Shuttle Reentry
Maley, Paul D. (PMALEY@GP808.jsc.nasa.gov)
Fri, 26 Jan 1996 13:29:56 -0600
As a long time shuttle observer (in orbit and during reentry) from Houston,
I would like to make the following comments. Maybe this will put to rest
some of the discussion on the net.
0. The shuttle reentry is one of the most spectacular things to be seen in
the night sky.
Reentries are land- limited to the a few southern states in the USA, Baja
California and northern Mexico for normal 28.5 degree inclination flights.
For higher inclination missions, some northwestern, central and eastern
states can witness the reentry if it occurs at night (and also parts of
Canada). But reentries do not normally occur at local night. With an 8-inch
telescope it is possible to observe what has been described as a non uniform
shape during the reentry process from Texas. The reentry resembles that of a
fireball with the head being the orbiter surrounding by glow with color(s)
that have ranged from yellow to white to red depending on the eye witness.
The tail looks like a Roman candle and also displays the same coloration
which varies by observer and location.
1.All of us here have been frustrated for years by the NASA decision to hold
back on publicizing reentry information but there are good reasons for this.
If you controlled the reentry, you would likely be faced with similar
concerns. We work within the system to try to get the best data when it
becomes available. A number of conditions could change the decision to
deorbit causing the orbiter to come down along a different track, time, or
day. In the public interest, NASA does not want to "cry wolf" and say the
shuttle WILL come down at a set time way in advance. More often than not
there is always a change in the deorbit time. The last mission (STS-72) was
a rare exception-----not just an exception, but a RARE exception. The
reentry was along the pre-mission predicted track, right on time in complete
darkness. Most reentries have been close to dawn in Texas with only dark
skies to our west. Most have historically involved a one orbit wave off or
even a day or more delay. Oops! a one orbit wave-off now pushes the track
into sunlight for most of the ground track restricting observations to
souther California and Arizona and always at low elevations above the
horizon. The decision to deorbit is always kept to the very end, since
weather at the Kennedy Space Center is the primary limiting factor.
2.I hope to bring a video to the next EUROSOM showing my first shuttle
reentry encounter in 1984. AVIATION WEEK AND SPACE TECHNOLOGY also published
the first photo of such an event which I took that year. The video reveals
some things of interest. There is more than one "trail". I have found two
parallel trails, one fainter than the other. The trail is tied in knots and
dissipation is not uniform. Last year one of the astronauts provided me with
a photo taken out of the overhead window during reentry which shows a twin
tail wake, confirming the double trail for the first time. When the vehicle
reaches our longitude , it is traveling about Mach 16 and at an altitude of
about 195,000 feet, 15 minutes before landing. We may or may not hear the
sonic boom(s), but more often than not we do hear them. Last year I sighted
the reentering orbiter with the sun 3 degrees above the horizon at Hobby
Airport here in Houston. It was barely visible to the eye. Just as we were
stepping onto the transfer bus to get from the parking area to the terminal,
we clearly heard the double boom. I presented a paper at a conference as
long ago as 1984 in Switzerland documenting the shuttle reentry phenomenon
but, of course, nobody was interested back then. Since then I have seen 8
reentries here.
3.The persistent trail is left due to heating of air molecules which are
excited during the combination process. Depending on where you are along the
ground track the persistence can last seconds to minutes in duration. From
our longitude (95 deg W) the duration has been shown to be around 90
seconds, though this too varies. From California, the persistence has been
reported up to 11 minutes at the lowest light levels (and the highest Mach
numbers). Brightness of the reentry can range from -4 in Baja Calfornia down
to +2 as it approaches Florida.
4. The shuttle is generally visible even through clouds, unless the decks
are very thick and multiple in nature. I have found that sound waves from
the sonic boom shock front can be detected by frogs croaking on a pond
before humans can detect the boom. Observers have seen the brightness
pulsate. In shuttle reentry videos taken by past crews this pulsation is
quite evident and occurs differently along the reentry track both in
intensity and in pulse rate. Pulsations can be due to RCS jet firings.
5. I have tried for years without success to convince NASA to do a better
job of publicizing reentries. All to no avail. Demand and pressure from
many sources is the only way to make it happen. The news media will be
interested once or twice but not consistently. Also there have been some
past experience with numerical errors in reporting the data to the public
which have provided some disappointments. . Since the decision to deorbit
will likely always be made at the last minute, the best way to know
when/where the reentry track will be is to watch the NASA select cable
channel for the latest decisions, ground tracks and times. If the sky is
dark it is usually possible see the reentry track within 200 to 250 miles
from the track itself. Though SEESAT readers may not accept this, the cable
TV channel is NASA's answer to a public response for such information. In
reality this works really well if one is able receive that cable channel.
I hope this will provide some useful information to SEESAT readers.
Paul Maley