NASA Technical Reports Server (NTRS) 20080038679: Ares V and Future Very Large Launch Vehicles to Enable Major Astronomical Missions

0.15 MB·English

by  NASA Technical Reports Server (NTRS)

8-25-08 draft
59th International Astronautical Congress

IAC-08-A5.3.6

ARES V AND FUTURE VERY LARGE LAUNCH VEHICLES TO ENABLE MAJOR
ASTRONOMICAL MISSIONS

Harley A. Thronson
Astrophysics Science Division
NASA Goddard Space Flight Center
[email protected]

Daniel F. Lester
Department of Astronomy
University of Texas
[email protected]

Stephanie R. Langhoff
Chief Scientist
NASA Ames Research Center
[email protected]

Randy Correll
Ball AeroSpace
[email protected]

H. Philip Stahl
NASA Marshall Space Flight Center
[email protected]

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ABSTRACT

The current NASA architecture intended to return humans to the lunar surface includes the Ares
V cargo launch vehicle, which is planned to be available within a decade. The capabilities
designed for Ares V would permit an 8.8-m diameter, 55 mT payload to be carried to Sun-Earth
L locations. That is, this vehicle could launch very large optical systems to achieve major
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scientific goals that would otherwise be very difficult. For example, an 8-m monolith
UV/visual/IR telescope appears able to be launched to a Sun-Earth L location. Even larger
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apertures that are deployed or assembled seem possible. Alternatively, multiple elements of a
spatial array or two or three astronomical observatories might be launched simultaneously.

Over the years, scientists and engineers have been evaluating concepts for astronomical
observatories that use future large launch vehicles. In this presentation, we report on results of a
recent workshop held at NASA Ames Research Center that have improved understanding of the
science goals that can be achieved using Ares V. While such a vehicle uniquely enables few of
the observatory concepts considered at the workshop, most have a baseline mission that can be
flown on existing or near-future vehicles. However, the performance of the Ares V permits
design concepts (e.g., large monolithic mirrors) that reduce complexity and risk.

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1. WORKSHOP BACKGROUND AND
GOALS (4) Is there a trade-off between mass and
complexity that could reduce launch risk and,
On April 26th and 27th, 2008, NASA Ames thereby, the cost of building large telescopes?
Research Center, directed by Dr. Peter
Worden, hosted a two-day workshop entitled The results of this workshop has been
“Astronomy Enabled by Ares V.” The provided to the National Research Council’s
primary goal of the workshop was to begin Science Opportunities Enabled by NASA's
the process of bringing the Ares V designers Constellation System study effort, which is
together with senior representatives of the currently underway.
astronomical community to discuss the
feasibility of using the Ares V cargo launch 2. WORKSHOP SUMMARY
vehicle, a major element in NASA’s
Constellation Program, to launch large The workshop began with an overview of the
observatories. This paper reports on the Constellation program and the role of future
essential results and recommendations of the NASA launch vehicles. One of the essential
workshop. A complete report of the ground rules of the workshop was that
workshop has been produced by Langhoff, changes in the Ares V cargo launch vehicle
Lester, Thronson, and Correll (2008) [1]. cannot compromise its primary mission of
transporting the Altair lander and supplies to
All presentations and discussions at the the lunar surface.
workshop referred to the Ares V design
concept at that time. The design and even The large lift mass capability of Ares V
the name “Ares V” could change in the (approximately 55 metric tons to Sun-Earth
future. Nevertheless, we believe this L ) and large fairing (8.8 meter interior
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workshop opened up astronomical trade diameter) opens up new telescope design
space for any future generation heavy-lift possibilities that could significantly enhance
launcher. the future scientific missions of many kinds.
As it transpired, most of the observatory
Key questions at the workshop included: concepts that were discussed at the workshop
are limited by volume, not mass, and many
(1) Are there telescope concepts or missions of the missions could take advantage of a
capable of breakthrough science that are “taller” fairing than the baseline design. The
either enabled or significantly enhanced by length of the Ares V fairing is ultimately
the capabilities of an Ares V cargo launch constrained by the height of the Vehicle
vehicle? Assembly Building (VAB).

