Howorth, Lark

NASA Technical Reports Server

1JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
Human Spaceflight 
Conjunction Operations
History, Lessons Learned, and Future 
Improvements for ISS Debris Avoidance
SpaceOps 2011 Workshop
June 14-16, 2011
Lark Howorth
Lead, Space Station Trajectory Operations Group
Mission Operations Directorate, Johnson Space Center
https://ntrs.nasa.gov/search.jsp?R=20110012457 2019-08-30T15:56:10+00:00Z
2JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
Evolution of Conjunction Assessment
• Before 1986 Challenger accident, no Shuttle mitigation actions were taken 
for conjunctions based on assumption that predictions were too imprecise
• After Challenger, analysis led to a “box” method for Shuttle protection
Initial Operations:  Space Shuttle
• The 24/7/365 nature of ISS operations meant the box method 
from Shuttle could not be an option
• Work began at NASA/JSC in early 1990’s to develop infrastructure for “Pc” method
• NASA provided support to USSPACECOM to develop covariances, computing 
capability, and “JSpOC” staffing for conjunction detection
• NASA started trial conjunction assessment operations with Russia 
in 1996 on Mir (“Phase I” program, upon arrival of Americans 
onboard)
• Box method always used as trigger to notify Moscow during Mir, and no Russian 
avoidance maneuver was expected or performed
Preparation for ISS operations:  the Probability Method
3JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
Evolution of Conjunction Assessment:  
ISS Operations
Maneuver thresholds determined for ISS
• Balance safety with minimum of operations disruption
• Red threshold of 1-in-10,000:  Always maneuver if possible; 
place crew in Soyuz if maneuver not possible 
• Yellow threshold of 1-in-100,000:  Maneuver if convenient
Invented Orbital Conjunction Message (OCM) 
format with USSTRATCOM
Developed “validity criteria” on OCMs to ensure 
maneuver decisions based on good data
• Requires multiple updates, with prediction variations within 
expected bounds
Shuttle program and Goddard later adopted 
probability method for conjunctions
0.0 1.0 2.0 3.0 4.0 5.0 6.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Pm=10
-4
Pm=10
-3
Pm=10
-5
Pm=6x10
-6
 
3x10-5
3x10-4
Fractional Residual Risk vs Annual Maneuver Rate
Maneuvers per year
Fr
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annual debris flux = 0.3 obj/km2/year
2260 meter2 cross section vehicle
Pm for 60 meter radius sphere
for 8 hour propagation
Example of risk evaluations used to 
determine ISS maneuver thresholds
4JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
Developing ISS Operations
International coordination 
required to establish ISS 
DAM capability
• ISS has had 6 different vehicles to act 
as propulsion module, each requiring 
varying kinds of coordination:
• FGB (Zarya) until SM arrival
• Service Module (Zvezda)
• Progress:  aft and nadir
• ESA’s Automated Transfer Vehicle (ATV)
• Soyuz (contingency; never used)
• Shuttle
• Each method (except Shuttle) relies 
on a planning template requiring >28 
hours
5JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
12 ISS debris avoidance maneuvers attempted 
7 performed by ISS
1 attempted by ISS, but failed (ISS’s first attempt)
4 performed by Shuttle during mated operations
ISS Debris Avoidance Maneuver History
Date Debris Object Vehicle Notes
06/03/99 1844 (SL-3 R/B) FGB Maneuver Execution FAILED.  Two PC violations (1E-2 and 1E-3) early in event based on bad data
10/26/99 25422 (Pegasus R/B) FGB
First successful ISS DAM and only DAM performed by FGB.
Highest PC = 1E-03. 
09/29/00 5143 (SL-3 R/B) Progress 1P Maneuver on Yellow threshold violation
02/10/01 87610 (Unknown Debris) STS-98 (5A) Shuttle Box method used
12/15/01 5730 (SL-8 R/B) STS-108 (UF-1) Shuttle PC method used
05/15/02 23279 (SL-8 R/B) Progress 7P Pc = 1E-03 (red threshold violation)
05/30/03 25722 (MEGSAT) Progress 10P Pc = 9E-04 (red threshold violation)
08/27/08 33246 (COSMOS 2421 Debris) ATV-1 Pc =2E-02 (red threshold violation) and largest PC calculated to date
03/22/09 26264 (CZ-4 Debris) STS-119 (15A)
Red threshold violations on a multi-repeating conjunction.  TCAs were during an EVA, so a 
Retrograde DAM was executed early by having orbiter hold attitude.
