This document proposes a Phobos and Deimos sample return mission that would launch as a co-manifested payload on NASA's Space Launch System (SLS) rocket. The spacecraft would separate from the SLS after the Trans-Lunar injection burn and travel to Mars orbit. It would then rendezvous with Phobos and Deimos, obtaining samples from each moon. After completing its science operations, the spacecraft would depart Mars orbit and return the samples to Earth. Launching on SLS enables a dual-objective exploration mission by providing enough lift capacity to carry multiple payloads to different destinations on a single launch.

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Copyright ©2022 The Boeing Company, Published by the American Institute of Aeronautics and Astronautics, Inc., with permission
A Phobos and Deimos Sample Return Mission Launched
as a Co-manifested Payload on the NASA SLS Launcher
Benjamin Donahue1
, Matt Duggan2
The Boeing Company, Huntsville, AL, 35758, and Houston, TX, 77058, USA
Abstract – A combined Phobos and Deimos sample return mission spacecraft concept is presented as a mission
that would fly as a secondary, co-manifested payload on the NASA Space Launch System launcher. The
spacecraft separates from the SLS primary payload (the Orion capsule) after the Trans-Lunar injection burn.
After transfer to Mars parking orbit the spacecraft rendezvous with Phobos, obtains samples and then
transfers to Deimos, where other samples are obtained. After Deimos, the spacecraft departs for its journey
back to Earth. SLS’ heavy lift capability enables dual launch of high science value exploration missions.
The NASA Space Launch System
The NASA Space Launch System’s (SLS) heavy lift
capability and large fairing will enable a variety of
exciting new crewed and uncrewed missions over the
next 30 years. The SLS Block-2 configuration is capable
of launching with Orion co-manifested payloads of up to
10-mt. One such mission is described in the following.
The Phobos / Deimos Sample Return Mission
In this dual mission launch, a co-manifested payload
of 7.2 mt is launched with the crewed Orion spacecraft on
an SLS Block2. The payload, a Phobos/Deimos
spacecraft, is carried underneath the Orion in the SLS’s
10-m high, 8.4-m diameter (at its base) Universal
Spacecraft Adaptor (USA, Fig. 1). The SLS launches the
the Orion to Trans-Lunar Injection (TLI, C3= -2.0
km2/s2). Afterward, the Orion and co-manifested
spacecraft separate from each other and the Exploration
Upper Stage (EUS). The Orion continues on its 5-day
transit to the Moon, eventually injecting itself into a cis-
Lunar orbit. The co-manifested spacecraft loops around
the moon (Fig. 3) making one revolution of the Earth (10
days). As it returns to, and passes Earth at perigee, it is
properly aligned for its Trans-Mars Injection burn (TMI)
C3=12 km2/s2, and a 7-month journey to Mars.
The spacecraft consists of four elements, the Descent
Module (DM), the aerobrake, the Ascent Module (AM)
and the Earth Return Capsule (ERC). At arrival, the
Phobos-Deimos spacecraft does a series of aerobraking
maneuvers to place itself in a highly elliptical Mars
parking orbit (250 by 82,173-km, Fig. 8, Ref. 1). Once
captured, the aeroshell is jettisoned. From its Mars
parking orbit, the spacecraft will transfer to, rendezvous
with, and land on Phobos. Phobos is in a 9,376 by 9,376-
km orbit (Fig. 7). A science package is deployed, which
includes a rover. Samples are placed in a sample canister
on the ERC. Once the mission on Phobos is completed,
the spacecraft’s DM fires its engines again to transfer the
spacecraft to the higher orbit of Deimos (23,458 by
23,458-km, Fig. 7). Once in Deimos orbit it will
rendezvous and land. On Deimos it will deploy its second
science package and obtain samples. After Deimos
operations, the AM lifts-off and ascends back to its Mars
parking orbit. From there it departs Mars for its return
journey. At Earth arrival, the ERC separates from the AM
and reenters. A spacecraft concept is illustrated in Fig. 2-
5. The dual Earth departure trajectories (TLI and TMI) for
this mission are illustrated in Fig. 6.
Vehicle Elements
As mentioned, the Phobos/ Deimos vehicle consists of the
DM, the Mars aerobrake, The AM and the ERC.
Descent Module (DM)
The DM uses pressure-fed NTO/MMH space storable
propellant with 319-sec Isp engines. The DM supports
the AM/ERC at launch (Fig. 3). After aerocapture the
Aeroshell is jettisoned. The DM’s Delta-Velocity (dV)
budget is shown in Table 2, mass in Table 1.
Ascent Module (AM)
The AM uses 319-sec Isp NTO/MMH engines. The
AM supports the ERC at launch (Fig. 3). AM dV is
shown in Table 2, mass in Table 1. The AM carries
both the Phobos and Deimos science packages.
Earth Return Capsule (ERC)
The ERC (Figure 3,4,5,) will carry the sample canister.
The ERC is carried by the AM; after the AM does an
Earth entry aim point maneuver, the ERC will separate
and fly its entry flight corridor.

