Collision Avoidance Operations in a Multi-Mission Environment
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AIAA 2014-1745 SpaceOps Conferences 5-9 May 2014, Pasadena, CA Proceedings of the 2014 SpaceOps Conference, SpaceOps 2014 Conference Pasadena, CA, USA, May 5-9, 2014, Paper DRAFT ONLY AIAA 2014-1745. Collision Avoidance Operations in a Multi-Mission Environment Manfred Bester,1 Bryce Roberts,2 Mark Lewis,3 Jeremy Thorsness,4 Gregory Picard,5 Sabine Frey,6 Daniel Cosgrove,7 Jeffrey Marchese,8 Aaron Burgart,9 and William Craig10 Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450 With the increasing number of manmade object orbiting Earth, the probability for close encounters or on-orbit collisions is of great concern to spacecraft operators. The presence of debris clouds from various disintegration events amplifies these concerns, especially in low- Earth orbits. The University of California, Berkeley currently operates seven NASA spacecraft in various orbit regimes around the Earth and the Moon, and actively participates in collision avoidance operations. NASA Goddard Space Flight Center and the Jet Propulsion Laboratory provide conjunction analyses. In two cases, collision avoidance operations were executed to reduce the risks of on-orbit collisions. With one of the Earth orbiting THEMIS spacecraft, a small thrust maneuver was executed to increase the miss distance for a predicted close conjunction. For the NuSTAR observatory, an attitude maneuver was executed to minimize the cross section with respect to a particular conjunction geometry. Operations for these two events are presented as case studies. A number of experiences and lessons learned are included. Nomenclature dLong = geographic longitude increment ΔV = change in velocity dZgeo = geostationary orbit crossing distance increment i = inclination Pc = probability of collision R = geostationary radius RE = Earth radius σ = standard deviation Zgeo = geostationary orbit crossing distance I. Introduction PACECRAFT operators are concerned about close approaches between their spacecraft and other operational Sspacecraft or orbital debris, and need to be prepared to execute thrust or attitude maneuvers aimed towards reducing the risks of an on-orbit collision, if those capabilities exist. The space around Earth is rather crowded already, but even in less congested environments, there is a non-zero probability for collisions. Although only few Downloaded by UNIV OF CALIFORNIA LOS ANGELES on July 23, 2014 | http://arc.aiaa.org DOI: 10.2514/6.2014-1745 spacecraft currently operate in lunar orbits, a close encounter with another object could have catastrophic consequences. 1 Director of Operations, Space Sciences Laboratory, University of California, Berkeley, AIAA Senior Member. 2 Ground Systems Engineer, Space Sciences Laboratory, University of California, Berkeley, AIAA Member. 3 Mission Operations Manager, Space Sciences Laboratory, University of California, Berkeley. 4 Lead Flight Controller, Space Sciences Laboratory, University of California, Berkeley. 5 Lead Scheduler, Space Sciences Laboratory, University of California, Berkeley. 6 Mission Design Lead, Space Sciences Laboratory, University of California, Berkeley. 7 Navigation Lead, Space Sciences Laboratory, University of California, Berkeley. 8 Flight Dynamics Analyst, Space Sciences Laboratory, University of California, Berkeley. 9 Flight Dynamics Analyst, Space Sciences Laboratory, University of California, Berkeley. 10 Project Manager, Space Sciences Laboratory, University of California, Berkeley. 1 American Institute of Aeronautics and Astronautics Copyright © 2014 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. The University of California, Berkeley (UCB) currently operates seven NASA spacecraft – five in Earth and two in lunar orbits – from its multi-mission operations center at Space Sciences Laboratory (SSL).1 A summary of salient mission characteristics is provided in Table 1. The Time History of Events and Macroscale Interactions During Substorms (THEMIS) mission, a NASA Medium-class Explorer, is a five-spacecraft constellation launched in 2007 to study the physics of the aurora.2 Three of the original THEMIS spacecraft, also referred to as probes, currently operate in highly elliptical, low-inclination Earth orbits with perigees between 600 and 1,100 km, and apogees of 66,500-66,800 km. Primary concerns include periodic crossings of the geostationary belt as well as encounters with low-Earth objects near perigee. The remaining two THEMIS spacecraft departed Earth orbits in 2009 and arrived in lunar orbits in 2011, forming a new mission called the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS).3-5 The low-energy transfer with Earth and lunar gravity assists and with