April 18, 2003
Reboost
Credit: The crew of ISS Expedition
6, NASA
Explanation: The International Space Station is
sinking.
There’s no reason to panic. This
happens to all spacecraft in low Earth orbit as they skim the
diaphanous outer regions of Earth’s atmosphere. Aerodynamic friction
saps orbital energy, and the space station sinks about 100 meters
per day–easily correctable by means of a reboost.
Pictured above is a reboost in
action on
April 4, 2003. Ground controllers at Mission Control Moscow
ignited the thrusters of a Progress
rocket docked to the station’s Zvezda Service Module. The
14-minute firing raised the average altitude of the station by
about 3 km. One of the crew captured this
picture of the yellow-glowing thrusters from a window in
the Service Module.
The layer of Earth’s atmosphere
that causes the ISS to sink is called the
thermosphere. Although considered to be part of the atmosphere,
the thermosphere is space-thin–about 1012 times less
dense than air at sea level. The thermosphere responds to the
Sun: When solar activity
is high, the thermosphere puffs up like a marshmallow held over
a fire. It envelops low-orbiting spacecraft, accelerating orbit
decay. The 11-year
solar cycle crested most recently in 2001-02. For the next
few years, solar activity will wane, reducing the need for reboosts.
So far this month two Progress
thruster firings have lifted the altitude of the station’s orbit
a total of 5 km.
Note: The many white speckles in this digital image
are not stars. They are camera artifacts. | 
