Vishal Ray, University of Colorado Boulder; Tom E. Berger, University of Colorado Boulder; Zach C. Waldron, University of Colorado Boulder; Eric Sutton, University of Colorado Boulder; Greg Lucas, University of Colorado Boulder; Delores J. Knipp, University of Colorado Boulder; Jeffrey P. Thayer, University of Colorado Boulder; Siamak G. Hesar, Kayhan Space; Daniel J. Scheeres, University of Colorado Boulder
Keywords: atmospheric drag, very low Earth orbit (VLEO), Starlink, maneuvers
Abstract:
There’s recently been an increased interest in exploring the very low Earth orbits (VLEO) for sustained satellite operations and as parking orbits before raising the satellite to its operational altitude. With increasing congestion in Low Earth Orbit (LEO) and its associated collision risks, VLEO could provide an additional orbital regime where satellites could reap the benefits of the LEO regime. There are significant advantages in terms of launch costs as well as increased resolution of communication and optical payloads [1]. Additionally, the increased effects of atmospheric drag imply an easier and faster end-of-life deorbiting capability. But on the flip side, the large effects of atmospheric drag also translate to an increased maneuver cost for orbit maintenance along with increased atomic oxygen erosion of spacecraft materials [2]. This is made even more difficult due to the large variability of the thermosphere in response to space weather disturbances. It is crucial to understand the range of variations of the thermospheric density at such low altitudes to design robust orbit maintenance systems. In this work, we analyze the effects of high drag environments on satellite orbits and associated fuel costs for orbit maintenance and orbit raising. The effects of density increase due to solar and geomagnetic storms on the maneuver cost and the performance of the propulsion systems will be thoroughly studied. Thrusters being highly directional, the performance of the propulsion system is highly dependent on the Attitude Determination and Control (ADC) system. Drag at such low altitudes acts as a large disturbance torque and can present significant challenges to the ADC system [3]. In the extreme case, during space weather disturbances, the increased drag can saturate the attitude control and the spacecraft might not be able to reorient itself temporarily. If this coincides with a requirement to thrust for orbit raising, there can be disastrous consequences. The unfortunate reentry of around 40 newly launched Starlink satellites during the Feb 2-4 geomagnetic storm period illustrates this point [4]. Even though the storm was classified as a minor geomagnetic storm, the almost 50 % increase in density prevented the Starlink satellites to switch from “safe mode” to “thrusting mode”. The error margins around nominal attitude control and orbital maneuvers need to be analyzed carefully and should take into account potential thermospheric disturbances due to space weather events. We will calculate the fuel margins and attitude control torques required to compensate for drag disturbances at VLEO altitudes for increasing levels of solar and geomagnetic activity. An in-depth analysis of the Starlink event will carried out as a case study. Our goal with this work is to increase the awareness of the significant effects of space weather events on satellite orbits, particularly at VLEO altitudes, and that missions at such altitudes should give serious consideration to space weather impacts during the design stage.
References
[1] N.H. Crisp, P.C.E. Roberts, S. Livadiotti et al. The benefits of very low earth orbit for earth observation missions. Progress in Aerospace Sciences, vol. 117, 2020.
[2] Lance McCreary. A satellite mission concept for high drag environments. Aerospace Science and Technology, vol. 92, 2019.
[3] Joseph Virgili-Llop, Halis C. Polat, Marcello Romano. Attitude Stabilization of Spacecraft in Very Low Earth Orbit by Center-Of-Mass Shifting. Frontiers in Robotics and AI, 2019.
[4] https://www.technologyreview.com/2022/02/10/1045202/spacex-just-lost-40-satellites-to-a-geomagnetic-storm-there-could-be-worse-to-come/
Date of Conference: September 27-20, 2022
Track: Atmospherics/Space Weather