07/18/09 84180 (Unknown Debris) STS-127 (2 JA)
TCA occurred 15 hours after STS-127 docking (during crew sleep).  Due to the docking 
perturbations, a red threshold violation was computed and a reboost was performed by the 
Shuttle before the crew went to sleep.
10/26/10 37195 (UARS Debris) Progress 39P
Pc = 5E-03 (red threshold violation). Probabilities were yellow & red threshold violations 
throughout the event.
4/2/11 34443 (Cosmos 2251 Debris) ATV-2 Conjunction with dozens of repeating passes, high drag.  Prime TCA shifted later during event
6JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
Visiting Vehicle Protection
The Trajectory Operations group (TOPO) at MCC-Houston integrates the 
trajectories of ISS and all vehicles coming to ISS
In recent years, the TOPO office began implementing debris avoidance support 
for the operators of these visiting vehicles
• ESA’s ATV (1 mission per year)
• JAXA’s HTV (1 mission per year)
• Russia’s Soyuz and Progress (approximately 7 missions per year)
• Commercial vehicles (SpaceX Dragon and Orbital Cygnus) (1-2 missions per year each)
• This protects not only the vehicles in rendezvous, but keeps the ISS environment safe as well.
TOPOs use close relationship with JSpOC to help provide the highest-quality data 
for use in protecting these vehicles
• Maintain single human spaceflight point of contact with JSpOC to provide data for partners to assess risk
• As with ISS, all visiting vehicles screen their planned maneuvers against debris catalog
• In development of conjunction operations, TOPO has provided expertise in
• Negotiating ephemeris formats and notification thresholds with JSpOC on behalf of partners
• Providing primary or backup collision probability calculations during mission
• Currently, operational responses are still the responsibility of the operating partner and vary for each vehicle
• With high levels of vehicle traffic expected for many years, ops standardization might become more important
7JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
Implemented Conjunction Improvements for ISS
Russian pre-maneuver command verification improved
Developed “validity criteria” to ensure good Pc 
calculations so unnecessary actions aren’t taken
Improvements in notification criteria to reduce false 
alarms
Implementation of capability to cancel a DAM almost up 
to the time of ignition
Crew Soyuz “shelter in place” capability for late 
notifications where DAM not possible
Ongoing improvements in orbit determination capabilities 
at USSTRATCOM/JSpOC
8JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
ISS Lessons Learned
•“1844 Incident” (1999) proved the need for a very well-defined 
process, well-trained people, and software adequate to the task
– A sequence of errors occurred among all parties, from conjunction 
identification, to “debris clearing” of the DAM, to failed execution of the 
maneuver.  Actions taken:
• Ground processes between Houston and Moscow made clearer
• Russian maneuver command testing implemented
• NASA and USSTRATCOM began routine familiarization cross-training
•Constant improvements to balance safety and mission viability are 
beneficial
– Excessive conjunction notifications from Houston to Moscow early in 
