Welcome to the
Rosetta RPC-LAP Technical Pages
Swedish Institute of
Radiation during the 1st Rosetta Earth
Eriksson, Magnus Billvik and Lennart Åhlén
Swedish Institute of Space Physics,
Department of Astronomy and Space
Physics, Uppsala University
13 December 2004
Is the radiation environment encountered in the first Earth flyby so
severe that we should turn off our instruments? One should note that
radiation belts almost coincides with the plasmasphere of the Earth,
which is the most important region to cover during the Earth flyby for
the RPC instruments LAP, MIP and MAG, because of maximum plasma density
and magnetic field providing unique opportunities to verify instrument
There are two issues to consider:
total dose acquired in the radiation belt passages is taken into
account in the requirements in the EID-A, so all
instruments are designed for this. Nevertheless, in this note we
calculate the total
modelled radiation dose expected, and find it to be rather small.
- Long term degradation of electronics (mainly total dose effects)
- Perturbation on instrument operations (often dose rate effects)
For issue 2, which is of more interest for operations planning, we
should consider the probability of different kinds of single event
effects (SEEs). The major source of SEEs use to be heavy ions and
protons, while the electrons encountered in the outer radiation belts
are usually less efficient. Heavy ions at high energy are mostly due to
galactic cosmic rays, and hence less common inside the shielding
magnetosphere than outside. However, SEEs, particularly so-called
single event upsets (SEUs), are not unknown on spacecraft in the
radiation belts. Modelling SEE rates require
detailed modelling of components, so we instead compare the expected
fluencies (time-integrated fluxes) of particles in the tens of MeV
range during the flyby to what we can expect in interplanetary space,
and to known effects on other s/c.
We use the SPace ENVironment Information System (SPENVIS,
a web-based unified interface to a set of models for the space
environment and its effect on spacecraft. For the Rosetta trajectory,
we use the AUX files provided by ESOC on the DDS. Presently, the
SPENVIS web interface only includes the possibility to define the orbit
in terms of orbital elements of closed orbits, but thanks to the
cooperation of Daniel Heynderickx of the SPENVIS team at the Belgian
Institute for Space
Aeronomy, we could feed the Rosetta trajectory file, converted to
geographic coordinates, into the system.
SPENVIS includes the NSSDC
models AP-8 and AE-8 for trapped protons and electrons in the
terrestrial radiation belts. We present the results from runs using the
AP-8 and AE-8 for solar max conditions here, but the difference when
running for solar min conditions was very small in this case, certainly
much smaller than the prediction uncertainties.
For the estimate of total radiation dose (energy deposited in the
target), we used the SPENVIS implementation of
the SHIELDDOSE-2 model (v 2.10), assuming a silicon target
inside a sphere of 1, 2 or 3 mm aluminium.
Predictions of SEE phenomena, like SEU rates, are much more dependant
specifics of individual electronic components. Though SPENVIS provides
facilities for such modelling, we here only draw some general
conclusions, avoiding detailed modelling of selected components.
Calculated fluxes of particles during the Rosetta Earth flyby are shown
in Figure 1. The time-integrated flux for the full flyby, i.e. the
total number encountered or the particle fluence, is illustrated as a
cumulative plot in Figure 2 and tabulated in Table 1. The estimated
radiation doses are listed in Table 2.
1. Predicted fluxes of protons above 10 MeV and above 30 MeV, and
of electrons above 1 MeV and 5 MeV, from the SPENVIS implementation of
the AP-8 and AE-8 models for solar max conditions.
2. Predicted fluence of protons above 10 MeV and 30 MeV, and of
electrons above 1 MeV and 3 MeV, from the SPENVIS implementation of the
AP-8 and AE-8 models.
Table 1. Predicted fluencies of
high energy particles.
|Protons > 10 MeV
|Protons > 30 MeV
|Electrons > 1 MeV
|Electrons > 3 MeV
Table 2. Radiation dose
expected for the 1st Rosetta Earth flyby, for a silicon target within
an aluminium sphere of given thickness , or behind a semi-infinite 1 mm
Al plate. The "total dose" column also includes small contributions
from bremsstrahlung and solar protons, though the radiation belt
particles clearly dominate.
The exact fluencies can of course vary a lot with the
actual magnetospheric conditions, but the results above nevertheless
baseline for estimating the possible impact of the radiation belts. To
put them into perspective, we can compare to what we normally expect to
find in interplanetary space. Feynman et al. (1990) suggest typical
yearly averaged fluxes of solar proton event particles varying between
10^7 and 10^10
protons/year above 30 MeV, with 10^9 a reasonable number a few years
after solar max. This would suggest that for the protons, the radiation
belt passage gives a dose equivalent to what we may expect to get in
about a week of operations in interplanetary space. There is no reason
to worry about total dose effects.
For the RPC electronics box, a relevant model may be a 0.5 mm thick Al
sphere, representing the RPC-0 electronics box, behind a 1 mm
semi-infinite Al slab, representing
the spacecraft. The last row of Table 2 may thus be taken as an upper
limit to what may be expected. The total dose on RPC main electronics
should thus be small, below 100 Rad. As noted above, the effects
of this dose are largely independent on whether we are on or off, so
this has little impact on operations.
What about SEUs? According to the above, we may expect below
100 Rad in total, most of it within some 20 minutes, which would
indicate dose rates of order 0.1 Rad/s, with perhaps up to 1 -- 10
peak values, most of it due to electrons. We should here be safe for
latchups, and SEUs appears unlikely but cannot be
For a high-voltage instrument, which can actually be damaged by the
effects of a bit-flip at the wrong place, turning off in the rad belts
may be considered. For RPC-LAP, which has no high voltages and which
be killed by any conceivable bit-flip, the worst effect of an SEU would
that the operational mode runs wild and we lose data. Though the SEU
risk appears low, it therefore seems prudent for us to include a few
the operations timeline.
Thanks to Ali Mohammadzadeh at ESA for good comments.
last modified onFriday, 08-Jun-2007 15:30:01 CEST