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Using a solar sail
for a plasma storm
early warning system

Adapted from IAA-96-IAA.3.3.06


Authors

Jean-Yves Prado, Alain Perret, Guy Pignolet
Union pour la Promotion de la Propulsion Photonique (U3P)
1 Avenue Edouard Belin - 31055 Toulouse Cedex FRANCE
prado@cst.cnes.fr, alain.perret@cst.cnes.fr
Iannis Dandouras
Centre d'Etude Spatiale des Rayonnements (CESR)
9 Avenue du Colonel Roche - 31029 Toulouse Cedex FRANCE
dandouras@cesr.cnes.fr

CONTENTS
Introduction
Solar sail basics
Space environment and space weather prediction
The VIGIWIND system
Mission design
Instrumental payload requirements
System design
Development perspectives
Conclusions
References and links

Introduction


The Sun is permanently emitting into space a wind charged with ions, electrons and protons which reach Earth at speeds of 400 kilometers per second in average. The interaction of the solar wind with the magnetosphere is that of a Magnetohydrodynamic generator, that converts a part of the solar wind kinetic energy into electromagnetic energy, which is transferred to the magnetosphere. This coupling can become so efficient that the transmitted power can be as high as 1013 watts, equivalent to worldwide power consumption.

The side effects of important magnetospheric storms are well known. Some are beautiful, like the aurorae, but many others are harmful to terrestrial electricity transmission lines (they provoke huge power surges), to spacecraft (they can corrupt or even destroy their service) and even dangerous for astronauts health. As there is no way to counteract this amount of energy, the most that can be done is to monitor the space environment and to rely on accurate predictions of its characteristics to take countermeasures.

In situ measurements are required and the further upstream from Earth they are performed, the longer will be the warning delay. A good location, that is already used, is the L1 Libration point lying 1.5 million kilometers upstream from the Earth. Augmenting the delay would mean to orbit closer from the Sun but then, with a classical spacecraft design, the angular velocity of the spacecraft would be higher than the Earth's so that the configuration would not remain stable with time, unless a permanent pull is given to the spacecraft.

This cannot be done by chemical engines for a long duration, so the door is opened for using the photonic radiation pressure with a solar sail as a way to provide most of the needed thrust.

The station-keeping conditions can be obtained with a two impulse strategy, but, as the sail is also an efficient way for navigation, it would be much cheaper to use this capacity also for placing the vehicle at its working location.

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Solar sail basics

· typical utilizations

Today, we know that the light of the Sun provides, at the range of the Earth, a thrust of 9 micro-newton per square meter of sail, which is the weight of a 900 gram mass applied to a square kilometer area. This could appear negligible, but when applied in space on a spacecraft, for which all the other forces are balancing each other, it becomes a leading effect. In the best case, photonic propulsion will not provide a better acceleration than a few mm/s2. However, unlike the case of conventional propulsion with chemical propellants, the thrust will last as long as desired. Quick calculations demonstrate that a solar sail in Earth orbit could reach escape velocity within a resonable period of time and then set for interplanetary travel.
· technological main features

Like for any other spacecraft, a solar sail design is mainly driven by its attitude control system. It can be 3 axis stabilized, then demanding a rigidization system for forming the sail shape, or spin stabilized, the rotation of the sail providing the required tension.
Both concepts have been tested in orbit. In february 1993, the Russian 'Consortium Space Regatta' [9] conducted the 'Znamia' (Flag, in russian) experiment from a progress vehicle while leaving the MIR station. The deployed aluminized surface (diameter 25 meters, spinning rate ~10 rpm) was correctly deployed and shaped by centrifugal tension.

More recently (may 96), the NASA astronauts deployed from the Shuttle the Spartan 207/IAE 14 meter diameter inflatable antenna dish [8]. Although not intended to be used for solar sailing but rather for communication antennas, this experiment paved the way for the 3 axis stabilized solar sail technology.