(2) What demands do large telescopes place While an Ares V cargo launch vehicle
on the payload environment of the Ares V, uniquely enables a few of the telescope
such as mass, volume, fairing shape, concepts considered at the workshop, most
cleanliness, acoustics, etc.? have a baseline mission that can be flown on
existing or near-future heavy-lift launch
(3) What technology and environmental vehicles. However, the large fairing and lift
issues need to be addressed to facilitate capabilities of the Ares V open up new
launching observatories on an Ares V? design flexibility, for example large mono-
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lithic mirrors of reduced complexity and The workshop clearly showed that the Ares
have no risk of deployment. Such larger- V has considerable potential to do
aperture telescopes offer much higher breakthrough astronomy. It is also likely that
sensitivity and spatial resolution than it could advance the Earth science and
telescopes that can be launched with current planetary science goals of NASA. A follow-
launch vehicles. This is particularly on workshop on solar system science
important for studies of the early Universe applications was held at NASA Ames August
and for imaging exosolar planetary systems. 17 and 18, 2008. The results of that
workshop will be reported on separately.
While it is too early in the design cycle of
the Ares V for a definitive understanding of 3. WORKSHOP PRESENTATIONS
its launch environment, designing to a AND DISCUSSION
launch environment comparable or better
than the Shuttle was considered a good 3.1. The Ares V Cargo Launch Vehicle
metric. Since the main engines can be
throttled, it is expected that the acoustic and The workshop began with an overview of the
dynamic loads can be kept within acceptable Constellation Program by Steve Cook, Ares
limits. Project Manager at NASA MSFC. The
discussion here attempts to capture some of
One recurring theme that had not been the key points made in his presentation, and
included in the workshop agenda was the to set the stage for the science cases that
importance of on-orbit servicing of follow. More in-depth and authoritative
astronomical observatories. The recent accounts of the rapidly unfolding
success of the Defense Advanced Research Constellation and Ares programs exist on the
Projects Agency’s (DARPA) Orbital web, for example, at
Express demonstrated that on-orbit servicing http://www.nasa.gov/mission_pages/constell
can be done autonomously if the telescopes ation/ares/aresV.html. The key focus of the
are designed for standard servicing Constellation Program is to deliver both
functions. For almost two decades, the cargo and humans to the lunar surface by
Hubble Telescope servicing missions have 2020. At the same time, it has been
dramatically increased the scientific value of increasingly recognized that such transport
the telescope by implementing improved systems can straightforwardly access other
instruments and detector technology. The interesting destinations such as
Ares V cargo launch vehicle and other Geosynchronous Earth Orbit (GEO), the
Constellation assets could enable servicing Sun-Earth and Earth-Moon Lagrange
of satellites either with robots or with (libration) points, and some asteroids.
astronauts. It is precisely because Ares V
and other future large vehicles can launch Key elements of the Ares V cargo launch
extremely large, capable, and expensive vehicle were adopted for the workshop. The
telescopes that on-orbit servicing to repair payload fairing is being designed to carry
and upgrade those telescopes appears to add the Altair lunar lander. One of the primary
considerable value. This subject is further focuses of the workshop was to determine
discussed in Section 3.5.1 and is what demands launching large astronomical
recommended to be the subject of follow-on observatories might place on the size of the
workshops. fairing. There may be some design
flexibility in the fairing as long as it carries
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out its principal mission of transporting concepts for all of the key elements of the
Altair to the lunar surface. Other elements of Ares V including the EDS, the core stage,
the Ares V presented at the workshop the notional instrument unit, the EDS J-2X
include the Earth Departure Stage (EDS), a engine, the SRBs, and the upgraded RS-68
loiter skirt, an interstage, and then the core engine. As all this information is available
stage powered by five Delta IV derived RS- on-line
68 LOX/LH2 engines and two solid rocket (http://www.nasa.gov/mission_pages/constel
boosters. The Ares V presented at the lation/main/index.html) and not critical to
workshop is being designed to use many of how an Ares V could be used to launch large
the major components being developed for telescopes, we omit the details here.
the Ares I crew launch vehicle. For example, However, the element of the Ares V that
the first-stage 5-segment solid rocket seems most important for astronomical
boosters, the J-2X upper stage engine, and missions is the shape and interior dimension
the instrument unit, will all have heritage on of the upper stage fairing. Sumrall presented
Ares I. This adaptation is intended to a shroud shape trade study that they had
greatly reduce schedule and cost risks, as done within the restriction of a 9.7-m barrel
well as development and life-cycle costs. height. This barrel height is required to
accommodate the current Altair lunar lander
3.2. Overview of Ares V Performance configuration. They considered many
shapes, but selected the biconic shroud
Phil Sumrall, the NASA MSFC Advanced shown as their baseline. A critical dimension
Planning Manager for the Ares Projects is the 8.8-m diameter interior of the barrel.
Office, gave a two-part presentation on Ares The maximum length of the barrel is
V, providing first a mission and vehicle constrained to 18.7 m by the height of the
overview, and then a description of per- Vehicle Assembly Building (VAB) at
formance. Again, we emphasize that this is a Kennedy Space Center (KSC). Increasing
current snapshot of an on-going program. the barrel length to this maximum reduces
The Ares V cargo launch vehicle, which is the payload mass capability slightly. For
primarily being designed to place large example, the payload to Sun-Earth L is
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masses on the lunar surface, is intended to reduced from 55.8 to 55.1 mT by using the
have greater payload capacity to low Earth extended fairing. For astronomical missions,
orbit (LEO) (~140 metric tons (mT)) than the longer notional shroud was generally
any previous vehicle, including the Saturn favored (see later discussion), because these
V. The Orion crew exploration vehicle (CEV), missions are usually constrained by volume,
which is launched separately on an Ares I not payload mass.
crew launch vehicle, performs a rendezvous
and docking with the EDS. Finally, the EDS Preliminary analyses indicate that the
trans-lunar injection (TLI) burn sends the payload environment (e.g., acoustic loads,
Altair lunar lander/CEV on to the Moon. A vibration, cleanliness, etc.) should be
loiter skirt, which is connected to the EDS, comparable to other heavy launch vehicles
supports a four-day loiter period in LEO.
and thus unlikely to negatively impact
Presumably, on an Ares V mission used to
launching large telescopes. This is discussed
launch an observatory, this loiter skirt and
in more detail for specific missions in the
loiter period would not be required, which
following sections.
would add further to the payload capacity.

