700km, showing the mesosphere, thermosphere and ionosphere (MTI) regions. It also shows various MTI atmospheric phenomenon and the atmospheric range in which they occur. The TIMED spacecraft orbits at an approximate altitude of 680 km.

  • TIMED satellite explores little-known region high above Earth
  • Important for studying effect on the Sun on our planet
  • Spacecraft still healthy and mission extended for a fourth time

Nine years after beginning its unprecedented look at the gateway between Earth’s environment and space—not to mention collecting more data on the upper atmosphere than any other satellite—NASA’s Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission has been extended again.

Before the launch of TIMED, the mesosphere and lower thermosphere/ionosphere—which help protect us from harmful solar radiation—had been one of the least explored and understood regions of our environment.

“The middle part of the atmosphere was the part we kind of ignored,” says John Sigwarth, the deputy project scientist for TIMED at NASA’s Goddard Space Flight Centre in Greenbelt, MD.

“It’s too high for balloons and too low for spacecraft. So the understanding of this middle atmosphere and its impact on the upper atmosphere has been tremendously increased due to TIMED.”

The primary science objective of the TIMED mission is to understand the energy transfer into and out of the Mesosphere and Lower Thermosphere/Ionosphere (MLTI) region of the Earth’s atmosphere (energetics), as well as the basic structure (i.e., pressure, temperature, and winds) that results from the energy transfer into the region.

Artist's concept of TIMED spacecraft

Artist's concept of the TIMED spacecraft, which orbits Earth at 680km altitude.

The mission will now continue to study the influences of the Sun and humans on our upper atmosphere. TIMED began its extended mission on October 1, 2010, and will collect data through 2014. This is its fourth extension since the original 2-year mission began in January 2002.

A new focus

TIMED will focus this time on a problem that has long puzzled scientists: differentiating between human-induced and naturally occurring changes in this atmospheric region. This extension also allows TIMED to continue collecting data for longer than a full 11-year solar cycle.

“The Sun is a variable star with an 11 year cycle,” says Sigwarth. “So, if things change in the mesosphere, you don’t know if it’s because the Sun changed or because human activity has caused the change.”

“By getting back to the same point in the cycle, we can compare what it was like then, and what it’s like now, and see if there’s a long term trend of changes that’s not solar related.”

The key instrument performing this work is known as SABER (or Sounding of the Atmosphere using Broadband Emission Radiometry), built by Hampton University in Hampton, Virginia. SABER can remotely sense composition and temperature in the mesosphere.

In addition to checking for effects from humans, TIMED scientists would like to understand how cooling temperatures in the middle atmosphere are causing the thermosphere to become less dense and its composition to change.

With fewer particles in the thermosphere, there’s less drag on satellites in space, which affects how long spacecraft and space debris stay in orbit—information that must be integrated into calculations for orbit models.

Composition changes in the thermosphere can also alter ionospheric structures that affect radio wave propagation and communications. To help with this is an instrument called SEE (or the Solar EUV Experiment) built at the University of Colorado, which looks at the Sun’s x-rays and extreme ultraviolet rays to see how they impact our atmosphere.

TIMED will also collaborate with NASA’s newest eye on the Sun, the Solar Dynamics Observatory, which provides continuing solar radiation measurements and new views of how solar activity is created.

Adapted from information issued by NASA GSFC / JPL/APL.

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Surfing an alien atmosphere

Artist's impression of Venus Express in orbit around Venus

Venus Express has two solar power wings with two panels per wing. As Venus is closer to the Sun, there is twice as much sunlight in Venusian orbit as there is in Earth orbit.

The European Space Agency’s Venus Express spacecraft has completed an ‘aerodrag’ campaign that used its solar “wings” as sails to catch faint wisps of the planet’s atmosphere.

The test used the orbiter as an exquisitely accurate sensor to measure atmospheric density barely 180 km above the hot planet.

During the five aerodrag measurements, Venus Express’ solar arrays and control systems were operated as one big flying sensor, with the solar arrays rotated at various angles to the direction of flight.

The special configuration exposed the wings to the faint wisps of atmosphere that reach to the boundary of space around the planet, generating a tiny but measurable aerodynamic torque, or rotation, on the satellite.

This torque can be measured very accurately based on the amount of correction that must be applied by reaction wheels, which counter-rotate inside the spacecraft to maintain its orientation in space.

The solar panels rotated through five daily-changing sets of orientations. While one panel remained perpendicular to the direction of flight, the other rotated in steps, gradually increasing the torque to be counter-balanced by the reaction wheels.

On the last day, the solar arrays were rotated at plus and minus 45° to the atmospheric flow, mimicking the vanes of a windmill, in order to gather additional information on the behaviour of the molecules of the atmosphere bouncing off the solar wings.

Venus Express seen during pre-launch testing

Solar arrays on Venus Express seen during pre-launch testing in September 2005.

More tests planned

“The aerodrag campaign went without problem, and conclusively demonstrated that Venus Express can be securely and accurately used to sense the density of the planet’s atmosphere. Venus Express has shown once again that it is a very capable satellite,” said Spacecraft Operations Manager Octavio Camino.

The mission operations team will study the results to develop an optimised configuration for more aerodrag campaigns in October 2010 and in 2011. Aerodrag testing was also conducted in 2008, 2009 and February 2010.

Continued positive results may enable Venus Express to conduct more sophisticated investigations deeper in the atmosphere, which would be of immense interest to planetary scientists.

The solar array on Venus Express comprises two symmetrical wings supporting gallium-arsenide solar cells. Their combined 5.7 sq m can generate up to 1400 W of power in Venus orbit.

Adapted from information issued by ESA.