ISS program caused personnel overwork
– In response, partners implemented smarter conjunction notification 
criteria
•Flexibility in planning avoidance maneuvers is key in reducing the 
likelihood of needing to execute them
9JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
ISS vs. Cosmos 2251 Debris
• April 2011 conjunctions with 
Cosmos 2251 had over 40 
registered close passes
• Highest concern TCA changed 
over time
• 0.5 m/s DAM was executed to 
protect highest Pc cases
• Scenario shows value of flexible 
maneuvering capability
• There may be better ways to 
assess risk for repeaters in the 
future
TCA (GMT) TCA-30 (CDT) U V W R 
Time to 
TCA Pc 
2011 091 (Fri, 01 Apr) 03:41:59.541 2011 089 (Wed, 30 Mar) 04:42 PM CDT 2.6 29.5 -7.7 30.6 44.7 3.54E-13 
2011 091 (Fri, 01 Apr) 04:27:40.973 2011 089 (Wed, 30 Mar) 05:27 PM CDT -2.1 133.1 34.2 137.4 25.9 0.00E+00 
2011 091 (Fri, 01 Apr) 05:13:39.612 2011 089 (Wed, 30 Mar) 06:13 PM CDT 2.5 -64.4 16.6 66.5 15.7 0.00E+00 
2011 091 (Fri, 01 Apr) 05:59:18.063 2011 089 (Wed, 30 Mar) 06:59 PM CDT -1.5 106.0 27.1 109.4 27.4 1.90E-10 
2011 091 (Fri, 01 Apr) 06:45:16.485 2011 089 (Wed, 30 Mar) 07:45 PM CDT 1.7 -87.4 22.4 90.2 17.2 1.23E-19 
2011 091 (Fri, 01 Apr) 07:30:52.375 2011 089 (Wed, 30 Mar) 08:30 PM CDT -2.7 121.0 30.8 124.9 22 9.61E-06 
2011 091 (Fri, 01 Apr) 08:16:51.811 2011 089 (Wed, 30 Mar) 09:16 PM CDT 1.1 -90.9 23.3 93.9 24.2 5.77E-15 
2011 091 (Fri, 01 Apr) 09:02:30.236 2011 089 (Wed, 30 Mar) 10:02 PM CDT -2.1 85.9 21.8 88.6 19.5 2.16E-06 
2011 091 (Fri, 01 Apr) 09:48:28.154 2011 089 (Wed, 30 Mar) 10:48 PM CDT 0.4 -107.5 27.4 110.9 25.7 9.66E-08 
2011 091 (Fri, 01 Apr) 10:34:06.606 2011 089 (Wed, 30 Mar) 11:34 PM CDT -1.9 68.5 17.3 70.7 21 1.85E-12 
2011 091 (Fri, 01 Apr) 11:20:05.111 2011 090 (Thu, 31 Mar) 12:20 AM CDT -0.5 -135.5 34.4 139.8 30 0.00E+00 
2011 091 (Fri, 01 Apr) 12:05:42.765 2011 090 (Thu, 31 Mar) 01:05 AM CDT -1.6 53.1 13.4 54.8 22.6 4.96E-17 
2011 091 (Fri, 01 Apr) 12:51:41.170 2011 090 (Thu, 31 Mar) 01:51 AM CDT -1.3 -149.8 37.9 154.5 31.5 0.00E+00 
2011 091 (Fri, 01 Apr) 13:37:18.883 2011 090 (Thu, 31 Mar) 02:37 AM CDT -1.4 39.3 9.9 40.5 24.1 0.00E+00 
2011 091 (Fri, 01 Apr) 15:08:54.994 2011 090 (Thu, 31 Mar) 04:08 AM CDT -1.3 27.0 6.8 27.9 25.6 0.00E+00 
2011 091 (Fri, 01 Apr) 16:40:30.795 2011 090 (Thu, 31 Mar) 05:40 AM CDT -1.3 17.3 4.3 17.9 27.2 0.00E+00 
2011 091 (Fri, 01 Apr) 18:12:06.496 2011 090 (Thu, 31 Mar) 07:12 AM CDT -1.3 8.4 2.1 8.7 28.7 0.00E+00 
2011 091 (Fri, 01 Apr) 19:43:42.272 2011 090 (Thu, 31 Mar) 08:43 AM CDT -1.3 2.0 0.5 2.5 30.2 0.00E+00 
2011 091 (Fri, 01 Apr) 21:15:17.857 2011 090 (Thu, 31 Mar) 10:15 AM CDT -1.2 -1.8 -0.4 2.2 31.7 0.00E+00 
2011 091 (Fri, 01 Apr) 22:46:53.255 2011 090 (Thu, 31 Mar) 11:46 AM CDT -1.1 -3.7 -0.9 4.0 33.3 0.00E+00 
2011 092 (Sat, 02 Apr) 00:18:28.483 2011 090 (Thu, 31 Mar) 01:18 PM CDT -1.0 -3.3 -0.8 3.5 34.8 0.00E+00 
2011 092 (Sat, 02 Apr) 01:50:03.495 2011 090 (Thu, 31 Mar) 02:50 PM CDT -0.8 -1.1 -0.3 1.4 36.3 8.96E-20 
2011 092 (Sat, 02 Apr) 03:21:38.420 2011 090 (Thu, 31 Mar) 04:21 PM CDT -0.5 2.5 0.6 2.6 37.8 4.28E-09 
2011 092 (Sat, 02 Apr) 04:53:13.332 2011 090 (Thu, 31 Mar) 05:53 PM CDT -0.2 7.5 1.8 7.7 39.4 8.58E-05 