3.2 Lessons from HST: Maximizing the
Sumrall discussed in detail the design
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Value of Large Investments the apex of discovery potential. The
importance of on-orbit servicing in
Frank Cepollina (NASA GSFC) presented a extending the lifetime and science
paper discussing the lessons that have been capabilities of a telescope could not have
learned from the Hubble Space Telescope been made more dramatically, according to
(HST) servicing missions. [2] He began with Cepollina.
a quote from NASA Administrator, Michael
Griffin, who observed, “It is dumb to launch Looking toward the future, Cepollina
complicated, expensive telescopes into outlined a concept for a piloted Orion
space that cannot be serviced.” With the servicing vehicle. He discussed the
opportunity that Ares V and other future possibility of a servicing arm attached to
large systems presents to launch very large Orion and controlled telerobotically by
and potentially expensive telescopes, it is onboard astronauts. His mission concept was
especially useful to consider on-orbit to replace an instrument of a large telescope
servicing as a means of expanding their at its operational location at Sun-Earth L
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scientific productivity. point or, alternatively, at the more accessible
Earth-Moon L jobsite [Ref. 3 and this
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The thesis of Cepollina’s talk was that HST conference] using an augmented Orion and
servicing missions have dramatically Lunar Surface Access Module (LSAM).
increased the scientific value of the Cepollina reported on a study that compared
telescope. Periodic changes in instruments to costs of a series of expendable new
focus on new scientific questions, and the telescopes versus a regularly serviceable
implementation of new instrument detector telescope, which indicated that the servicing
technology, result in a continuing scenario was less costly overall. He ended by
rejuvenation of the scientific performance of emphasizing that the time is now to start
this popular mission, and an enhanced pace studying how elements of the Constellation
of innovation and discovery. Cepollina program such as the Orion crew exploration
described the history of on-orbit servicing vehicle could contribute to the major science
missions beginning with the Solar goals by extending human and robotic
Maximum Repair Mission. To date, there servicing missions to the next generation
have been nine servicing missions to a telescopes; that is, if we can do servicing
variety of LEO satellites, which have shown missions with the Shuttle, why not with
that a satellite’s design (e.g., standardized Orion?
modularity) is the single most significant
aspect of cost apart from launch costs. Of 3.4. Telescope Concepts/Missions
these nine there have been four servicing
missions to HST, each time resulting in new The Ares V cargo launch vehicle could be a
scientific discoveries and an explosion of significant enhancement for astronomical
scientific papers. The SM4 servicing observations by allowing for much larger
mission planned for late 2008 will add the telescopes, which in turn enables much
Cosmic Origins Spectrograph and Wide fainter objects to be studied. In this section,
Field Camera 3, again extending the science we summarize the presentations on seven
capabilities of the telescope. To date, there astronomical concepts that are either enabled
have been over 6000 refereed papers by or significantly enhanced by the
generated from HST data. With the availability of an Ares V cargo launch
completion of SM4, Hubble will again be at vehicle.
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A key message of Lester’s presentation was
3.4.1 Emerging Pathways for the the importance of servicing the spacecraft.
Single-Aperture Far-Infrared For this to be feasible, the spacecraft would
Telescope (SAFIR) with an Ares V. have to be modular to facilitate the
replacement of the focal plane science
Dan Lester (University of Texas) discussed instruments, spacecraft systems, and the solar
how an Ares V cargo launch vehicle could shield. Instrument upgrades are particularly
enhance the Single Aperture Far Infrared important in the FIR, for which detector
Telescope (SAFIR) concept. The large sensitivity and array sizes are undergoing
interior diameter of the Ares V shroud rapid improvement. Lester described a
allows for a large single-substrate servicing concept where the telescope is
