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.