The material that is used for solar sail can be a very thin (a few microns) plastic film which is aluminized (by vapour deposit under vacuum) on one face or both.

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Space environment and space weather prediction


The interaction mechanisms between the interplanetary solar wind and the Earth's magnetosphere are very complex [e.g. 5] and still the target of many scientific space missions. As seen from above, as is the case of the Ulysses mission from ESA, the magnetic field lines emanating from the Sun's surface have a spiral shape. Seen from the Ecliptic plane, the interplanetary magnetic field can be depicted as a sheet rotating from the Sun [11] and crossing the ecliptic plane in different regions, each crossing place corresponding to a change in the sign of the North-South component Bz of the interplanetary magnetic field, which is a fondamental player of the energy exchange game.

The power provided to the Earth magnetosphere by its interaction with the solar wind can be determined from two observable parameters
· the magnetic field, fluctuating in direction, of magnitude between 5 and 10 nT,
· the solar wind particles velocity
using a semi-empirical formula [20].

Given the importance of reliable 'space weather' predictions for many activity areas, the United States have dedicated an operational centre for such tasks [7].

The Space Environment Center (SEC) provides real-time monitoring and forecasting of solar and geophysical events, conducts research in solar-terrestrial physics, and develops techniques for forecasting solar and geophysical disturbances. SEC's parent organization is the National Oceanic and Atmospheric Administration (NOAA). SEC is one of NOAA's 11 Environmental Research Laboratories (ERL) and one of NOAA's 7 National Centers for Environmental Prediction (NCEP). SEC's Space Weather Operations is jointly operated by NOAA and the U.S. Air Force and is the national and world warning center for disturbances that can affect people and equipment working in the space environment.

The spacecraft of which data are used for space weather monitoring are scientific spacecraft such as SOHO or WIND, located at the L1 libration point (1.5 Million of kilometers upstream). The Advanced Composition Explorer spacecraft [12] is planned to be launched in 97 and be located also at L1. From the 9 instruments of its payload, 3 will be dedicated to a 21hours per day service of one-hour advance notice of approaching magnetospheric storms.

The primary mission of these spacecraft is not space weather monitoring (but solar physics and plasma science). What is proposed now is a space system fully dedicated to space weather monitoring, with a 2 hours advance warning of impending geomagnetic activity: the VIGIWIND system.

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The VIGIWIND system
· Mission design


The overall mission can be split into three phases:
- the launch and the near Earth phase, where the spacecraft is mostly under the influence of the Earth gravitational field,
- a heliocentric leg, roughly covering half of an ellipse, the aphelium being the Earth departure and the perihelium the operational station keeping location. This phase is about 6 months long,
- the station keeping phase, where the spacecraft actually fulfills its mission and which is limited in time by the ageing of the sail material and the possible failures of the onboard equipment.

As each phase is constraining the one preceding it, they are addressed in the reverse order.
a) station keeping
From the Kepler's third law, the requirement to define the sail performance is straightforward: the term (a3/µ) has to be equal to the Earth's one. Considering a circular orbit of radius r, the pull from the Sun is µ/r2 ( 5.9 mm/s2 at 1 AU from the Sun) so the radial acceleration gr to be applied to a spacecraft in order to have an Earth synchronous orbit around the sun, is

gr = µ/a2 [(1-h3)/h2]

h being the ratio between the distance from the sail to the Sun and the Earth orbit radius, a .
Defining the lightness number, l, of a sail as the ratio of solar thrust to the solar gravitational force, we obtain the required performance of a sail
l= 1-h3
As a dimensionning target for a 'first generation' solar sail, a range of 3 Millions of kilometers is considered, corresponding to a characteristic acceleration of 0.37 mm/s2, or a lightness number of 0.0588. This figure corresponds to a Surface to Mass ratio (S/M, S in square meters, M in kilograms) of about 40. For a targetted total mass of 175 kilogrammes, this leads to a surface of 6.400 m2 that is a circle of radius about 45 meters, depending on the considered efficiency of the reflective material.
It must be noticed here that the equivalent DV that would have to be provided by a more conventional propulsion system is worth 32 m/s per day. After three months, the amount of wasted fuel would be the equivalent of a circularization impulse from a GTO to a geostationary orbit...


