IMPLICATIONS OF PROLONGED SOLAR MINIMUM CONDITIONS FOR THE SPACE DEBRIS POPULATION Hugh G. Lewis(1) and Timothy Horbury(2) 8QLYHUVLW\RI6RXWKDPSWRQ$VWURQDXWLFV5HVHDUFK*URXS)DFXOW\RI(QJLQHHULQJDQGWKH(QYLURQPHQW 6RXWKDPSWRQ62%-8QLWHG.LQJGRP(PDLOKJOHZLV#VRWRQDFXN ,PSHULDO&ROOHJH/RQGRQ6SDFHDQG$WPRVSKHULF3K\VLFV*URXS'HSDUWPHQWRI3K\VLFV)DFXOW\RI1DWXUDO 6FLHQFHV/RQGRQ6:$=8QLWHG.LQJGRP(PDLOWKRUEXU\#LPSHULDODFXN 1 INTRODUCTION ABSTRACT The familiar solar sunspot cycle is a manifestation of a dynamo in the Sun’s interior, with the solar magnetic Observations of the current solar cycle show the likely field effectively flipping every 11 years. Low sunspot continuation of a long-term decline in solar activity that numbers – solar minimum – occur when the global field began during the 1980s. This decline could lead to is relatively dipolar-like, while solar maximum conditions similar to the Maunder minimum within 40 corresponds to a much more complex and variable field years [1], which would have consequences for the space configuration. debris environment. Solar activity is a key driver of atmospheric mass density and, subsequently, drag on Since the earliest systematic sunspot observations in the orbiting spacecraft and debris. Whilst several studies seventeenth century, it has become clear that the have investigated potential effects on the global climate, sunspot cycle is itself variable, both in period and no assessment has been made of the impact of a amplitude (Fig. 1). The most celebrated change in the Maunder-like minimum on the space debris population cycle is the Maunder minimum, from around 1645 to in Low Earth Orbit (LEO). Consequently, we present a 1700, when very few sunspots were observed. The new study of the future debris environment under weaker Dalton minimum, from around 1790 to 1830, Maunder minimum conditions and provide an comprised a set of cycles with sunspot maxima assessment of the possible consequences to the LEO significantly lower than those around them. space debris population and space operations. More recently, approximately coinciding with the start The University of Southampton’s Debris Analysis and of the Space Age, the Sun entered a period of enhanced Monitoring Architecture to the Geosynchronous activity known as a Grand Solar Maximum. Since the Environment (DAMAGE) has been used to analyse the mid-1980’s, however, activity levels have reduced. The consequences of a Maunder minimum of approximately minimum before the start of the current Cycle 24 was 50 years duration and to quantify the impact on the longer than expected and as the sun approaches its effectiveness of debris mitigation measures. Results maximum, the cycle is much weaker than those from these studies suggest an increase in collision immediately preceding it, suggesting a continued activity and a corresponding, rapid growth of the debris decline (see [1] for a recent review). Indeed, there is population during a Maunder minimum period, in spite some evidence from helioseismology measurements of on-going mitigation efforts. In the best case, the (e.g. [2]) that precursors of the next cycle within the DAMAGE results suggest that the population of debris solar interior are also weaker than at this point in the > 10 cm could double in number by the end of Maunder cycle, suggesting that Cycle 25 might be weaker again. minimum conditions. However, the rapid growth in the Solar cycle changes in its magnetic field have effects population is followed by a strong recovery period on elsewhere, for example in modulating the cosmic ray exit from a Maunder minimum. The recovery is flux at the Earth as a result of alterations in the characterised by a decrease in the debris population, interplanetary magnetic field. This makes it possible to which can be to a level similar to that seen before the use secondary measurements, such as cosmogenic onset of the Maunder minimum, if mitigation efforts are isotope production, to extend estimates of solar activity sustained. As such, prolonged solar minimum conditions levels beyond the earliest systematic sunspot may have relatively benign implications for the long- observations. Such reconstructions show that solar term evolution of the debris environment. However, the activity levels are indeed variable over a range of scales, risks to spacecraft from collisions with debris during a that the Sun can enter an extended period of low Maunder-like minimum would be considerably higher activity, such as the Maunder minimum, after a Grand than present, and exacerbated by any deficiency in Solar Maximum and that this occurs after around 1 in debris mitigation efforts, with corresponding impacts on 10 of such maxima [3]. space operations. _____________________________________ Proc. ‘6th European Conference on Space Debris’ Darmstadt, Germany, 22–25 April 2013 (ESA SP-723, August 2013) It is therefore unlikely but not impossible that the Sun is Atmospheric decay remains the only effective sink in the process of entering a low activity state, similar to mechanism for space debris up to