Related images
(click to enlarge)

[You must be registered and logged in to see this image.]NASA



The
world's eyes are on the Upper Atmosphere Research Satellite (UARS)
headed toward re-entry into Earth's atmosphere. The satellite is
currently predicted to re-enter sometime on the afternoon of Friday,
Sept. 23, 2011, but it hasn't been easy to precisely determine the path
and pace of UARS despite the fact that scientists well understand how
satellites move through space. The problem lies in the fact that space
itself changes over time -- the upper layers of Earth's atmosphere can
warm up and, more importantly, puff up in response to incoming energy
and particles from the sun. Satellites experience drag as they move
through the outer reaches of Earth's atmosphere, a large region of hot
gas known as the thermosphere. Like a marshmallow held over a campfire,
the thermosphere puffs up when heated by solar ultraviolet and
x-radiation. The more the thermosphere swells, the more drag satellite
experience. For satellites at lower inclinations and at low latitudes
near the equator, this increase in energy mostly comes from sunspots
and solar flares. The number of photons at the shorter wavelengths can
increase by up to 100 times or more within a few minutes due to a
single flare, and can then last up to a day before returning to
pre-flare levels.

Over the last few weeks a variety of solar events have affected the
density of the thermosphere. With an increase in active sunspots,
there's been a sharp uptick in extreme ultraviolet photons being
deposited into Earth's upper atmosphere. In addition, a large flare,
categorized as an X1.4 class flare, peaked on September 22 at 7:01 AM
ET. The output of this particular flare could increase the draft on
satellites at heights of 300 miles by up to about 50%, but at UARS'
current altitude of about 110 miles, it will only experience a change
in drag of under 1%, but it nevertheless represents the varying solar
activities that make for an environment that is difficult to categorize
one moment to the next. And that environment has everything to do with
the amount of atmospheric drag on UARS and, consequently, its re-entry
time. New events occurring on the currently very active Sun might
affect the re-entry of UARS even more.

Solar activity driving premature reentry of satellites is not new.
Strong solar activity caused the early reentry of the Skylab orbiting
observatory, and the famous "Halloween Storms" of Oct-Nov 2003 caused
the Student Nitric Oxide Explorer (SNOE) satellite to reenter earlier
than expected before these storms occurred.

Measurements by Solar Dynamics Observatory and other spacecraft in
NASA's heliophysics fleet are currently helping to refine how
scientists and spacecraft operators can predict drag on all satellites.]