(monolith) primary mirror diameter of brought back from its optimal operating
approximately 8 m. SAFIR is designed to location at Sun-Earth (SE) L to Earth-Moon
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obtain spectra in the far infrared (FIR) (~20- (EM) L , which is much more convenient for
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300 μm) using high-performance focal plane human travel. Transit from SE L to EM L
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FIR sensors. The entire telescope is cooled requires only a few tens of meters/sec delta-
to less than 10 K by both passive and active V [3].
means. SAFIR will explore the FIR universe
with higher sensitivity and spatial resolution 3.4.2. ATLAST: The Roadmap
than previously achieved, providing new to an 8-m to 16-m UV/Optical
insights into the cosmic history of star Space Telescope
formation and nucleosynthesis. [4, 5]
Marc Postman (Space Telescope Science
With an Ares V, the single-substrate 8-m Institute) described a concept for both an 8-m
mirror could be launched with no and 16-m UV/Optical Space Telescope that
deployment mechanisms required. This could be deployed from an Ares V. The
could significantly reduce complexity, risk, Advanced Technology Large-Aperture Space
deployment design, and integration and Telescope (ATLAST) would have
unprecedented sensitivity and angular
testing, which translates into reduced cost.
resolution in the optical region. It could
Since SAFIR is diffraction limited at 20 μm,
investigate a wide range of important
the optics will be lighter than, for example,
astronomical issues, such as how the present
the James Webb Space Telescope (JWST).
Universe formed, how galaxies form, and how
The launch mass is only 10 mT or ~20% of
planetary systems form from circumstellar
the launch capacity of Ares V for a mission
disks. [6] In conjunction with a separately
to Sun-Earth L . This would allow
2 launched external occulter or a high-
significant augmentations to the observatory
performance coronagraph to block the light
such as more/larger instruments, as well as
from the central star, it would be able to
an enhanced spacecraft. Simple scaling of
characterize the atmospheres of exosolar
the SAFIR baseline concept, but with a
planets. The 16-m ATLAST, in particular,
JWST-like deployable primary mirror,
would be able to obtain spectra of the
would allow a ~20-m diameter telescope atmospheres of Earth-like planets in the
with 10 times the sensitivity and 2–3 times habitable zone of thousands of candidate stars.
the spatial resolution to be accommodated
by an Ares V cargo launch vehicle. As in the previous talk by Dan Lester,
Postman also discussed the tradeoff between
mass and mission complexity. Reducing
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complexity reduces risk and cost, thus an The Stellar Imager concept calls for a space-
Ares V could reduce the overall cost of the based UV/Optical Fizeau interferometer with
mission by allowing a less-complex a variable maximum baseline of 100 up to
observatory. For example, the Ares V 1000 m. The interferometer is proposed to be
enables a fully deployed 8-m or folded located near Sun-Earth L to enable precision
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segmented 15- to 20-m telescope in a single formation flying. The baseline concept
launch. However, this would require the consists of 30 1-m mirror elements focusing
“tall” option Ares V fairing. Without an light into a single beam-combining hub. The
Ares V, multiple launches, complex folded baseline concept can be launched on a Delta-
optics, and/or on-orbit assembly would be IV Heavy. The Ares V cargo launch vehicle,
the only alternatives for deploying a with its much larger fairing volume, enables
telescope larger than about 7 m in diameter. larger mirror elements, which dramatically
Postman ended his presentation by improves sensitivity and reduces observation
discussing the technology developments that times. This increases the science
need to be made prior to launch. Designing productivity, especially for fainter extra-
the mission to enable on-orbit servicing was galactic sources and for astroseismic observa-
also identified as a high priority. tions. In addition to larger mirrors, a single
launch of an Ares V could also include more
3.4.3 Stellar Imager (SI): Viewing than one hub and a reference
the UV/Optical Universe in High metrology/pointing control spacecraft. This
Definition would greatly increase the operational