altitudes of about 600 the Maunder or Dalton minima. If such a change, km. The drag acceleration, D', on a satellite with unprecedented since the beginning of the Space Age, cross-sectional area $ and mass 0 is a linear function of were to occur, it would have significant effects on the the local mass density, U, space environment of the Earth and near-Earth space, 1 $ (1) 2 , for example in enhanced cosmic ray flux. It would also D' &' UYU reduce the F10.7 EUV flux incident on the Earth’s 2 0 atmosphere, and through lowered heating rates reduce where &' is the drag coefficient and YU is the velocity of drag on objects in Low Earth Orbit. Such a reduction in the satellite relative to the atmosphere. A decrease in drag could increase the orbital debris population and mass density will, thus, produce a corresponding hence increase the risk to operational assets. decrease in the drag acceleration on a satellite, leading to an increase in the orbital lifetime. Solar irradiance is a key driver of mass density change in the thermosphere. Changes in solar irradiance over the 11 year solar cycle cause corresponding mass density changes of up to an order of magnitude [5]. These changes in density cause large variations in atmospheric drag, from (1), and the rate at which orbiting objects re-enter the atmosphere. )LJXUH \HDUV RI 6XQVSRW REVHUYDWLRQV ,PDJH In DAMAGE, the total mass density and density scale FUHDWHG E\ 5REHUW $ 5RKGH *OREDO :DUPLQJ $UW height are stored as look-up tables for discrete altitudes KWWSZZZJOREDOZDUPLQJDUWFRPZLNL)LOH6XQVSRWB1 and exospheric temperatures. Log linear interpolation is XPEHUVBSQJ used to extract density and scale height estimates from the look-up tables at the perigees of all objects within 2 THE DAMAGE MODEL the LEO region for )10.7(W) values throughout the projection period. The exospheric temperature, 7H[(W), at The Debris Analysis and Monitoring Architecture to the time W is Geosynchronous Environment (DAMAGE) is a three- , (2) dimensional computational model of the full LEO to 7H[ W 1.125 379 >3.24)10.7 W @ 59.89 '7H[ W GEO debris environment. It includes source models for objects down to 10 cm but is capable of evolving where )10.7(W) is the solar flux at a wavelength of 10.7 populations of objects down to 1 mm. cm and '7H[(W) is a correction factor for the semi-annual variability. The constant (59.89) is a correction due for DAMAGE is supported by a fast, semi-analytical orbital geomagnetic activity, which is based on a 20-year propagator, a breakup model, several collision average represented by Kp = 2.13. Projected solar prediction algorithms including a method based on the activity is described in DAMAGE typically using a CUBE approach adopted in NASA’s LEO-to-GEO pseudo-sinusoidal model, but has been modified for this Environment Debris model (LEGEND) [4], and a work to account for a prolonged period of low solar satellite failure model, which accounts for mission activity. failures resulting from the non-catastrophic impacts of small particles. Projections into the future are typically 2.2 DAMAGE Collision Probability performed using a Monte Carlo (MC) approach to Estimation account for stochastic elements within the model and to establish reliable statistics. DAMAGE normally uses an approach based on NASA’s CUBE algorithm [6] for predicting collision 2.1 DAMAGE Orbital Propagator risk. The advantage of this approach is that it involves a simple calculation and scales linearly with the number The DAMAGE orbital propagator includes orbital of objects. Whilst the implementation developed by perturbations due to Earth gravity harmonics (J2, J3, and NASA assumes a non-zero collision probability only for J2,2), luni-solar gravitational perturbations, solar objects residing within the same “cube”, the DAMAGE radiation pressure and atmospheric drag. The drag algorithm ensures that all close approaches within a model assumes a rotating, oblate atmosphere with high- user-specified distance from a target are identified and altitude winds. Atmospheric density and density scale analysed. This is achieved, essentially, by calculating height values are taken from the 1972 COSPAR the position of the target and the debris in a geo-centric International Reference Atmosphere (CIRA), which Cartesian coordinate system and then by “binning” the includes semi-annual and diurnal variations. values in the x-, y- and z-directions. The aim is then to identify all cases where a target spacecraft and a debris V 6 6 2 6 6 (4) particle reside in the same cube bin [6]. By expanding 7 ' 7 ' the search to include cases where a target spacecraft and for a target area, 67, and a debris area, 6', assuming debris reside in neighbouring cubes – in Cartesian both objects to be spherical. The calculation of collision space, there are eight neighbouring cubes – and then probability assumes that the target object can reside at applying a distance constraint, the DAMAGE approach any point within the cube volume with uniform identifies all cases where a target spacecraft and a debris probability.