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b) heliocentric orbit transfer

The same way as sea sailors can use or not their diesel motor to leave the harbour before unfurling their sail, two different types of mission are considered:
- a two impulse mission, where the sail is not deployed before being delivered in its operational orbit,
- a purely sail based strategy without any help of a classical motor after the Earth departure.

A two impulse mission is actually a Hohmann transfer between the Earth's orbit and the operational orbit. The velocity at departure, V·, has to be about 300 m/s lesser than the Earth's velocity to reach the targetted perihelium, in the rear direction with respect to the Earth motion.

At arrival on the operational location, a braking maneuver of about 950 m/s is needed to provide the required velocity. Then, before the sail can be unfurled, a 90° steering maneuver has to be executed in order to direct the spin axis in the sunward direction.

The main drawbacks of this solution are the mass of propellant needed for the maneuvers (about half of the launched mass) and the distance which is limiting the telemetry data rate between the Earth and the sail at the most crucial phase of the mission, when the sail is unfurled.

On the other hand, a purely sailing strategy would allow to decrease the launched mass and also to unfurl the sail quite close from the Earth. The main drawback of this solution is that two steering laws have to be implementd on board, as the angle between the normal to the sail and the velocity vector should be about 35 degrees for an optimal deceleration efficiency during the transfer phase and 90° for the operational phase. As the performance level of the sail corresponds to 32 m/s per day, during the 6 months transfer phase, the sail has a braking capacity of more than 5 km/s so a far from optimal strategy can be used. So we select as the reference design the purely sailing option.

c) launch and Earth departure

Using a dedicated launcher offers to be given the required infinite velocity V· by the launcher. But it is also possible to be launched, as a secondary passenger, on a geostationary transfer orbit. Then a perigee motor is needed to deliver an impulse of about 770 m/s in order to provide a C3 less than 1 km2/s2, the direction of the V· vector being of secondary importance.

b) Sail design

· Instrumental payload requirements


From E=µo-1 V B2 sin4 ( q / 2) lo2 the quantities V, B and q have to be measured.
- Two 3 axis Flux gate magnetometers on a 2m boom, one at the top, one at the middle to determine the magnetic field intensity and direction (B and q). Two magnetometer sensors are needed in order to be able to correct the measurements from the magnetic pollution caused by the sail and its plateform electronics.
- Hot Ion Analyser HIA will measure the solar wind particles velocity
- Positive ion gun (such as the ASPOC instrument on CLUSTER) for performing an active spacecraft potential control.

· Instrument operating modes and telemetry output
A spinning period of 64 seconds is assumed. Then:
FGM: 3 vector components per second=>6 bytes/s
HIA: 32 energy channels, 8 angular directions, 16 measurements (1 byte) per spin => 64 bytes/s
Adding housekeeping telemetry, an amount of 100 bytes / second is expected.

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· System design

a) Orbit control
After the early departure phase, the sail will enter a braking phase of about 6 months, in order to reach its operational position with the required velocity. The angle between the sail surface normal and the velocity vector that is optimal for reducing the orbital velocity is 35°.

Then, the sail has to be rotated sunwards in order to acquire its operational orientation.

