This document summarizes a lecture on space debris and the increasing risk of collisions. It discusses how the number of predicted close approaches between debris fragments is rising even as the number of fragments decreases over time. Data is presented on close approaches related to debris from the 2007 Fengyun-1C anti-satellite test and the 2009 Iridium-Cosmos collision. While around 27,000 close approaches under 5km were predicted per week in 2019, by 2021 that number had risen to around 46,000 per week. Similarly, close approaches under 1km rose from 1,100 to 2,400 per week. This increasing risk is likely due to the dispersing and expanding cloud of fragments over time.

1. Space Environment
Lecture 28 – Space Debris (Vol. 2)
Consequences (Part 1)
Professor Hugh Lewis
SESA3038 Space Environment

2. Overview of lecture 28
• In this lecture we start to look at the impacts of space debris on spacecraft
operations. In particular we focus on close approaches (also known as
“close passes” or “conjunctions”):
– Collision assessment is a key service provided by government and
commercial organisations (e.g. Space-Track and LeoLabs)
• First, we look at the predicted conjunctions associated with the fragments
from major debris-producing events (data from SOCRATES):
– The January 2007 ASAT test (Fengyun 1C)
– The February 2009 Iridium 33-Cosmos 2251 collision
• We investigate the trends and ask why the number of conjunction events
associated with these fragments is increasing when the number of
fragments is actually decreasing
Space Environment – Space Debris (Vol. 2)

3. COMBO Space Environment – Space Debris (Vol. 2)
Computation of Miss Between Orbits
United States Space Control Center (US SSC)
“All-on-all” analysis
Results from Payne (1996) and Payne et al. (2005):
Number of close approaches per day
• Averages based on three days in each year:
1996 2005
< 100 km 631,766 713,739
< 25 km 42,281 40,292
< 10 km 6,864 7,814
http://adsabs.harvard.edu/full/1997ESASP.393..597P

4. Space-Track Space Environment – Space Debris (Vol. 2)
• https://www.space-track.org/auth/login
• Space situational awareness service
provided by US Space Force, 18th Space
Control Squadron
• Free to register and access the API:
• Orbital element sets (TLEs)
• Satellite Catalog
• Re-entry predictions
• Conjunction predictions

5. LeoLabs Data Platform Space Environment – Space Debris (Vol. 2)
• Offers data and analyses:
• Radar measurements
• Objects state estimations
• Object propagations
• Conjunction screening and alerts
– 3 million conjunctions < 100 km each day
– 800,000 conjunction messages per day
– Satellites can query the API and receive
conjunction information
– E.g. Starlink
https://www.hou.usra.edu/meetings/orbitaldebris2019/orbital2019paper/pdf/6096.pdf

6. SOCRATES Space Environment – Space Debris (Vol. 2)
Celestrak: Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space
http://celestrak.com/SOCRATES/

7. Iridium-Cosmos collision Space Environment – Space Debris (Vol. 2)
Data from SOCRATES (note that these are predictions for a 7-day period)
• Conjunctions < 5 km
involving COSMOS 2251
DEB increasing by ~400
per report per year (R2 =
0.80)
• ~14 per report per year
for conjunctions < 1 km
• Conjunctions < 5 km
involving IRIDIUM 2251
DEB increasing by ~115
per report per year (R2 =
0.43)
• ~5 per report per year
for conjunctions < 1 km

8. Fengyun 1C ASAT test Space Environment – Space Debris (Vol. 2)
Data from SOCRATES (note that these are predictions for a 7-day period)
• Conjunctions < 5 km
involving FENGYUN 1C
DEB increasing by ~650
per report per year (R2 =
0.80)
• ~27 per report per year
for conjunctions < 1 km

9. Major debris-producing events Space Environment – Space Debris (Vol. 2)
Why is the number of conjunctions
increasing when the number of fragments
is decreasing?
• Prediction of the percent of the
total FENGYUN 1C DEB
population decaying from orbit
over time
• Assumptions:
– All fragments large enough to
be tracked by US SSN
– All fragments are the same
size and mass (0.38 kg)
– All fragments have the same
density
http://celestrak.com/events/asat.php

10. Major debris-producing events Space Environment – Space Debris (Vol. 2)
The total number of predicted
conjunctions is increasing:
• Total number of predicted
conjunctions < 5 km has increased
from ~27,000 per week in March
2019 to ~46,000 per week by
February 2021
• ~50% increase in nearly 2 years
• Total number of predicted
conjunctions < 1 km has increased
from ~1100 per week in March 2019
to ~2400 per week by February
2021
• ~120% increase in nearly 2 years

11. Major debris-producing events Space Environment – Space Debris (Vol. 2)
Data from SOCRATES (note that these are predictions for a 7-day period)
1-in-2.95
1-in-4.04

12. Major debris-producing events Space Environment – Space Debris (Vol. 2)
Data from SOCRATES (note that these are predictions for a 7-day period)
1-in-2.98
1-in-5.46

13. Recap of lecture 28
• In this lecture we began our look at the impacts of space debris on
spacecraft operations. In this topic, we are focusing on close approaches
• First, we looked at the predicted conjunctions associated with the
fragments from major debris-producing events using data from
CelestrakSOCRATES:
– The January 2007 ASAT test (Fengyun 1C)
– The February 2009 Iridium 33-Cosmos 2251 collision
• We investigated the trends and asked why the number of conjunction
events associated with these fragments is increasing when the number of
fragments is actually decreasing
• In the next few lectures we will look at other sources of conjunctions –
those associated with particular space systems
Space Environment – Space Debris (Vol. 2)

14. Activity:
• Recent SOCRATES data has
been provided for you on the
Blackboard site:
• You can look at these data
• You can analyse these data (maybe
produce some graphics similar to
those used in this lecture)
• Visit the SOCRATES website:
• http://celestrak.com/SOCRATES/
• To understand how these data are
generated and what each event
means
Space Environment – Space Debris (Vol. 2)