The Risk of Space Junk

Image credit: NASA


The November 2019 issue of Prism (published by ASEE) includes an article by Thomas Grose about the risks associated with space junk in low earth orbit (LEO).  The likelihood of a collision continues to increase: 4000 satellites and the International Space Station (ISS) travel in LEO, which has 128 million bits of junk; 20,000 pieces have a diameter greater than 10 cm.  The consequences of a collision could be more debris, a damaged satellite (which could interrupt communication or other services), or a casualty on the ISS.   A possible worst case is the Kessler syndrome (a chain reaction of debris-generating collisions).

The current mitigation efforts include rules to reduce the growth of space junk and a system for detecting possible collisions (so that spacecraft can be moved out of the way).  NASA has a technical standard, "Process for Limiting Orbital Debris," that requires space programs to assess and limit the likelihood of generating debris during normal operations, explosions, intentional breakups, and collisions.  "Orbital debris" is defined as follows:

Artificial objects, including derelict spacecraft and spent launch vehicle orbital stages, left in orbit which no longer serve a useful purpose. In this document, only debris of diameter 1 mm and larger is considered.

The NASA standard also discusses reducing the likelihood of collision by reducing a spacecraft's cross-sectional area.

New systems for tracking more space junk more precisely (e.g., the Space Fence) could lead to an automated "traffic control" system that warns a spacecraft operator when a collision is imminent while reducing the likelihood of false alarms.  An alarm is costly because it disrupts normal operations, and the spacecraft must burn fuel to move away from the space junk and then return to its normal position.

Researchers are also developing spacecraft that can capture space junk, which could reduce the likelihood of a collision.

The article mentions few efforts to reduce the consequences of a collision.  Astronauts in the ISS can head to a shelter if a close call is imminent.  But hardening a satellite would require more mass, which makes it more expensive.  Perhaps we need "shatterproof" materials or designs for spacecraft.

Robust Multiple-Vehicle Route Planning

Planning problems with multiple vehicles occur in many settings, including the well-known vehicle routing problem (VRP) and drone delivery operations, including the flying sidekick problem.   Most approaches to these problems assume that the vehicles are reliable and will not fail.

In the real world, however, a vehicle could fail, in which case the other vehicles would have to change their routes to cover the locations that the failed vehicle did not visit.  In recent research done at the University of Maryland, Ruchir Patel studied this challenge and developed approaches for generating robust routes for multiple-vehicle operations.  His thesis, Multi-Vehicle Route Planning for Centralized and Decentralized Systems, describes the results of his research.  The key idea is to consider the worst-case performance of a solution instead of the best-case performance.  A solution with better worst-case performance is more robust and will perform well even if a vehicle fails.

He found that a genetic algorithm (GA) could find high quality solutions, but the computational effort was substantial because evaluating the robustness of a solution required evaluating all possible failure scenarios and generating a recovery plan for each one.  His approach used Prim's Algorithm to generate a minimum spanning tree and construct a recovery plan quickly.  Although the computational effort may be acceptable for pre-mission planning, faster approaches could be useful for real-time, online planning.

Managing the risk of wildfires

In the August 25, 2019, issue of the The Washington Post, Steven Pearlstein wrote a column about how San Diego Gas & Electric (SDG&E) is pro-actively managing the risk that their power grid will cause a wildfire.  Some wildfires are caused when trees or branches fall and hit a transmission line and then the energized power line sparks a fire on the ground.  The power company's approach addresses two potential problems: damage to the transmission line, and the transmission line sparking a wildfire.

For the first risk, SDG&E is burying transmission lines in high-risk areas in the mountains and backcountry.  This is a preventive action because buried transmission lines can not be damaged by falling trees and branches.

For the second risk, SDG&E has installed a system that monitors transmission lines in remote areas, detects when a transmission line is damaged, and turns off the power to that transmission line instantly.  This contingency plan (turn off the power if the line is damaged) prevents the lines from sparking a wildfire.

Moreover, SDG&E is monitoring the conditions in the high-risk area and calculating the conditional probability that a spark would lead to a large wildfire in those locations.  If that conditional probability becomes too high, then the risk has increased to an unacceptable level, and the power company cuts the power to that transmission line (even though it is not damaged).  From Pearlstein's column:

“We are doing with wildfires what the National Weather Service has done with hurricanes,” said Scott Drury, the utility’s president. ... The wildfire model gives SDG&E the confidence to shut off power because it can pinpoint precisely where and when the risks are at levels that have resulted in disastrous fires in the past.

Pearlstein's article also discusses how people in the area are managing the risks associated with a power outage: parking outside their garages, storing water, and buying generators.