A Notional Scenario for Improved Support of International Space Station Construction
E.1 VISION OF AN ISS CONSTRUCTION MISSION SUPPORTED BY RELIABLE, ACCURATE RADIATION FORECAST MODELS DURING THE SOLAR MAXIMUM
Present models do not include all of the links that connect sources at the Sun to the radiation environment at the shuttle or the ISS. Moreover, they are primitive and offer only general guidance. Their forecasts of SPE occurrence and size are therefore too inaccurate to cause a flight director to alter a preset schedule of mission activities. Instead, an SPE forecast activates a situational analysis for preparing defensive responses. To illustrate the potential value to NASA of having much better information from space weather services, this section presents a counterfactual situation in which SPE forecasts are accurate enough to cause a flight director to alter a preset schedule of mission activities. One ancillary point made by this illustration is that high solar activity does not necessarily mean high radiation doses. Whereas the ability to make accurate SPE forecasts is usually associated with a decision to postpone or terminate an activity, it could also lead to a decision to initiate an activity. Even when radiation is elevated and further increases threaten, accurate numerical forecast models, which must still be developed and put into operation (see Appendix A), could nonetheless allow a flight director to maintain operations if the models forecast that the environment will remain safe for ISS workers.
E.2 THE WAY THINGS OUGHT TO WORK
It is afternoon on October 15 at the Johnson Space Center in Houston. Inside the control room, a flight director monitors the progress of AS13, the thirteenth U.S. shuttle flight carrying a team of astronauts to construct the International Space Station (ISS). AS13 has 2 full days left, and all has gone well so far. Just one EVA, scheduled for tomorrow afternoon, remains to accomplish mission objectives. But the flight director is worried. A huge active region on the Sun, which produced solar particle events (SPEs) on each of two earlier rotations, is making its third pass across the Sun's maculae-pocked face. Although the region is now approaching central meridian—the most dangerous time for SPEs—no very large events have occurred so far. But space weather forecasters at the NOAA Space Environment Center in Boulder note that a large X-ray sigmoid, a CME precursor, has developed in the center of the maelstrom. They predict that the region will probably erupt in a solar storm in a day or two. Forecasters at NOAA's
SEC and at the NASA Space Radiation Analysis Group (SRAG) console at JSC are communicating. SRAG is following the activity on its monitors in Houston as well as receiving alerts and forecasts from SEC.
What follows is a best-case scenario in which the course of events is controlled by an advanced capability for using space weather forecasts to support ISS construction. It envisages a time when accurate, reliable forecast models—which again, to emphasize the point, must still be developed and "transitioned" into operation—are in place to model the transport of coronal mass ejections from the Sun to Earth, to model the effects of shocks on interplanetary particle acceleration, and to accurately model the interaction of the disturbed solar wind with Earth's magnetic field, especially its effects on the size of the zones accessible to solar energetic particles. Given all this, the history of Flight AS13 unfolds as follows:
15 October, 1730 Houston time (HT), 2330 UT
The Space Environment Center receives data showing a class X flare at S10 W15. The GOES solar X-ray sensor measures a peak flux of 10–3W/m2, and a time-integrated energy of 0.7 J/m2 in the 1 to 8 nm X-ray band is accumulated in the first 10 minutes of the event. The solar X-ray imager (SXI)(scheduled launch date: October 2000) registers the event and shows large coronal changes and loop structures, indicating a substantial coronal mass ejection (CME) has occurred. Ground-based solar observatories report that the event includes an optical flare and large radio bursts.
SEC notifies SRAG that the alert criteria for a major X-ray flare and an integrated X-ray event flux have both been reached. SRAG notifies the flight surgeon that a major solar event has occurred.
15 October, 1735 HT, 2335 UT
SEC runs its updated PROTONS 5 Model. It produces a forecast of a large SPE with a 95 percent chance of occurrence. The first increase in 30 MeV protons is forecast to begin in 25 minutes. SEC relays this information to SRAG.
15 October, 1750 HT, 2350 UT
The X-ray event is still in progress. The X-ray-integrated flux reaches 1.5 J/m2 on the GOES satellite. A new run of PROTONS 5 forecasts a total event fluence of 107p.f.u. with energy (E) > 30 MeV. The peak flux is forecast to occur on 16 October around 0900 UT. SEC reports this information to SRAG. SRAG relays it to the flight surgeon and the flight director.
15 October, 1825 HT, 16/0025 UT
Energetic particle sensors on GOES begin to show an increase in energetic particle fluxes over the range 10 to 100 MeV.
15 October, 1840 HT, 16/0040 UT
The on-board radiation instrumentation, including the tissue equivalent proportional counter (TEPC) and the internal vehicle charged-particle directional spectrometer (IV-CPDS), which have been temporarily moved about the habitable cabin for a background radiation survey, are to be returned to their usual location. The TEPC alarm is turned on and set at a low threshold, and real-time telemetry from all radiation subsystems is activated.
15 October, 1855 HT, 16/0055 UT
The Large Angle and Spectrometric Coronagraph Experiment (LASCO) coronagraph data, combined with radio and plasma wave data from the WAVES instrument on the Wind spacecraft, are
used as input to the advanced Shock Propagation Model (SPM). Its output includes the prediction of a shock arriving at Earth between 1200 and 2000 HT on October 16. It also includes a forecast of B field structure and shock strength. This information is used to run the advanced SPE Profile Forecast Model (SPEPFM). The model forecasts that SPE fluxes will remain below 500 p.f.u. for particles with energies of 10 to 30 MeV until about 16/1600 HT. At that time, the particle fluxes are predicted to begin to increase and to rise by two orders of magnitude over the following 4 to 5 hours. The version 3.0 Neural Net Fluence Model (NNFM) predicts an integrated fluence of 108 to greater than 109 p.f.u. with E > 30 MeV for the entire event. SEC reports this information to SRAG.
