The energetic EUV and x-ray radiation from solar flares, and the dumping of tons of charged particles into the Earth’s atmosphere by coronal mass ejections, heat up the Earth’s atmosphere and make it expand. Satellites sense this expansion as an increase in density at the altitude at which they are flying. This atmospheric drag slows the satellite down, similar to your hand moving much slower through water than through air. Consequently, the satellite gradually loses height and –if not raised back to its normal altitude- eventually will return to Earth, mostly burning up during its passage through the atmosphere.
A well-known example is the premature re-entry of Skylab (image above) in 1979 due to the larger-than-anticipated solar activity during the rise of solar cycle 21. The re-entry occurred only 2 years before the launch of the first space shuttle, which could have raised it higher up into the Earth’s atmosphere. Another less dramatic, but certainly equally impressive example concerns the loss in altitude by the International Space Station (ISS) during the Bastille day event (14-15 July 2000). Instead of its usual daily 40-90 m loss in altitude, the station lost about 12 km (12.000 m) in altitude in just one single day! Fortunately, the ISS regularly gets a boost to keep it in its 330-410km high orbit.
Over the last few years, the Sun has been very benign towards satellites’ orbits. Indeed, compared to previous solar cycles, the solar activity during the ongoing solar cycle 24 has been quite low, with –so far- only a handful of strong X-class solar flares, and no extreme geomagnetic storms. This is good news for satellites: They can stay longer in the intended orbit and need much less boosts.
However, it also means that all the space debris, such as rocket bodies and other fragments, keeps lingering much longer in the atmosphere. Most of the fragments are small, but are -on the average- also travelling at high speeds, thus posing a continued collision risk to the operational satellites and astronauts during space walks (images underneath). Ground based radar and optical networks permanently can track the somewhat bigger space junk (up to 5 cm) and issue alerts if any dangerous collision might happen. That way, evasive manoeuvers can be taken similar to those taken by NASA last year to avoid a collision between the Fermi gamma-ray telescope and a defunct Russian Cold War spy satellite (see this NASA news item).
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