|Risk assessment and operations strategy for ISRO satellites during Leonid storm encounter|
Leonid meteoroids are associated with Comet 55P/ Tempel-Tuttle whose orbital period is 33.3 years. The meteors are named Leonids as they appear to emanate from the constellation of Leo due to perspective. The comet spews low density debris on and around its track like a wheel barrow due to sublimation of ice and dust, when the comet is in close proximity to the sun. The dissipation of the comet forms the annulus of meteoroids called `stream' with the genes of orbital characteristics inherited from the parent comet. The distribution of these cosmic spherules in the stream is governed by the `Poynting-Robertson Effect' by which the smaller particles sort themselves closer to the sun, whereas the larger ones keep away, provided all are of equal density. The earth on its annual journey on the ecliptic around the sun, encounters this stream during 14-20 November every year (Figure 1). In the case of Leonid, the shower takes the shape of storm once in 33.3 years, when the earth stirs the hornet's nest of debris swarm, just left behind by the perihelion passage of Tempel-Tuttle with the concomitant increase in deposit of debris1 (Table1).
The Leonid storm of current interest has dominated the contemporary storm record, with seven of them since 1799 (ref. 2) (Table 2). In 1965, Tempel-Tuttle passed the perihelion but the earth experienced a Leonid storm on 17 November 1966, the most intense storm ever recorded. The 1966 storm did not evoke much interest to space industry as there were only a few spacecraft and hence there is no historical data about the extent of damage on the satellites. The Leonid storm is forecast for November 14-20, 1998 when the earth along with about 500 operational satellites (6% of the total of 8000 orbiting) in its gravitational cradle, plough head-on through the swarm of dust particles precipitated by the comet 55P/Tempel-Tuttle, which slid through the perihelion on 28 February 1998. While we predict the Leonid storm to peak on 17 November 1998 at 20:40UT, there is historical precedent for the storm to peak a year later on 18 November 1999. The Leonid storm is more severe for a couple of hours on either side of the peak3 (Table3).
It was clear even before the launch of Sputnik in 1957 that the satellites would be subjected to hyper- velocity impacts from meteoroids and the main thrust of the early space research was on how to safeguard the space assets from these rubbles. The extent of damage inflicted on the satellites depends upon the combined effect of several factors such as the impact probabilities, relative velocity of the meteoroids, vulnerability of the satellite subsystems, the area of cross section, sensitivity of instruments on-board, orientation of the spacecraft to the storm, fluence, mass distribution of meteoroids, angle of incidence, etc. The present study is to assess the impact probabilities and then arrive at an idea on how best the preventive/ protective mechanisms can be developed.
ISRO satellites and the risks involved
Remote sensing satellites (IRS) are in near circular, near polar, sun synchronous orbits at about 820km with 3 axis stabilization and the scientific satellite (SROSS-C2) at 430km x 588km elliptical orbit with spin stabilization. Communication satellites (INSAT) are in geostationary orbits. The satellites are state of the art in nature and have several microprocessors on board. The surface area of these satellites exposed to the storm are considerably small (12-24m2) compared to space station-MIR (500m2). Many of these satellites with miniaturized, processor-based electronic and sensitive electro-optical devices on board are vulnerable to physical assault and plasma-oriented spacecraft-charging. The damage could be a catastrophic rupture, secondary fracture, leakage, vapourific flash, deflagration, deformation, reduced residual strength, fluid contamination, thermal insulation damage, erosion and plasma-caused microprocessor upsets, glitches, etc. The analysis of the Leonid storm effect using the available data shows that all Indian satellites serving scientific, communication and remote sensing missions are certain to undergo this ordeal with Leonid storm for the near full duration and earth will not protect the satellites as the orbital planes will be exposed to the storm. The locations of IRS and INSAT satellites on the equator at the time of leonid storm peak are given in Figures 2 and 3.
What makes this ribbon of debris stand out high is their hyper-velocity encounter. At 71km/s, they can play havoc with the life of the spacecraft. The risk due to the storm on satellites could be of both physical/ mechanical and electrical. Impact damages may be due to direct mechanical cratering, plasma and ESD creation, leading to electronic noise, sudden current and voltage spikes and software anomalies4.
The meteors could cause physical damage to the satellite structures or components, pitting of optical surfaces and mirrors, degrading the performance of critical sensors, etc. However, the studies undertaken show that the probability of impact on our satellites ranges between 0.01% and 0.03%. Physical damage to the solar panels is very unlikely as the meteor storm approach is edge-on to the sun tracking solar panels5.
Meteors disintegrating upon impact with the spacecraft could generate electrically charged plasma cloud as a result of ionization. This could lead to electro-static discharge (ESD) on adjacent surfaces, induce electrical shorts and failures in sensitive components. The plasma can cause sudden electrical pulse (SEP) which can upset sensitive microprocessors.
The probability of impact
The probability of impact is a function of surface area exposed and fluence of the storm. Though the probability number is low, the consequences of the impact, if occurred, could be catastrophic and hence an in depth study is warranted. The nominal Zenithal Hourly Rate (ZHR) observed is about 15 to 20 particles, whereas it peaks to about 40 particles per second, as was witnessed during the 1966 storm.
The impact probability (IP%) of Indian satellites is calculated and given in Table 4. But the uncertainty in the estimation is due to the fluence, as the mass distribution index and the flux as a function of mass are not accurately known and can vary by an order of magnitude. For the purpose of computation of impact probability, storm duration of 5h and a limiting mass of 10-5g is considered2.
