Frequently Asked Questions


BeppoSAX Reentry FAQs


Q: How many objects re-enter the Earth's atmosphere?
A: On average, one piece of debris of small size every 2-3 days. One larger size 
object or intact debris every 10-12 days. Such intact objects could be rocket bodies
or satellites.

Q: Is BeppoSAX a controlled satellite?
A: No. The satellite was switched off in April 2002.

Q: How much material from the BeppoSAX satellite will survive at reentry? 
A: Generally, about 10-50 % of a satellite's mass will survive at reentry. 
After detailed analysis of the construction materials used, the satellite 
shape, size, and weight, the specific BeppoSAX percentage is found to be 47%. 
The high percentage is due to the several component made of stainless steel 
and titanium. 

Q: How many fragments will survive at reentry and which is the heaviest 
   fragment?
A: About 42 fragments, the biggest of them being about 120 kg.

Q: Where can the fragments fall to Earth's surface?
A: Taking into account the satellite orbit inclination and the debris
dispersion during the reentry, any location included in the latitude belt of
+4.4 or -4.4 degrees might be reached by the surviving fragments.

Q: At what altitude does the final reentry phase start and how long does it
take from then for the fragments to reach the ground? 
A: The reentry phase will start at an altitude of120 km and will take about 40 min 
for the fragments to touch the ground (firstly a fragment of about 3 kg and 
3 minutes later, the last of them, of only about 6 grams).

Q: What is the longitude range involved during last orbit? 
A: The longitude range during all of the reentry phase (from 120 km 
altitude until 0 km) will involve a little bit less than about 2/3
of an entire orbit (222 degrees in longitude).

Q: Will the satellite be monitored during its reentry on ground?
A: The reentry in the dense layer of the atmosphere will be possibly monitored until 90
km of altitude. Unless the satellite re-enters above a specially equipped
radar site, no real-time sensor data will be available during the last phase of
the flight.

Q: Is there an independent source of information for this event?
A: The orbital parameters are available daily through the NASA Goddard Space
Center web site as Two Line Elements.

Q: When will the satellite re-enter the Earth and what is the relative error of
the time window?
A: The nominal reentry is now foreseen between 19 April and 21 June 2003.

Q: How precise is the forecast for the reentry time window?
A: The reentry window has a confidence level of approximately 90%. This means
that there is a probability of approximately 90% that the reentry will occur
inside such a window. The amplitude of the window is a function of the
uncertanties in the solar activity predictions, atmospheric models and
satellite aerodynamics.

Q: How large is the impact debris footprint? 
A: Debris that survives reentry will impact within a impact debris footprint
(i.e. the area on the ground that contains all of the debris fragments), of
320 km along track and plus or minus 42 km across track. It is possible to estimate the
size of the footprint, but very difficult to predict where the footprint will
be on Earth's surface or where specific pieces of debris will land.

Q: Can we predict where the satellite fragments or debris will land? 
A: It is very difficult to predict where all BeppoSAX debris will actually
land. The few surviving pieces of the object will be spread over a long
footprint, so that two nearly identical fragments may impact many miles apart.
Since predicting where specific pieces will land is very difficult, analysts
generally predict the location of the debris impact footprint to indicate the
general area where debris will land. Unfortunately, predicting where the
debris footprint will be on Earth's surface for the BeppoSAX fragments reentry
is also difficult. The primary difficulty with predicting the footprint
location relates to the uncertainties in predicting the object's lifetime in
orbit. Given sufficient tracking of the object during the orbit decay combined
with measurements of the sun's activity over extended time (solar activity can
have dramatic effects on the upper atmosphere), it is possible to obtain a
fairly good approximation of the date and time (but not location) of an
object's final reentry. In general, there is about a ±10 percent
uncertainty of the time of the final reentry. Considering that orbiting
objects are traveling at more than 7 km/sec, a prediction made at the
beginning of the object's last orbit, which could take about 90 minutes to
complete, could be off by as much as ±9 minutes, equivalent to more than
7,000 km on the ground. This means that the debris footprint could be located
anywhere along this 7,000-km-long path. In practice, the last trajectory data
available before re-entry may precede the event by several hours, implying an
uncertainty of ±45 minutes in the predicted reentry time, corresponding to
a full orbital track.

Q: Is the reentry material toxic for humans or bodies of water? 
A: There might be hydrazine propellant or battery chemicals which are toxic
agents. Do not touch or inhale them.

Q: Is the reentry material of strategic value? 
A: No.

Q: How can the risk on transportation on the ground, at sea and on planes be minimized?
A: The day before the last, an additional NOTAM will be issued with precise
trajectories (latitude and longitude) and risk time windows. Local civil
protection departments should minimize the risk of secondary effects.

Q:  What kind of alert from the launching country is foreseen before the reentry?
A:  A number of alert statements will be issued at increasing frequency, as
reentry will approach. They will be available at this web site. In general
risk assessments will be given daily in the last week, in terms of risky
time windows for each location flown over by the satellite.

Q: How will the fragments fall down and reach the ground? 
A: All of them will fall down vertically. Some small horizontal components may
be added by local winds, especially for the lighter fragments.

Q: Could fragments be seen by the naked eye from the ground?
A: Probably not much during daylight. From sunset to dawn the reentry will be
visible as a bright meteor, breaking up in many branches as the fragments
detach from the main body of the spacecraft.  The phenomenon will be visible
for a few minutes until 2 or 3 minutes before the debris impact with the
ground.

Q: What are the velocity ranges that all fragments have when they hit the
ground?
A: From 60 to 460 km/h.

Q: What should do the local contact point be when a country is found to be
involved in the reentry?
A: Alert the population to stay inside concrete houses (if possible) for a period of
about 10 minutes as specified in this web site.

Q: What is the evaluated risk of individual injury?
A: The BeppoSAX risk that an individual will be hit and injured is
estimated to be less than one in five thousand. Reentry risk estimates are
supported by the fact that, over the last 40 years, more than 1,400 metric
tons of material are believed to have survived reentry with no reported
casualties (of course, it is possible that casualties have occurred somewhere
in the world, but have not been reported).

Q: Has anyone been hit by falling debris?  
A: Reportedly, only one individual has been struck by debris from a
reentering spacecraft. There have been several noteworthy reentries affecting
populated areas. For example, the February 7, 1991 reentry of the USSR's
Salyut 7/Kosmos 1686 space station, with a mass of 36,700 kg, occurred over a
populated region. Salyut 7 reentered over Argentina, scattering much of its
debris over the town of Capitan Bermudez, 400 km from Buenos Aires. The
townspeople observed the reentry of Salyut 7's debris in their night sky as
incandescent meteors, traveling from the southwest to the northeast. The next
day, metal fragments were found dispersed throughout the town. Luckily, no one
was hit or hurt. The 1979 reentry of the U.S. vehicle Skylab was similar. NASA
controllers modulated the attitude of Skylab during the final orbits to
encourage reentry in an orbit that passed over as few populated areas as
possible. The final reentry of Skylab rained small bits of debris over a town
on the southern Australian coast. Heavier debris landed inland. Again, no one
was hurt.

Q: Who will pay the eventual damages?
A: By international treaty, the launching country is responsible for all proved
damages that are directly related to the satellite reentry.