extended operations in lunar libration point orbits culminated in the direct insertion into highly elliptical lunar orbits. The THEMIS and ARTEMIS probes are spinning instrument platforms to measure fields and charged particles. The spin-plane wire booms that are part of the Electric Field Instrument (EFI) extend up to 50 m end-to-end. All five spacecraft carry propulsion systems and can execute ΔV maneuvers to avoid close approaches. The Nuclear Spectroscopic Telescope Array (NuSTAR), a NASA Small Explorer operating in low-Earth orbit (LEO), is a three-axis stabilized astrophysics observatory, consisting of a pair of hard X-ray telescopes with focusing optics, mounted at the end of a 10-m long mast.6 NuSTAR does not carry a propulsion system, hence the only practical way to reduce the risk of a collision is by performing an attitude maneuver to minimize the cross section for a particular conjunction geometry. Such an attitude maneuver is designed to orient the long axis of the observatory parallel or anti-parallel to the relative velocity vector between NuSTAR and the approaching object. Table 1. Overview of Characteristics of NASA Missions Currently Operated at UCB. THEMIS / ARTEMIS • Constellation of 5 spin-stabilized probes (P1-P5), spin rates 14-20 rpm • Electric & magnetic field and charged particle sensors, 5 instruments per probe • Mission science: Magnetospheric Physics and Heliophysics • Launch: February 17, 2007 • Two probes (ARTEMIS P1, P2) transferred to lunar orbits, beginning in 2009 • Lunar orbit insertion achieved in 2011 • Current Earth orbits (P3, P4, P5): 600-1,100 × 66,500-66,800 km, i = 8-13° Image credits: NASA • Current lunar orbits (P1, P2): 50-1,200 × 15,900-17,500 km, i = 169° and 36° • Hydrazine propulsion systems with 4 thrusters for orbit and attitude control • Total mass per spacecraft: 127 kg (wet at launch), 78 kg (dry) • Spacecraft dimensions, tip-to-tip: 50 × 40 m in spin plane, 7 m along spin axis NuSTAR • Three-axis stabilized platform • Dual focusing hard X-ray telescopes, 10 m focal length • Mission science: Astrophysics • Launch: June 13, 2012 • Current Earth orbit: 613 × 630 km, i = 6° deg Downloaded by UNIV OF CALIFORNIA LOS ANGELES on July 23, 2014 | http://arc.aiaa.org DOI: 10.2514/6.2014-1745 • Reaction wheels (4), no propulsion system, total mass 324 kg • Spacecraft dimensions: 11 m long axis, 4 m solar array, 1.2 m spacecraft body RHESSI • Sun-pointed, spinning platform, spin rate 12 rpm • Imaging spectrometer for X-ray and gamma ray wavelengths • Mission science: Heliophysics • Launch: February 5, 2002 • Current Earth orbit: 514 × 535 km, i = 38° deg • Magnetic torque bars (3), no propulsion system, total mass 300 kg • Spacecraft dimensions: 5.5 × 5.5 m solar arrays, 2.5 m long imager tube 2 American Institute of Aeronautics and Astronautics The Ramaty High Energy Solar Spectroscopic Imager (RHESSI), another NASA Small Explorer, is a spinning, Sun-pointed observatory studying solar flares at X-ray and gamma ray wavelengths.7 It also operates in LEO and does not carry a propulsion system. The observatory’s cross section is not very strongly dependent on attitude. Due to torque limitations of the attitude control system the spacecraft can also not be reoriented efficiently on relatively short notice. RHESSI therefore does not have the ability to actively reduce the risk of a collision. This paper describes the planning activities and procedures that UCB developed to respond to conjunction warnings, as well as experiences and lessons learned with the process. II. Operating in Crowded Space A. General Concerns Operators of spacecraft in the crowded LEO environment must expect to be faced with close encounters without more than a week advance notice, sometimes even shorter. This situation was aggravated further as a result of several significant spacecraft break-up events. The most notable events creating large debris clouds were the Fengyun-1C disintegration on January 11, 2007, and the on-orbit collision of Iridium 33 and Cosmos 2251 on February 10, 2009.8,9 Due to differential precession, the debris clouds from these events have spread out around the Earth more or less evenly since the break-ups occurred. Pieces associated with these debris clouds alone account for more than 4,200 objects currently tracked by the United States Strategic Command (USSTRATCOM). Corresponding orbital elements are publicly available via the CelesTrak web site (www.celestrak.com). NuSTAR and RHESSI are operating within the crowded LEO regime. The three Earth orbiting THEMIS probes also pass through this regime near perigee of each orbit. B. Special Concerns for THEMIS Of special concern for THEMIS are seasonal crossings of the geostationary belt. The three Earth orbiting THEMIS spacecraft, THEMIS