2011 092 (Sat, 02 Apr) 06:24:48.222 2011 090 (Thu, 31 Mar) 07:24 PM CDT -0.2 15.2 3.7 15.7 40.9 1.31E-04 
2011 092 (Sat, 02 Apr) 07:56:22.833 2011 090 (Thu, 31 Mar) 08:56 PM CDT -0.3 25.9 6.3 26.7 42.4 7.54E-05 
2011 092 (Sat, 02 Apr) 09:27:57.249 2011 090 (Thu, 31 Mar) 10:27 PM CDT -0.3 38.5 9.3 39.6 43.9 1.44E-04 
2011 092 (Sat, 02 Apr) 10:59:31.460 2011 090 (Thu, 31 Mar) 11:59 PM CDT -0.2 53.6 12.9 55.1 45.5 2.25E-06 
2011 092 (Sat, 02 Apr) 11:45:25.418 2011 091 (Fri, 01 Apr) 12:45 AM CDT -8.6 -19.7 4.8 22.0 76.7 3.54E-13 
2011 092 (Sat, 02 Apr) 12:31:05.538 2011 091 (Fri, 01 Apr) 01:31 AM CDT -0.1 70.4 16.9 72.4 47 7.01E-14 
2011 092 (Sat, 02 Apr) 14:02:39.616 2011 091 (Fri, 01 Apr) 03:02 AM CDT -0.2 88.9 21.2 91.4 48.5 0.00E+00 
 
Summary of ISS conjunctions with object 34443
10JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
 Frequency of notifications varies substantially depending on debris population at vehicle 
altitude.  Number of ISS conjunctions:
 390 from ISS first element launch through July 2008 (~0.8 conjunctions per week)
200 from August 2008 through April 2009 (~5 per week)
COSMOS 2421 – Three breakup events in 2008:  14-March, 28-April, 9-June
 COSMOS 2251 / Iridium 33 collision 10-February-2009
191 from May 2009 through present (~2 conjunctions/week)
As ISS raises permanent altitude from 350 km to 410 km this year, conjunction frequency 
may increase
Lesson: Conjunction Frequency vs. Debris Flux 
11JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
Conjunction Operations Considerations for ISS
•Maximum consequences if we fail
•Size of vehicle: 140 meters between farthest points of solar arrays
•Low altitude, high-drag environment means making assessments for 
“correct” operations difficult
– Late-notifications (“false negatives”)
– Unnecessary concern for some objects
– Highly erratic predictions for small debris
– High Pc for objects very far away (>100 km)
•Many requirements on ISS orbit to satisfy visiting vehicle success
– Space Shuttle rendezvous on flight day 3
– Space Shuttle maximum rendezvous altitude
– Soyuz/Progress rendezvous on FD3; daylight landing in Russian zone
– HTV perigee limits
– ATV mission design limits
– Future vehicle requirements?
– All of these must be considered when designing a DAM for safety
12JSC/MOD/Flight Dynamics    Lark Howorth (281) 483-2754                william.l.howorth@nasa.gov  
Improvements for the Future
•Improving covariance realism
•Implementing late-notification DAM capabilities 
– Allows critical decisions to be made when uncertainties are small
•Solving the repeating conjunction problem
•Developing covariances for perturbing events on ISS
– Docking/undocking
– Reboost
– Propulsive attitude maneuver
•Adapting operations/resources to an increasing debris population
– Future collisions, satellite breakups
– Improved tracking capability may find more, and smaller, objects
•Overhaul criteria to determine if a Pc is valid
•Investigate using covariance-based debris screening (instead of 
fixed volumes)


Human Spaceflight Conjunction Operations: History, Lessons Learned, and Future Improvements for ISS Debris Avoidance

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