3. In-Space Maneuvers
After TMI (C3=12 km2/s2), the Phobos spacecraft will
do a 9-m/s mid-course correction. At Mars arrival
aerocapture is utilized; subsequently the vehicle enters its
Mars parking orbit (250 by 82,173-km, Fig. 8, Ref 1). After
aerobrake jettison the DM performs a 30-m/s correction
burn. Later the spacecraft does a burn to transfer to, and
rendezvous with Phobos (538-m/s +30 dispersions). After
the Phobos portion of the mission is completed, the DM’s
engines fire again to lift off (escape velocity is 11-m/s). The
250-kg Phobos science package is left behind (a separate
75-kg science suite is carried for Deimos.) The vehicle
ascends, transfers to, and rendezvous with Deimos
(dV=778-m/s +20 corrections). After the Deimos mission,
the AM/ERC combination lifts-off (escape velocity 6-m/s);
and transfers to Mars parking orbit (dV=604-m/s +72
dispersions). The AM’s engines are used again for Trans-
Earth Injection (TEI) (dV=1,150-m/s). An inbound
correction is done (9-m/s). At Earth arrival the AM does an
aimpoint maneuver (5-m/s), before being separated from
the ERC. Total spacecraft dV (with 10% margin) is 4,254-
m/s (Table 2). The highly elliptical staging orbit (Fig. 8)
represents an initial orbit a spacecraft would enter through
aerocapture after an interplanetary transfer from Earth with
periapsis close to Mars’ atmosphere and an arbitrarily high
apoapsis. The staging orbit is independent of heliocentric
transfer trajectory from Earth (short-stay or long-stay). The
periapsis altitude is 250 km, apoapsis 82,173 km, its orbital
period is 3 days, 7 hours and the speed at apoapsis is 202
m/s. Information on the staging orbit is taken from Ref. 1.
Element Masses
The Phobos/ Deimos spacecraft masses 7.17-mt (Table
1). Four 1-kg samples are taken, two each from Phobos and
Deimos. The aeroshell masses 1,433-kg. The DM stage
masses 4,335-kg (excluding payloads; which are 250-km
Phobos and 75-kg Deimos). The AM masses 1,071-kg and
the ERC 166-kg (excluding samples). The AM and DM
have a 20% mass growth allowance applied. As mentioned
previously, the SLS Blk-2 vehicle can launch, in addition
to the Orion, 9.80-mt to TLI. After factoring in the mass of
the Payload Attach Fitting (PAF, 208-kg), a robust launch
margin of 2,421-kg exists for this mission. (Table 1). In
Fig. 2 a post-aerocapture configuration of the spacecraft is
shown; the ERC can be seen attached to the side of the AM.
Also illustrated are the solar arrays, Deimos science
module, robotic arm and Phobos science module. On the
opposite side of the AM is the 2.5-m diameter high gain
antenna. A front view is illustrated in Fig. 2 (DM propellant
tanks are not necessarily to scale). Fig. 3 illustrates the
spacecraft stowed in the SLS USA, the 6-m diameter
aeroshell and the PAF. Fig. 2 shows the spacecraft in its
cruise configuration with its 2.2kW (at 1 AU) cruise solar
array.
Phobos and Deimos
Phobos (Figs. 9) gouged by an impact crater and beaten by
meteorite impacts, is the larger of Mars' two moons; it is 27
by 22 by 18 km in diameter. It orbits Mars three times a
day, and is so close to the planet's surface that in some
