ASTEROIDS IV MICHEL, PATRICK, DEMEO, FRANCESCA E., BOTTKE, WILLIAM F. PUBLISHED BY UNIVERSITY OF ARIZONA PRESS - DOIS
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Asteroids IV
Michel, Patrick, DeMeo, Francesca E., Bottke, William F.
Published by University of Arizona Press
Michel, Patrick, et al.
Asteroids IV.
University of Arizona Press, 2015.
Project MUSE. muse.jhu.edu/book/43354.
For additional information about this book
https://muse.jhu.edu/book/43354
[ This content has been declared free to read by the pubisher during the COVID-19 pandemic. ]Abell P. A., Barbee B. W., Chodas P. W., Kawaguchi J., Landis R. R., Mazanek D. D., and Michel P. (2015) Human exploration of near-Earth
asteroids. In Asteroids IV (P. Michel et al., eds.), pp. 855–880. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816532131-ch043.
Human Exploration of Near-Earth Asteroids
P. A. Abell
NASA Johnson Space Center
B. W. Barbee
NASA Goddard Space Flight Center
P. W. Chodas
Jet Propulsion Laboratory, California Institute of Technology
J. Kawaguchi
Japan Aerospace Exploration Agency
R. R. Landis
NASA Wallops Flight Facility
D. D. Mazanek
NASA Langley Research Center
P. Michel
Lagrange Laboratory, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS
Due to a number of factors, including a recent U.S. presidential directive, the successful
return of an asteroid sample by the Japanese spacecraft Hayabusa in 2010, and the high-visibility
airburst impact event over Chelyabinsk, Russia, in 2013, scientific and exploration interest in
near-Earth asteroids (NEAs) has never been greater. In particular, NASA and the Japanese
and European space agencies have begun expending serious effort to discover and identify
appropriate NEA targets for a wide variety of spaceflight activities, including both robotic and
human missions. These missions are particularly attractive as they will yield an unprecedented
amount of knowledge about the formation of the solar system, provide a stepping-stone ap-
proach for future human exploration missions to Mars and beyond, identify materials for in
situ resource utilization (ISRU), and test techniques for deflecting potentially hazardous objects
that threaten Earth.
1. INTRODUCTION pdf, accessed July 19, 2015). In response to these findings,
President Barack Obama directed NASA to include NEAs as
Asteroids have generated significant worldwide interest in destinations for future human exploration with the goal of
recent years as a result of flyby, rendezvous, and sample return sending astronauts to an NEA in the mid to late 2020s. This
missions. Robotic missions to near-Earth asteroids (NEAs) directive became part of the official National Space Policy
have already been completed by several space agencies, and of the United States of America as of June 28, 2010. More
subsequent robotic missions to these bodies are under devel- recently, on June 18, 2013, President Obama gave NASA a
opment or consideration. In addition, human-led exploration Grand Challenge to “find all asteroid threats to human popu-
missions to these destinations are now also being seriously lations and know what to do about them.” This Grand Chal-
considered. In 2009, the Review of the United States Human lenge, combined with the directive for a human NEA mission,
Spaceflight Plans Committee, also known as the Augustine forms the basis of NASA’s Asteroid Initiative (http://www.
Commission, identified NEAs as high-profile destinations for nasa.gov/asteroidinitiative, accessed June 10, 2015) (Fig. 1).
human exploration missions beyond the Earth-Moon system Near-Earth asteroid targets suitable as destinations for hu-
as part of the “Flexible Path” approach (http://www.nasa.gov/ man exploration tend to have orbits very similar to Earth’s,
pdf/384767main_SUMMARY%20REPORT%20-%20FINAL. and also have a statistically greater chance of impacting our
855856 Asteroids IV
Asteroid Initiative order to maximize operational efficiency and reduce mission
risk. These precursor missions will help fill crucial strategic
knowledge gaps (SKGs) (see section 6.1.2) concerning the
Asteroid Grand physical characteristics of NEAs that are relevant for human
Mission Enhanced Challenge exploration of these relatively unknown destinations.