These two phases are presented here as fully separated ones for clarity reasons. In fact, the transition between the first phase and the second can be much smoother. Further studies will search for an
optimal sail steering law in order to obtain the operational characteristics as early as possible.
b) Sail design

The present sail design relies on inflatable technology. The inflatable structure includes a torus and 4 out of plane radial masts, of smaller diameter, that give the sail its final shape. These inflatable pipes are made of prepreg Kevlar, rigidized under the action of solar heating and of a gaseous catalyst mixed to the pressurant [18].
The reflecting material, chosen for its lightness characteristics, is aluminized Mylar', manufactured by Dupont de Nemours, of thickness 1 µm (1.4 g/m2). This film is reinforced by a mesh of strips.
c) Attitude control

During the first phase of the mission, the scientific payload is not intended to provide usable data so the spinning rate can be different of its operational value. A lower spinning rate will enable a cheaper and more precise counterbalance of the torques applying to the sail.

The following figure highlights the effect of a sail curvature (with two simple models) upon the torque for different values of the solar aspect angle q and the non-planeity of the sail. In this example, the sail is assumed to have a 50 meter diameter.
For instance, a 0.1 rpm spinning rate would lead to a very low average consumption of 10 grams of hydrazine per day to maintain the required steering angle.

To cope with the scientific requirement of having a spinning rape close to 1 rpm, the sail has to be spun up after its 35° rotation maneuver. Once this has been achieved, the curve of the sail is sufficient to ensure a permanent sunwards pointing. The only attitude control maneuvers then will be dedicated to maintain the spin rate.

In order to keep the design as simple as possible, the attitude will be controlled by jet thrusters. A couple of small thrusters (a few Newton thrust) with a lever arm of about 1 meter will be sufficient to provide attitude control for both phases of the mission. The total mass budget allocated to the attitude control system is 10 kilograms.


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d) Payload accommodation

e) Communications and ground segment

Uplink and downlink will be done using S band, with a low gain antenna on-board. A second antenna of the same type can be needed on the sunward face depending on the geometry of the near Earth phase.

The ground antennas that are considered are 9 meter dishes, which is quite a common size. Three such antennas are needed for a round the clock coverage during the operational phase.

A Mission and Control centre, limited to one workstation performing all the tasks, will be set up.

Telemetry link budget
dB
Frequency 2300 MHz
Tx power 10 W 10.00
Onboard loss 2.00
Antenna gain 3.00
Satellite EIRP 11.00
Range 3,000,000 km
Free space loss 229.22
Polarisation loss 3.50
Pointing loss 0.00
Atmospheric loss 0.20
Rain loss 0.10
Ionospheric loss 0.30
Total loss233.32
Ground station diameter 9 meters
G/T 21.00
Boltzmann's constant -228.60
C/No 27.28
Degradation 2.00
Bit rate60 b/s 17.78
Eb/No 7.50
Required Eb/No 4.20
Margin 3.30
f) Power

As the sail will always face the Sun, the power system can be kept simple. A fixed solar panel of about 1 m2 will deliver the required power (~100 W). No eclipses are planned so there is no need for any battery. The power for the early phases can be provided by dry cells. Everything will be switched on after sail deployment and all the electrical systems will remain permanently in operation.
g) Data handling

The onboard data handling system will be kept very simple. Very few commands will be uplinked:
- sail unfurling
- electrical system switch-on
- time tagged thrust pulses
- payload commanding


The attitude restitution will be performed on ground from the Sun and Earth sensor data and the orbit restitution by Doppler analysis.

The only intensive data processing onboard task will be data compression since a compression ratio of 10 is needed to match the instrument data flow with the downlink telemetry capacity.
h) Data dissemination

A wide use of Internet will allow all the interested users to retrieve the environment data and predictions. In case specified values are expected to be overstepped, the registered users will receive in real time warning messages by electronic mail.