15 October, 1930 HT, 16/0130 UT
Using input from SOHO, SEC refines the Shock Propagation Model forecast to an arrival time of 16/1630 HT (16/2230 UT). The advanced Polar Cap Model predicts an opening of the polar caps down to 48 degrees geomagnetic latitude. The NNFM forecasts the time-integrated particle flux to exceed 109, with less than 10 percent error in the forecast.
15 October, 2000 HT, 16/0200 UT
SRAG reports the SEC forecasts to the flight surgeon and the flight director. A morning rest period is scheduled for the astronauts on the following morning and a 6-hour EVA is to begin on October 16 at 1300 HT. All three parties confer. SRAG reports the doses on an EVA for these fluxes will be low through the morning of the 16th but will be much higher in the afternoon, after 1600 HT. The flight surgeon begins to review the radiation records of individual crew members.
15 October, 2300 HT, 16/0500 UT
Telemetry is live from the TEPC, EV-CPDS, and IV-CPDS. SRAG compares the observed dose profiles with the projected doses from satellite particle data and finds them to be within 15 percent agreement. SRAG and SEC review the observed SPE profile, which is within 20 percent of the forecast from the SPEPFM.
16 October, 0200 HT, 0800 UT
After conferring with the flight surgeon and SRAG, the flight director decides to move the EVA to 0900 the following morning.
16 October, 0800 HT, 1400 UT
Spacecraft dosimeter readings and dose calculated from energetic particle data are in agreement. The dose rate is low. Telemetry continues live from TEPC, EV-CPDS, and IV-CPDS. In case of space telemetry shutdown because of the effects of the SPE on system hardware, doses calculated from satellite SPE observations will be used on the ground to monitor the crew dose. The EVA begins.
16 October, 1053 HT, 1653 UT
The TEPC alarm sounds. After consultation between the flight director, the flight surgeon, and SRAG, it is determined that although the dose rate has gradually reached the alarm level it is still low in terms of crew exposure. SRAG consults with SEC and decides the rapid increase is still not expected before 1600 HT and 51/2 hours are left before the dose rates rise rapidly.
16 October, 1200 HT, 1800 UT
Satellite particle fluxes begin to increase rapidly at 1430 UT. They reach 15 times the background level before the increase. Doses inside the shuttle increase even more rapidly because of
the hardening of the event spectra. The crew goes to the docking module for additional protection during the five polar cap passes that impart the most exposure.
16 October, 1330 HT, 1930 UT
The dose rate has continued to increase slowly but remains low as measured by the on-board dosimeters being monitored on the ground by mission control. The EVA is completed 30 minutes early and the crew returns immediately to the spacecraft.
16 October, 2400 HT, 17/0600 UT
SPE fluxes and dose rates return to the levels of early on October 16. Reentry is still as scheduled, the next morning.
By using a (hypothetical) full-up space weather capability, with integration of advanced forecast modeling capabilities, the ISS has completed its mission and avoided an extra day in orbit, which would have been necessary if the last EVA had been carried out as originally scheduled. In that alternative case, the EVA would probably have begun and then been cancelled as the radiation levels rose or the crew would have suffered excessive radiation doses while trying to complete the EVA.
E.3 THE MISSING PIECES
A number of advanced models are posited in this scenario:
Updated PROTONS 5 Model, which predicts SPE from solar X-ray inputs;
Advanced Shock Propagation Model, which predicts shock arrival time and post-shock parameters;
Advanced Profile Forecast Model, which predicts flux profiles as the event proceeds for a standard set of energies;
Version 3.0 Neural Net Fluence Model, which predicts integrated fluence for the event; and
Advanced Polar Cap Model, which specifies energetic particle access zones from predicted postshock and CME conditions.
All of these models, which have been given descriptive hypothetical names, are in various stages of development. None is up to the task assigned it in this scenario, but as Appendix A describes, each is a goal of the National Space Weather Program (NSWP) and of scientists involved in space weather modeling. The models will not be ready in time to make the scenario described here a reality during the planned construction phase of ISS. NSWP and the community of space weather scientists and service providers intend, however, to improve the state of space weather services by the time the operational phase of ISS has been reached so that it approaches the level of capabilities assumed for these ideal models.
E.4 TIMETABLE FOR IMPLEMENTING THE REPORT'S RECOMMENDATIONS
A variety of actions are recommended in this report to support the construction of the ISS during a solar maximum. Figure E.1 gives a timeline keyed to the ISS construction schedule (as it was known in July 1999) for the implementation of the recommendations of this report. As shown in the figure, there are recommendations whose implementation should be completed right away (R1, R3a, R4, R5b, and R6); recommendations that will require one or two years to implement (R2 and R3b); and recommendations that will take several years to implement (R3c and R5a). The research needed to improve space weather services in support of manned missions (described in Appendix A) is also shown on this figure. Research activities are organized into two groups: those that can be implemented within 5 years (A1), with some activities being implemented within 1 year, and those requiring more time to implement (A2).