Can Hubble be Moved to the International Space
Station?1
On January 16, NASA Administrator Sean O’Keefe informed scientists and engineers at
the Goddard Space Flight Center (GSFC) that plans to service Hubble in 2006 with the
space shuttle had been cancelled. While there was no single deciding factor in this
decision, the administrator reviewed the primary recommendations of the Columbia
Accident Investigation Board (CAIB) for return to flight: that there should be a method to
inspect the thermal insulation on the underside of the shuttle, that there be a method to
repair damage to this insulation, and that there be a “safe haven” in the event that repairs
could not be performed by the astronauts. It is easier to respond to these
recommendations for trips to the International Space Station (ISS) than to Hubble
because the ISS itself acts as the safe haven, and ISS facilities could be used to assist in
inspection and repair of the shuttle thermal insulation.
Even prior to this announcement, the astronomical community that uses Hubble had been
lobbying heavily for an additional servicing mission, beyond the now-cancelled 2006
mission. The rationale for this was the fact that Hubble instruments could gain yet
another factor of 10 in capabilities with current technology, the fact that a servicing
mission in 2006 was unlikely to be sufficient to make Hubble survive until the launch of
the James Webb Space Telescope, and the fact that NASA had already decided that it was
necessary to return to Hubble once more to install a propulsion module to ensure a safe
de-orbit.
In response to this earlier debate, the suggestion to move Hubble to the ISS had already
surfaced. This was studied by NASA, as well as organizations outside of NASA. The
Space Recovery Corporation promoted the use of its Spacecraft Life Extension System
(SLES), under development for commercial applications, to accomplish the task. This
proposal even achieved prominence in the January 2004 issue of Popular Science.
However, at least one high official within NASA is quoted as saying “that ignores the
laws of physics.”
We briefly review the HST-to-ISS option, examining both whether it is possible without
violating the laws of physics, and what some of the challenges might be if it were to be
done. The basic conclusion is that the orbit transfer is possible, but would require
enormous solar arrays (comparable to the ISS arrays), a chemical propulsion module with
lower thrust than standard “upper stages,” and/or a hybrid of chemical and electric
propulsion. There are significant technical challenges even after the orbit transfer is
accomplished, however there is also tremendous science potential.
1
A summary prepared by the scientific staff of the Space Telescope Science Insititute, which is Operated by the Association of
Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555. Please direct questions or
comments to Henry Ferguson [email protected].
Basic Orbital Mechanics
Orbital inclination changes are costly. They are also common. Most satellites launched
into geosynchronous orbit transfer from a 28° inclination orbit when launched to a 0°
inclination orbit that keeps them over the equator when they are operating. To move to
the ISS orbit, Hubble would have to accomplish a similar inclination change, from 28°, to
51°. However, typical communications satellites are much lighter than Hubble, and the
inclination-change maneuver requires much less propellant at geosynchronous altitude
(35,800 km) than at Hubble’s altitude (590 km).
The energy requirements for changing orbits are usually discussed in terms of the
quantity ∆v, the change in velocity required to move from one orbit to another. For the
classic Hohmann “two-kick” transfer, the change in inclination requires a velocity change
∆v = 2v sin β / 2, where v is the orbital velocity and β is the angle between the two orbital
planes. This works out to 3.1 km/s for the HST-to-ISS inclination change. Less efficient
orbital transfer maneuvers would require a larger ∆v. In contrast, changing from the HST
orbital altitude of 590 km to the lower ISS altitude of 390 km would require much less
energy, with a ∆v of only 0.13 km/s.
The mass of propellant required to perform the inclination changed can be calculated
from the rocket equation:
m fuel = ( m Hubble + m booster )( e ∆ v / v exhaust − 1).
The mass of Hubble mHubble is 11,100 kg. The mass of a chemically propelled booster
suitable for this operation would be in the range 5000 kg (e.g. the Centaur IIA upper
stage with associated control and interface hardware). With typical exhaust velocities of 5
km/s for chemical propulsion, the required fuel mass is approximately 14,000 kg, which
is within the capacity of large upper stages currently used to launch geosynchronous
satellites or interplanetary missions.
Unfortunately, the thrust from most large upper stages would exceed the structural limits
of Hubble, which is roughly 13000 Newtons (N). For example, the Centaur IIA thrust is
about 19000 N. A gentler solution is
required. Gentler chemical propulsion
modules exist, but their fuel capacity is
currently below that required. One
interesting possibility is to modify the
Interim Control Module, designed and built
by the Naval Research Laboratory, and now
in storage (shown at right). This was a
backup unit intended for the ISS that was
built at the time when there was uncertainty
in the Russians’ ability to deliver their
propulsion module.