efficiency and robustness of the mission. The
Ken Carpenter (NASA GSFC) discussed value of in-situ servicing, such as refueling
Stellar Imager, which is a space-based and repairing or replacing damaged
UV/Optical interferometer with over 200 hardware, was also emphasized for this
times the resolution of the Hubble Space mission.
Telescope. With its combination of high
angular resolution, dynamic imaging, and 3.4.4 Generation-X: A Mission
spectral energy resolution, it is capable of Enabled by Ares V
performing breakthrough science in the
UV/Optical spectral region. Science goals Roger Brissenden (Smithsonian
include an improved understanding of solar Astrophysical Observatory) discussed an X-
and stellar magnetic activity and ray telescope concept known as Generation-
understanding accretion mechanisms in X [7]. The science goals include studying
sources ranging from planet-forming the early universe where the first black
systems to black holes. The sub-milli- holes, stars, and galaxies formed, as well as
arcsecond angular resolution enables the their evolution with cosmic time. The
study of dynamical structure and physical telescope would also provide new insights
processes in currently unresolved sources into the physics of matter in extreme
such as active galactic nuclei (AGN), environments. Key parameters in the
supernovae, planetary nebulae, and baseline concept are that the effective area at
interacting binary stars. SI is also capable of 1 keV be 50 m2 and the angular resolution
imaging transits of exosolar planets across be 0.1 arcsec. To meet the effective area
their stellar disks. It is a candidate large- requirement of 50 m2 requires about 104 m2
class strategic mission for the mid-2020s. of glass area. This, in turn, requires thin
mirrors (~0.1–0.2 mm) to meet mirror
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spacing and launch mass requirements, combiner with a one-kilometer baseline
although the mass is less of an issue with the constrained by tethers. The interferometer is
Ares V launch payload capacity. Since the designed to achieve an angular resolution of
requirements on effective area imply a 12-m 50 milli-arcsec over a wavelength range of
diameter mirror, this would require either 40–640 μm, simultaneously obtaining
multiple launches and assembly, multiple spectral and spatial information using a
satellites, or a single monolithic mirror that double-Fourier technique. This would allow
could only be launched on an Ares V. spatial resolution in the FIR that is
comparable to HST at optical wavelengths.
The original mission concept using a Delta- Science priorities include studies of the first
IV Heavy called for six identical 8-m stars (whose light is redshifted into the FIR),
diameter telescopes, each carried as six galaxy evolution, star formation, and
segments to fit in the fairing. The Ares V planetary and debris disks. The spectrometer
enables a simplified and more cost-effective is designed with sufficient sensitivity to map
mission concept. The baseline concept calls high-redshift extra-galactic sources.
for a partially filled 16-m diameter mirror,
which folds to fit within a 10-m fairing. The The SPECS mission could be launched on a
X-ray telescope is delivered directly to Sun- Delta 4 Heavy, but the larger fairing of the
Earth L , and the estimated spacecraft mass Ares V could be significantly enhancing.
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of 22 mT is far less than the 55 mT Ares V This would allow different packaging
capability to this location. Since the options, which would in turn lead to a much
telescope is volume-limited, not mass- simpler deployment scheme. In addition, the
limited, the mass margin enables design Delta 4 Heavy launch limits SPECS to a pair
freedom for the optics, structure, supporting of 4-m monolithic mirror telescopes. While
electronics, and the science instruments. The this would achieve the SPECS stated science
ideal situation would be to have a fully filled goals, an Ares V would allow for both more
~12-m monolithic mirror that would need no and/or larger collector telescopes. Other
deployment, thereby reducing cost and risk. potential benefits would be to use the larger
launch payload of an Ares V to carry more
Brissenden ended his presentation by propellant for a longer mission lifetime and
discussing some of the technology to perhaps add a servicing spacecraft that
developments that would be needed to could do both refueling and repair and
