Your picture shows the plasma trail behind STS-135, the last space shuttle entry in history.
The trail you’re seeing is therefore not actually the Orbiter
streaking across the Earth! The plasma trail behind it fades with
time, so the trail is brightest near the Orbiter’s position and
fainter as you backtrack along its path. Think of it as an afterglow
of the passing of Atlantis.
Why does this happen? The air gets heated by the Orbiter’s ramming the
atmosphere at 20+ times the speed of sound. And contrary to popular
belief, it’s not friction that heats the air, but compression. When
you compress a gas it heats up (like when a bicycle pump gets hot when
you use it a lot), and the Orbiter is screaming through the atmosphere
at hypersonic speeds. That compresses the air a lot. A shock wave
forms in front of the Orbiter, and the air begins to glow as it gets
heated up to temperatures as high as 1260° C (2300° F).
That’s what you’re seeing above: the shocked, rammed, and glowing air
as Atlantis pounded through it at several kilometers per second. And
it did this many, many times over its life… until this one final time,
caught on camera by astronauts high above the Earth.
Plasma trails were also visible from the ground if one was in the right place at the right time, as in this incredible picture of the STS-93 entry over Johnson Space Center. I was standing outside watching this in person, but I did not take this picture, sadly.
Here is a picture of the STS-107 plasma trail over California, before things went visibly wrong.
I do not know what caused the bump in the trail, but possible causes are:
- a maneuver by the shuttle (roll reversal)
- high altitude winds or other meteorology