Mass budget
ItemContent Mass (kg)
Platform 57
Attitude sensors Sun sensor, CCD, Electronics 1
Comms Receiver, Transmitter, Antenna, Harness 2
Power Solar panel, Distribution 8
ThermalPaint 1
Stucture and mechanisms Central tube, Platforms 30
Attitude control Tank, Thrusters, Tubing 10
Miscellaneous Harness, Balancing mass5
Payload FluxGate Magnetometers, Boom, Hot ion analyzer, Ion gun 8
Sail 95
Inflatable pipes Torus (radius 45 m), 4 radial, 70 g/m 35
Sail material 6400 m2, Mylar ' 1µ, reinforcement20
Stowage elements * Structure, Mechanisms, Separation system 20
Pressurisation system * Tank, Gas, Valves, Plumbing 20
Total launched mass (no margin) 160
Margin 20 % 32
Launched mass (incl. margin) 192
Total sail initial mass (no margin) 120
Sail initial mass (incl. margin) 148
Operational S/M ratio 43
* jettisonned after sail deployment

Nota: In case of a GTO launch, a ~100 kg perigee engine has to be added.
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Development perspectives


It is expected that the development be done on a purely commercial basis, the potential sponsors being
- airlines (GPS reliability)
- electricity providers
- space system operators for TV, telecoms, PTT, defence...
- the space agencies which are involved in the International Space Station programme, caring about their astronaut health.
The instruments themselves will be almost identical to those manufactured for CLUSTER.

Conclusions


Solar sailing was first seen as an alternative to chemical or electric propulsion for interplanetary travels. New applications have then emerged:
- cities lighting (Znamia)
- star occultation for out of solar system planet discovery
- and now, magnetic storms prediction
From the presently available state of art, it is possible to double the solar wind storm notice. Using a higher lightness number, as soon as technology progress is available, the advance notice will be extended in an almost linear way, as shown by the following plot.
The progress in solar sail technology will have a direct impact on the plasma storm notice, and thus, on the safety of many Earth and space activities.

Finally, regarding the likely human exploration of Mars, it is worth to note that solar activity prediction will be mandatory for the human health on the surface of the planet since they will not be protected by a magnetosphere. The same early warning approach can be adopted by placing a sail between Mars and the Sun. Given the higher radius of its orbit, a sail with a given lightness number will be more than three times more efficient in terms of notice time for a martian use than for a terrestrial use.

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References and links


[1]- Starsailing: Solar sails and Interstellar travel L. Friedman John Wiley & Sons, Inc. [1988]
[2]- New Scientist magazine 3 February 1996
[3]- Failures and anomalies attributed to spacecraft charging August 1995, NASA Reference Publication 1375
[4]- CLUSTER mission, payload and supporting activities ESA SP-1159
[5]- Etude de la dynamique de la queue de la magnétosphère terrestre et des conditions de déclenchement des sous orages magnétosphériques State doctor thesis by Iannis Dandouras CESR october 1988
[6]- Earth Moon Race with solar sails (1995 U3P & Ecole Centrale de Lille )
[7]- SEC Home page
[8]- SPARTAN 207 Home page
[9]- Consortium 'Space regatta' booklet (in Russian and French)
[10]- Space News, january 13-26, 1992
[11]- ISTP/GGS server
[12]- ACE Real Time Solar Wind
[13]- Champs d'application d'un système d'alarme solaire - Rapport final juin 1995 - CSD
[14]- Space Experiment ZNAMIA 3 Brochure 1994
[15]- U3P Solar Sail Preliminary Mission Analysis, Tokyo, April 89 Alain Perret
[16]- Etude d'environnement du Voilier Solaire Européen, CERT-ONERA, july 1993, D.Mimoun
[17]- Solar Sail Design and the Earth Moon Race, Nagoya, November 1990, JY Prado
[18]- QUASAT industrial Phase A Study Final report ESTEC contract AERITALIA May 88
[19]- Solar Sail spacecraft historical development and mission applications Dr Colin McInnes Interdisciplinary science reviews, 1995, vol 20, No 4
[20]- Energy coupling between the solar wind and the magnetosphere, Space Science Review, Akasofu, 1981
[21]- Large scale response of the magnetosphere to a southward turning of the interplanetary magnetic field, J.A. Sauvaud et al., Journal of Geophysical Research, Vol92, N. A3, March 1987

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