Allocating Time on the Hubble Space Telescope

1
Allocating Time on the Hubble Space Telescope

• Olivia Gillham

2
Contents

• What is Hubble Space Telescope?
• Current scheduling framework
• Formalization as a scheduling problem
• Optimization of basic problem
• Additional issues
• Recommendations

3
What is Hubble Space Telescope?

• 2.4 meter mirror collects light (and some forms
  of radiation), directs it to scientific
  instruments
• Orbits 569 km above the Earth
• Solar-powered
• Location above Earths atmosphere provides
  unusual clarity - no distortion or absorbtion
• Completes an orbit every 97 minutes

4
Who gets to use Hubble?

• Anyone can apply for time on Hubble
• Proposals are submitted and reviewed annually
• Proposals are accepted based on their value to
  the scientific community
• Mostly professional astronomers, but amateurs are
  occasionally approved

5
How are proposals selected?

• Time Allocation Committee(TAC) composed of
  international experts selects proposals based on
  their value to the scientific community
• TAC tries to select even proportions of long,
  medium, and short projects
• Director of Space Telescope Science Institute
  (STScI) approves selection

6
How are projects scheduled?

• STScI scheduling experts form a tentative
  long-range schedule for the year
• Detailed schedules are created 3 weeks ahead of
  time and transmitted to Hubble weekly

7
Contents of a Phase I Proposal

• A summary of the observations requested
• The scientific justifications for the project.
• A list of targets with their celestial
  coordinates.
• The instrument(s) to be used with desired modes,
  filters, and dispersers.
• An estimate of the number of spacecraft orbits
  needed to accomplish the observing program

8
Basic Formalization

• Single machine
• pj orbits required
• wj scientific value
• All jobs must be completed within 1 year, so dj
  D 1 year 5422 orbits
• Maximize value gt minimize weight of tardy jobs
  sum of wjUj.
• 1/ djD /sum wjUj knapsack!
• Dynamic algorithm runs in O(nD)O(5.5mil)

9
Complications

• Snapshot proposals requiring 45 minutes or less
  (lt1/2 orbit) are prioritized
• Targets of Opportunity(TOO) occur at unknown
  times
• Unforeseen celestial events
• Set-up times (to lock on to new coordinates) vary
• STScI Director allots 10 of total time (usually
  to TOO and projects too large to be approved by
  the committee)

10

• Snapshots can be treated as normal jobs, pj .5
• Targets of Opportunity have unknown rj, dj, but
  wj and pj are known, so they can be included in
  the selection process
• Unforeseen events have unknown parameters -
  online scheduling scenario
• Weekly programming means we can address TOO and
  unknown jobs dynamically, as they come up
• Create original schedule for 90 of total time,
  or 4880 orbits

11
Further Complications

• Some jobs can only be completed at certain times
  of the year (differing rj and dj)
• Set-up time between jobs varies (locking onto
  guidance stars)
• Both can be incorporated into the formal model as
  stochastic elements, but complicate the solution
  enormously

12
Blue box set-up White box observation Pink
bars target blocked Red bars guide star
blocked
13
Preliminary Recommendations

• Re-allocate responsibility so that TAC expert
  committee assigns weights, STScI scheduling staff
  selects and schedules
• Include snapshots, TOOs, and too long projects
  in weighting and selection
• Dynamic knapsack algorithm on 4880 orbits
  provides initial optimal solution
• Use Directors time primarily for TOOs and
  unexpected events, possibly reducing from 10

14
QA
15
References

• Villard, Ray The Hubble Space Telescope
  Observing Program. http//www.nasa.gov/mission_pa
  ges/hubble/servicing/series/How_science_is_done.ht
  ml
• Hubble Essentials. Hubble Site
  http//hubblesite.org/the_telescope/hubble_essenti
  als/
• Space Telescope Science Institute, Hubble Space
  Telescope. http//www.stsci.edu/hst/