locations on Mars it cannot always be seen. Its orbital
period is 7-hours 39-minutes. Phobos’ most prominent
feature is the 9.7 km crater Stickney, its impact causing
streak patterns across the moon's surface. Stickney was
seen by Mars Global Surveyor to be filled with fine dust,
with evidence of boulders sliding down its sloped surface.
A second impact crater, within Stickney, is Limtoc. Phobos’
density to too low to be solid rock, and it is known to have
significant porosity. These results led to the suggestion that
Phobos might contain a substantial reservoir of ice.
Mapping by the Mars Express probe and subsequent
volume calculations might suggest the presence of large
caverns within the moon. The porosity of Phobos was
calculated to be about 30%, or nearly a third of the moon
being hollow. Deimos (Fig. 10) is the smaller of Mars' two
moons. Being only 15 by 12 by 11 km, Deimos whirls
around Mars every 30 hours. Phobos and Deimos have
extremely low gravity; Phobos gravity is 0.0057 m/s2.
Discussion
This mission will return samples from both moons of
Mars and characterize their surface and interior. This
mission will also demonstrate Mars aerocapture into an
elliptical orbit, demonstrate advanced autonomous
operations, validate round trip Mars operations and
demonstrate the potential of Phobos or Deimos as a
communication base for future crewed missions.
Summary
The SLS’s outstanding capabilities as a heavy lift
launcher will allow it to fly a number of exciting, dual
purpose missions with multiple payloads sent to different
destinations. The SLS can simultaneously fulfill its role as
the launch provider for Orion missions to the Moon while
also providing, on the same launch, a secondary mission to
a different destination. The Phobos/ Deimos Sample
Return mission is one such mission enabled by the SLS.
Acknowledgements
Many thanks to Jerry Horsewood of Spaceflight Solutions
Inc, for trajectory analysis and illustrations. Also, Michael
Elsperman and John Behrens.
References
[1] Foster, Cyrus, “Delta-V Budgets For Robotic and
Human Exploration of Phobos and Deimos,” NASA
Ames Research Center, Universities Space Research
Association, Moffett Field, CA, Second International
Conference on the Exploration of Phobos and
Deimos, March 14-16, 2011, NASA Ames Research
Center, Moffet Field, CA, USA.

4. Fig. 1 (Left) SLS Block 2 with Orion and USA. Fig. 2 (right) Phobos / Deimos Spacecraft
Post Aerobrake Jettison
Fig. 3 (Left) Phobos Spacecraft in SLS USA Fig. 4 (right) Outbound Cruise Configuration

5. Table 1 (above left) Phobos/ Deimos Sample Return Spacecraft Mass Statement
Fig. 5 (above right) Spacecraft in its Earth Return configuration
Fig. 6 SLS Dual Launch: Lunar Trajectory (yellow/green) and TMI Earth Departure (red)

6. Fig. 8 Highly Elliptical Mars Staging Orbit for Access to Phobos and Deimos (Ref. 1)
Fig. 7 Phobos Deimos Orbits

7. Table 2 Phobos / Deimos
Sample Return Mission DV
Budget
Fig. 9 False Color Image of Phobos – Limtoc (top) crater (within Stickney crater)

8. Fig. 10 Deimos
Table 3 Mission Objectives
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