Another possibility is to use electric propulsion, as envisioned for SLES. Various kinds
of electric propulsion modules exist, including the ion engine that was used for the Deep
Space-1 mission, Hall-effect engines that have been used in over 100 satellites, or arcjet
π 
∆v = 2v02 − 2v02 cos ∆i 
2 
thrusters. A summary of electric propulsion options can be found at
http://www.islandone.org/APC. Electric propulsion generally has a much higher vexhaust,
and hence requires much lower fuel mass than chemical propulsion. However the low
thrust is both a benefit and a problem. Changing the orbital inclination takes much longer
with low thrust. Furthermore, the thrust can only be applied on the bright side of the orbit
if the electric power is provided exclusively by solar panels. The low-thrust transfer
maneuvers require a larger ∆v to accomplish the same change in inclination. Computing
optimal orbit transfers is complicated, but we can get a ballpark estimate from
Edelbaum’s equation (1961, “Propulsion Requirements for Controllable Satellites,” ARS
Journal, 1079). This equation assumes a constant thrust that switches signs at the
antinodes. In the case of a simple change of inclination for a circular orbit, the equation
reduces to:
This formula implies ∆v = 4.7 km/s for the HST-to-ISS inclination change. For this
ballpark estimate, let us assume a thrust of 1N and an exhaust velocity of 20 km/s. This
plausibly within the range of state-of-the-art Hall-effect thrusters such as the Pratt &
Whitney T-220. Assuming the same mass for the booster, the fuel mass is now
approximately 4300 kg instead of the 14000kg required for chemical propulsion. The
time it would take to achieve the orbit transfer is roughly ∆v/a where a is the acceleration
from the thruster. With our putative 1N thruster, this acceleration is very small: 4.9×10-5
m/s2. It would take approximately 3.0 years to get to the
ISS. To make the transfer times reasonable would require
several of these thrusters (one year is perhaps a reasonable
upper limit to the acceptable amount of time.)
Alternatively, a higher-thrust unit, such as the NASA
457M (shown at right), which has tested at 3N, could be
used if it could be space-qualified for the required total
impulse (lifetime limitations are a serious technical issue
for this kind of thruster). As a very rough guide, it takes
approximately 20 kW of electric power to produce one
Newton of thrust. Each solar array wing on the ISS
delivers 64 kW of power, so this requirement is high, but
is not unheard of.
A serious technical issue is the fact that HST is in shadow nearly half the time. Thus the
constant-thrust assumption used above would only be valid if there is another source of
power during dark time. Perhaps simpler would be to use a hybrid of chemical propulsion
and electric propulsion, e.g. making use of the existing ICM chemical propulsion
capability.
Clearly, the amount of power or propellant required to move HST to the ISS orbit is very
large. The engineering challenge is no doubt significant. Nevertheless, transfer to the ISS
orbit is a budgetary and engineering problem, not a physics question. The most
significant challenge may be the autonomous rendezvous with HST, which is a challenge
that must be tackled even to bring HST back to earth.
Would Hubble and the ISS make good neighbors?
While changing the orbital inclination appears technically possible, it is not entirely clear
that Hubble can operate at the ISS altitude, or, if it could, whether that would be
desirable. Among the technical issues that would have to be addressed are the following.
• The density of the atmosphere near solar maximum is sufficiently high at 390 km
that Hubble may have difficulty pointing.
• Any rendezvous with the ISS would have to be done exceedingly carefully, perhaps
assisted by the shuttle.
• There are enough contamination concerns about the ISS environment that it is
unlikely that it will be desirable to operate Hubble near the ISS. Instead the ISS
would serve as a base for servicing.
• If Hubble were left at a higher altitude, there are several options for servicing: (1)
shuttle could visit the ISS, have the tiles inspected, and then proceed to Hubble; (2)
Hubble, equipped with a propulsion module, could transfer to the ISS orbit for
servicing. A major complication is the fact the orbits will precess at different rates,
imposing timing constraints for low-energy orbit transfers. At their current altitudes,
the orbits would precess such that the nodes would line up about once every two
years.2
• It is unlikely that a propulsion module with enough power to change the orbital
inclination could be left attached to Hubble for science operations. It probably
would be too massive and would affect the center of gravity. Instead, if Hubble were
to be serviced and then moved to a higher orbit, the propulsion module would have
to be stripped down. Parts such as solar panels could be re-used on the ISS itself.
Benefits of moving to the ISS orbital inclination
While the cost is likely to be considerable (although perhaps not compared with the
$400M estimated for a shuttle mission), and there are clearly technical issues concerning
servicing once the orbital inclination has been changed, there are nevertheless a host of
attractive features of this solution that make it worthy of continued technical study.
• This solution satisfies all of the CAIB requirements, to the extent that any mission
to the ISS satisfies those requirements.
• Servicing Hubble in this manner will realize the enormous scientific return on the
$250M invested in equipment that was to be installed on the next servicing mission.
• With Hubble in the ISS orbit it is possible to consider additional upgrades. With
current technology it is possible to realize yet another factor of 10 improvement in
capability. Concepts exist for an optimised coronagraph to detect planets in orbit
around nearby stars, or a wide-field imager to explore the mysterious dark-energy
that is responsible for accelerating the cosmic expansion.
22
The precession of the line of nodes is given by
dΩ − 3nJ 2 RE2 cos i
, where J2=1.08263×10-3 is a
=
2
2 2
dt
2a (1 − e )
coefficient describing Earth’s oblateness, RE is Earth’s equatorial radius, a,e, and i are the orbit semi-major
axis, eccentricity, and inclination (respectively), and n is the orbital mean motion (GME/a3)1/2.
•
•
•
De-orbiting Hubble when a suitable replacement is finally in orbit would simply be
a matter of docking it to the ISS and de-orbiting them together (the ISS has
propulsion already).
NASA is already planning for an autonomous propulsion module. That part of the
engineering challenge must be met whether or not the scientific mission is extended.
Servicing Hubble is noble work for the International Space Station.