enable the Generation-X mission. replacement. DARPA’s Orbital Express
Significant improvements to the mirror mission demonstrated such a capability in
figure control are required to achieve the 2007.
high-angular resolution mission parameter.
3.4.6. The Dark Ages Lunar
3.4.5 Submillimeter Probe of the Interferometer
Evolution of Cosmic Structure
(SPECS) Spectrometer Enabled by Joseph Lazio (NRL) gave a presentation on
Ares V the Dark Ages Lunar Interferometer (DALI)
concept, a telescope designed to conduct
Stephen Rinehart (NASA GSFC) discussed cosmological observations of the so-called
the SPECS far-infrared (FIR) interferometry “Dark Ages” of the early universe and,
mission [8,9] This system consists of 4-m potentially, of the Epoch of Reionization.
collector telescopes and a Michelson beam The Ares V is likely to be required for the
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DALI concept, both because the extreme blocked in this location. The current antenna
faintness of the desired signal requires concept consists of dipoles deposited on
substantial collecting area (and, hence, polyimide film. Each rover would unroll the
mass), and because the desired site for the polyimide film rolls for its station, then
telescope is the far side (i.e., anti-Earthward remain in place to serve as a “transmission
side) of the Moon. hub,” beaming the signals from its station to
a central processing facility. Elements of the
The ground state of the hydrogen atom has Constellation architecture, both the Ares V
the famous 21-cm hyperfine transition. After and the cargo version of the Altair lander,
recombination, about a half-million years present an attractive means of deploying the
after the Big Bang, the dominant component large launch mass of the telescope, which is
of the intergalactic medium (IGM) is atomic dominated by the antennas and rovers.
hydrogen, and the predicted temperature of
the gas eventually drops below that of the 3.4.7. Starshades in the Ares V
cosmic microwave background (CMB).
Even well into the Epoch of Reionization, Tupper Hyde (NASA GSFC) reviewed the
the IGM remains dominated by neutral next generation missions intended to
hydrogen, although a more complicated characterize exosolar planets using a star-
temperature evolution is predicted as the shade, which Ares V might enable. He
first stars and black holes form and heat and discussed the sequence of missions to
ionize the IGM. Depending upon the characterize exosolar planets beginning with
redshift, the hyperfine transition should be the baseline New Worlds Observer
seen in either absorption or emission against Spectroscopy mission to characterize planets
the CMB, and would serve as a at low spectral resolution. This mission could
cosmological probe in much the same way be carried out with current heavy-lift launch
that the CMB itself has been over the past vehicles. The next most advanced mission is
four decades. Importantly, the redshifted 21- Life Finder that would carry out medium
cm transition may offer the opportunity to resolution spectroscopy (R ~ 10000) of the
follow the evolution of the Universe during atmospheres of exosolar planets. This would
this crucial epoch. Secondary science require a large 8- to 16-m telescope in
includes studying the magnetospheric conjunction with a starshade. Here an Ares V
emission from exosolar planets and cargo launch vehicle would be enhancing, but
heliophysics. not absolutely essential. The next most
ambitious mission called Planet Imager,
The baseline DALI concept calls for a large which requires multiple telescopes to carry
number (hundreds) of antenna “stations,” out large-baseline imaging interferometry
with each station consisting of 100 antennas. plus starshades, could only be carried out
The nominal location is the Tsiolkovsky with an Ares V or an equivalently capable
crater. The stations would be deployed with vehicle.
robotic rovers, and signals from the stations
would be transmitted via laser links to a In all of these mission concepts, the
correlator. The Moon’s far side may be the starshade is used as an external occulter to
only place in the inner solar system where block the light from the star. The telescope
these observations can be carried out, due to needs to be large enough to collect enough
strong terrestrial (human-generated) light from the planet and needs to be far
emissions in the radio that are effectively enough away from the starshade to have a
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