TO: Brian O\'Connor, NASA Headquarters AA for Safety and Mission
Assurance

Wayne Frazier, NASA Headquarters Flight Assurance

FROM: Granville Paules

SUBJ: EO-1 Orbital Debris Status per NSS 1740.14

DATE: 11/22/04

Dear Brian/Wayne,

This memorandum is to inform you of our strong desire to continue flying
the Earth Observing One (EO-1) spacecraft beyond the point where it
would exceed the maximum allowable disposal period per the guideline
reflected in NSS 1740.14, Guideline 6-1, Option a.

EO-1 was launched in November 2000 as a one-year technology
demonstration mission. It is in a circular polar orbit following one
minute behind the Landsat-7 spacecraft. Prior to launch, an orbital
debris analysis was performed and a nominal uncontrolled re-entry
altitude was computed (610km X 700km) to comply with the 25-year maximum
disposal period. Recently the disposal analysis has been revisited
because EO-1 is still on-orbit, fully functional, and is a candidate for
supplying a major part of the Landsat gap filler data required by NASA
over the next 4-5 years. NASA has a legal requirement to "ensure Landsat
data continuity" while, at the same time, we seek to meet our
established disposal requirements. Presently Landsat-7 is experiencing
difficulties and a Landsat follow-on mission is in formulation. In the
meantime, we must provide a "gap filler" capability from existing
resources until the Landsat follow-on mission is operational. We seek a
course for EO-1 that enables us to support Landsat data continuity in
the near-term but will re-enter EO-1 as soon as that role is completed.

The EO-1 fuel balance is such that de-orbit maneuvers can be
accomplished anytime this year to stay within the disposal period
maximum of 25 years. However, to stay one minute behind Landsat-7
requires annual inclination precession compensation maneuvers that are
usually performed in early October by both missions to control the Mean
Local time drift and maintain flight on the World Reference System
(WRS-2) grid. The Delta-I maneuvers use most of the fuel burned during
the fiscal year.

It is essential to keep EO-1 one minute behind Landsat-7 to optimize its
gap filling utility. We have worked with Nicholas Johnson of the Orbital
Debris Program Office at JSC and Scott Hull of the GN&C Systems
Engineering Branch at GSFC to re-evaluate the disposal profile for the
EO-1 spacecraft. The results of those analyses are that:

-   The EO-1 Debris Survivability Casualty Area (5.9m^2^) will continue
    to remain well within the international standard of 1 in 10,000
    probability of causing significant injury to one person.

-   Using a cross-sectional area as 6.2933 m^2^ (which equates to 0.0115
    m2/kg for Area-to-Mass ratio), the calculated values of orbital
    lifetime for various initial conditions are as follows:

Disposal start to occur before Perigee x Apogee Uncontrolled disposal
duration

> Oct 2005 600x700 20.4 yrs (within Guideline)
>
> Oct 2006 620x700 25.8 yrs (exceeds guideline by 0.8 years)
>
> Oct 2007 650x700 28.1 yrs (exceeds guideline by 3.1 years)
>
> Oct 2008 670x700 34.9 yrs (exceeds guideline by 9.9 years)
>
> Oct 2009 680x700 35.6 yrs (exceeds guideline by 10.6 years)

 

Each of these potential de-orbit scenarios moves the EO-1 spacecraft out
of the way of the morning constellation of satellites and puts it in a
precessing orbit that does not cause future close encounter concerns
with the other satellites. Preserving enough fuel for this maneuver
requires that EO-1 conduct a disposal maneuver no later than in October
2009.

There is no increased likelihood of EO-1 exploding or otherwise causing
additional debris or on-orbit contamination to be generated due to the
possibility of taking longer to reenter. Therefore, other than the
increased duration of the disposal profile, no additional risk is
anticipated due to a decision to keep EO-1 on orbit beyond its guideline
maximum. It is to be noted that all EO-1 systems are presently operating
within specifications and there are no operating trends that indicate
any serious degradation of any life-limiting components. Therefore, it
is expected that all life-limiting components should work through
September 2009.

We strongly desire to allow EO-1 to exceed the guideline and continue
one minute behind Landsat-7 for the Delta-I that will occur next October
2005 and possibly beyond that date as the utility of EO-1 to perform
part of the Landsat-7 gap filling role is demonstrated. Should this role
fail to materialize, we would be willing to conduct a disposal maneuver
for EO-1 at the next opportunity.

Please let me know if you have any concerns as to this course of action.