Solar S'Mores
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May
30, 2000 -- As most Boy and Girl Scouts can testify, if you
hold a marshmallow close to a roaring camp fire it puffs up.
A well-roasted marshmallow can grow to nearly twice its normal
size, doubling its allure to a voracious sweet tooth.
Something similar happens to Earth's atmosphere every 11 years
when the sunspot cycle nears maximum. As solar activity increases,
extreme ultraviolet radiation (EUV) heats our planet's gaseous
envelope, causing it to swell and reach farther into space than
normal. While puffed-up marshmallows can lead to tooth decay,
our puffed-up atmosphere vexes satellite operators with a different
kind of problem -- orbit decay.
Above: The space shuttle orbits in the thermosphere,
a tenuous layer of our atmosphere that gets hotter and expands
during solar maximum. The puffed-up thermosphere increases drag
on Low Earth Orbit (LEO) satellites.
"Orbit
decay" happens as these Low-Earth Orbit
(LEO) satellites move through the thermosphere. Each time
a LEO satellite circles the globe, its perigee (closest
approach to Earth) becomes a bit lower as aerodynamic drag robs
the satellite of orbital energy. The most famous example of this
effect was Skylab,
which burned up in the atmosphere on July 11, 1979 after its
orbit deteriorated for 5 years.Left: Layers of the Earth's atmosphere. The troposphere is the first layer above the surface and contains half of the Earth's atmosphere. Weather occurs in this layer. Many jet aircraft fly in the stratosphere because it is very stable. The stratosphere contains the ozone layer. Meteors burn up in the mesosphere. Aurorae occur in the lower thermosphere. The thermosphere is also where the space shuttle orbits. [more information from the University of Michigan]
Some satellites, like the Compton Gamma Ray Observatory, have onboard jets to compensate for orbit decay. When perigee gets too low, they can nudge themselves back to a higher altitude.
Other LEO satellites need a little help. The Hubble Space Telescope (HST) has no jets or engines of any kind for propulsion, so the only way to restore the altitude is to grab it and move it. This can and has been done by the space shuttle during HST servicing missions.
Just last week, astronauts flying the space shuttle Atlantis used the orbiter's jets to raise the altitude of the 35-ton International Space Station by 27 statute miles.
"The ISS will sink a couple of kilometers per year in the future because of atmospheric drag - in its current configuration," says Larry Kos, a NASA/Marshall Space Flight Center engineer with experience in computer modeling of the space station's orbit decay. "These kinds of 'reboosts' are entirely normal. Eventually the station will have its own propulsion system to compensate for orbital decay, but until the facility has a propulsion module, it's going to need occasional lifts from the shuttle."
The solar cycle has a big effect on the thermosphere where
satellite drag takes place, agreed David Hathaway. "During
solar minimum, the gas temperature in the thermosphere is around
700 °C. That's high, but not nearly as high as the temperature
during Solar Max. When the Sun is active, high levels of solar
EUV raise the temperature of the thermosphere all the way to
1,500 °C."
Increased solar heating makes the thermosphere puff out as denser
layers from lower altitudes expand upward. The density
of the thermosphere can soar by a factor of 50 during solar
maximum, with a commensurate increase in atmospheric drag on
satellites.
Above: This image, courtesy of Dr. Judith Lean at the
US Naval Research Laboratory, shows three extreme ultraviolet
(EUV) pictures of the Sun captured by the ESA/NASA Solar and
Heliospheric Observatory at different times during the current
solar cycle. In 1996, near solar minimum, the EUV Sun was nearly
featureless. Now, near the peak of the cycle, the Sun is dotted
by fiery regions of hot gas trapped in magnetic fields above
sunspots and plages. These active regions produce copious numbers
of EUV and X-ray photons that are absorbed in outer layers of
our atmosphere before they reach Earth's surface. The red curve
in the image is a computer model of the solar EUV flux at 304
Angstroms derived from ground-based Ca K images made at the Big
Bear Solar Observatory.
"The extreme ultraviolet photons that heat the thermosphere
aren't the same as the UV rays that give you sunburns,"
says Dr. Judith Lean, a physicist at the US Naval Research Labs.
"They are much worse. Sunburns come from the UV-A and UV-B
bands around 3000 Angstroms. The photons that heat the thermosphere
are at least 10 times more energetic and they vary 100 times
more [between solar minimum and solar maximum]. It's good thing
they're all absorbed
by nitrogen and oxygen at high altitudes -- otherwise a day
at the beach would be no fun."
If the thermosphere is so hot, wouldn't astronauts feel uncomfortably
warm during space walks? 
No, says Hathaway. The air up there is
so tenuous that you can't really feel the heat. In fact, it's
so thin that scientists can't even measure the temperature directly.
Instead, they put orbital decay to good use by monitoring the
drag on satellites to estimate the density of the rarefied air.
Then they can use the density to calculate the temperature --
proof that every cloud has a silver lining!
Right: NASA/Marshall's Larry Kos used a computer model
of the ISS orbit to project its orbital decay resulting from
atmospheric drag. Left uncorrected, the space station would decline
in altitude by less than 6 km over the next 1500 days. The software
that produced this approximate prediction is called LTIME (i.e.,
Lifetime). It has been developed and improved, most recently
by Mr. Jim McCarter, over the last 30+ years at the Marshall
Space Flight Center. Input parameters include the assembly stage
of the space station (solar panels, e.g., would increase
drag) and the response of the atmosphere to the solar cycle.
Another positive result of orbit decay involves space debris. According to the Orbital Information Group at NASA's Goddard Space Flight Center, in April 2000 there were 6133 bits of unwanted debris in Earth orbit, far outnumbering useful satellites. Astronauts on the space shuttle occasionally have to make course corrections to avoid these derelict pieces of space junk. Atmospheric drag on these objects can be good because it helps clear out the littered neighborhood of low Earth orbit. On the other hand, the changing orbits of these objects as they slowly reenter the bloated atmosphere make them more difficult to track for collision avoidance.
Above: The decay rate of the Solar
Maximum Mission, which deorbited in December 1989, varied
with the Sun's 27--day rotation and the solar cycle. This image,
which originally appeared in The Sun's Variable Radiation
and its Relevance for Earth (Annual Reviews of Astronomy
& Astrophysics, 1997) is courtesy of Dr. Judith Lean, NRL.
For more news and updates about space weather and solar activity,
please visit SpaceWeather.com.
Also: Do you have a favorite satellite, or would you like to
know where the International Space Station is tonight? Visit
NASA
Liftoff's J-Track for a 3D view of more than 600 Earth-orbiting
objects.
SOHO is a cooperative project between the European Space Agency
(ESA) and NASA. The spacecraft was built in Europe for ESA and
equipped with instruments by teams of scientists in Europe and
the USA.
SOHO
home page
-real-time images of the Sun, screen savers, and more
International Space
Station -- home page
The Thermosphere -- from the University of Michigan
Boosting Compton -- June 6, 1997, GSFC Astronomy Picture of the Day
Orbits in Space -- from the NASA/Goddard Space Flight Center