Operational strategy to safeguard the satellites
The entire team of satellite controllers involved in the satellite operation are to be made aware of this event and its impact. The presence of senior spacecraft controllers and sub-system specialists should be ensured at the spacecraft control centre during the storm period. The operation team should have the support of the sub-system designers for a period of seven hours on either side of the storm peak. Discussions should be held with designers, mission and project teams so that the experts will be available on the day of storm to overcome any contingencies. Controllers should check the health of the satellites frequently, looking primarily for electrical anomalies and glitches, regulator voltage and currents, etc. during 14-20 November 1998 in a count-down mode.
Spacecraft health monitoring
Vigilant and frequent health-monitoring prior to, during and post-storm phase is mandatory. Intensive monitoring of critical and additional important satellite parameters is to be carried out during the storm period. As IRS-1C, 1D and P3 are processor-based satellites, these satellites need to be monitored with a bit of extra care6.
Pan camera steering for IRS-1C/1D in positive direction may be avoided during the storm period. It is preferable that Pan operations be carried out with nadir viewing or with negative tilt angle. X-ray payload operations of IRS-P3 and Gamma Ray Burst Detector payload operations of SROSS-C2 which involve high voltage, may be suspended for the period of storm. Remote sensing payload operations may be stopped during the storm period. If payloads are to be serviced on priority, such operations may be conducted without inhibiting the solar panels. On-Board Tape Recorder operations need not be scheduled for the orbits during the peak of the storm, as the instrument has a built in processor.
Configuration changes for the satellites
It is suggested to power off sensitive instruments and prefer to be in AOCE Hardware, wherever the option exists. It should be ensured that the satellites are configured for `safe mode'. In case of IRS-P3, favourable X-ray source should be selected in stellar-pointing mode to suit the safe orientation of the star sensor and X-ray detectors against the meteor storm. It will be expedient to operate the satellite in the earth-pointing mode. As the equipment panels of our satellites are generally of uniform area, re-orientation may not help in reducing the exposed surface area to the storm. In case of INSAT satellites gyros will be switched on, if not `ON' already to provide rate information.
It would be prudent to plan only the essential operations and not to schedule any special or critical operation during this period. Scheduling efforts should be made to allocate more TTC stations for processor-based and important missions. Scheduling priorities are to be assigned such that minimum visibility gap exists for IRS-1C, 1D and P3 satellites.
It is also essential that alternative communication links are at hand in case of any communication blackout from the connected GEO satellites. In case of any planned launch, the simple solution is to wait for the storm to blow over. NASA has also cancelled the space shuttle launch during the storm period. 14-20 November may be declared as Leonid campaign week for intensive health monitoring of satellites.
All Indian satellites are certain to undergo this ordeal with Leonid storm. The earth will not shield the satellites and most of the orbital plane will be exposed to the storm for full duration. The studies made show that the probability of impact on our satellites falls in the range of 0.01% to 0.03%. Physical damage to the solar panels is very unlikely as the meteor storm approach is edge-on to the sun-tracking solar panels. Though the probability number is low and our satellites are designed to be rugged with in-built fault-tolerant features, the consequences of the impact, if occurred, could be catastrophic and therefore the Leonid storm event cannot be taken lightly. As a measure of abundant caution, a strategy has been worked out to take adequate steps to safeguard the satellite fleet with minimal interruption in the routine operational services. As most of the satellites can switch over to `safe mode' in the event of any serious anomaly, loss of satellite is unlikely. Even if the satellites are found to be unaffected after the event, the Leonid storm of 1999 and 2000 can not be neglected as the intensity of the storm can be high in the subsequent years, as is evident from the precedence set by the previous storms.
Will our satellites come out unscathed ? An affirmative answer is hard to be made now. But, the heavenly experience and the challenges courageously faced will be the positive aspect of this great event. The experience gained will stand as in good stead in facing future situations. We await the rare spectacle of this century with a repeat next year too, which may drive the event of comet Shoemaker-Levy's impact on Jupiter into the back-seat. Let the flamboyant Leonid illuminate the winter sky without any sparkle on our inflammable space assets.
1.Glenn E. Peterson et al., Leonid Meteoroid Encounter Data.
2.Beech, M. et al., Meteor Storms: An Assessment of Satellite Impact Probabilities, 1996. Presented at 9th CASI Conference on Astronautics, Ottawa, Canada.
3.Rao, J., The Leonids: King of the Meteor Showers
4.Ailor, W. H. et al., The Upcoming Leonid Meteoroid Storm and its Effect on Satellites, May 1998. Testimony to the US house of representatives Committee on science, Sub-committee on Space and Aeronautics hearing on Asteroids, Perils and Opportunities.
5.The Leonid Meteoroid Storm, INTELSAT's Risk Mitigation Strategy, July 1998.
6.Celestial Extravaganza: Leonids Strike Again, Teletrack of ISTRAC, ISRO, October, 1998.
ACKNOWLEDGEMENTS.We thank Sri S. K. Shivakumar, Director, ISTRAC and Dr S. Rangarajan, Director, SCP, ISRO HQ, for their guidance and encouragement in bringing out this article. We thank Sri S. Sukumar, SCP, ISRO/HQ for his valuable suggestions. We also thank Shri Y. Prakasharao, Engr-in-Charge SOCS/ISTRAC for the valuable contribution in flooding us with relevant information.
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