NEO One such SKG that is considered critical for future human
Robotic Observation Diverse
Mission Campaign Stakeholder
spaceflight is the identification of resources that could be
Segment Engagement utilized in situ. Some NEAs have been identified via spec-
troscopic observations as having affinities with volatile- and
Learning How
to Manipulate
organic-rich carbonaceous chondrite mineralogies, which
Human contain significant amounts of OH and other volatiles bound
and Interact Mitigation
Mission
Segment
with a NEA Approaches within their geologic structures (Brearley, 2006; Pizzarello
et al., 2006; Weisberg et al., 2006; Burton et al., 2012). Suc-
cessful identification of these types of materials and the
subsequent extraction and utilization of the resources con-
tained within them will enable human missions to leverage
Fig. 1. NASA’s Asteroid Initiative. these resources and essentially “live off the land.” This will
reduce mission risk and allow more aggressive approaches
for conducting exploration missions beyond low-Earth orbit
(LEO). Such missions would not have to carry all the neces-
planet. Therefore, one of the major goals for NASA’s human sary supplies that would normally be used in a round-trip
spaceflight program and the Asteroid Initiative is to identify mission, thus freeing up extra mass that could be used for
these NEAs both from the standpoint of human exploration additional payloads and systems. This could eventually aid
and planetary defense. Ideally, once suitable targets have in the development of alternative mission architectures to
been identified, NASA would send astronauts beyond the Mars and other solar system destinations.
Earth-Moon system to NEAs within the coming decades. In addition, robotic precursor and human exploration mis-
These missions, and the activities required to identify and sions to NEAs would allow NASA and its international
characterize suitable targets, would pave the way for explora- partners to gain operational experience in performing complex
tion of the martian system, and would have the added benefit tasks (e.g., sample collection, deployment of payloads, re-
of finding potentially threatening objects and increasing our trieval of payloads, etc.) with crew, robots, and spacecraft
knowledge about the physical characteristics that will be nec- under low-gravity or microgravity conditions at or near the
essary to develop future NEA hazard mitigation techniques. surface of a small body. This would provide an important
Human-led missions to NEAs will undoubtedly provide synergy between the worldwide science and exploration com-
a great deal of technical and engineering data on spacecraft munities, which will be crucial for development of future
operations for future human deep-space exploration and international deep-space exploration architectures and has
planetary defense while simultaneously conducting in-depth potential benefits for future exploration of other destinations
scientific examinations of these primitive objects. Informa- beyond LEO. This experience would be directly applicable
tion obtained during a human mission to an NEA, combined to the moons of Mars, Phobos and Deimos, and could be
with ground- and spacebased observations and robotic leveraged to enable an eventual landing on Mars itself.
spacecraft investigations of asteroids and comets, will also Among all human spaceflight destinations currently under
provide a real measure of ground truth to data obtained from consideration, only NEAs encompass all the following critical
samples within the worldwide extraterrestrial material col- themes: exploration, science, resource utilization, and plane-
lections (e.g., meteorites and cosmic dust) and those col- tary defense. The scientific, in situ resource utilization (ISRU),
lected directly from spacecraft sample-return missions [e.g., and hazard mitigation benefits, along with the operational
Stardust, Hayabusa, Hayabusa2, and the Origins Spectral benefits of a human venture into deep space, make a human
Interpretation Resource Identification and Security-Regolith mission to an NEA a compelling prospect that has the poten-
Explorer (OSIRIS-REx)]. Major advances in the areas of tial to inspire future generations of scientists, engineers, and
geochemistry, impact history, thermal history, isotope explorers on an international level for decades to come.
analyses, mineralogy, space weathering, formation ages,
thermal inertias, volatile content, source regions, solar sys- 2. HISTORICAL BACKGROUND
tem formation, etc., can be expected from human NEA
missions. Samples directly returned from a primitive body The impetus for asteroid exploration is scientific, politi-
would lead to the same kind of breakthroughs for under- cal, and pragmatic, and the idea of sending human explorers
standing NEAs that the Apollo samples provided for under- to asteroids is not new. Piloted missions to these primitive
standing the Earth-Moon system and its formation history. bodies were first discussed in the 1960s, pairing Saturn V
Prior to sending human explorers to NEAs, however, rockets with enhanced Apollo spacecraft to explore what
robotic investigations of these bodies will be required in were then called “Earth-approaching asteroids” (Cole, 1963;Abell et al.: Human Exploration of Near-Earth Asteroids 857
Cole and Cox, 1964; Smith, 1966). The difference today, Moon. This was before NASA Headquarters established the
compared to nearly a half-century ago, is that the science Apollo Applications Program (AAP) in 1968; its intent was
and engineering communities are now much more aware of to develop a detailed planning guideline for the Manned
the small bodies in the vicinity of Earth. Prerequisite steps Spacecraft Center [now the Johnson Space Center (JSC)]
must be taken before mounting a robotic precursor mission, and Marshall Space Flight Center (MSFC) for piloted mis-
let alone piloted missions. The first such step, and the linch- sions utilizing the hardware developed for the Moon. The
pin of such an architecture, is a spacebased near-Earth object proposed flight schedule was ambitious, calling for 13 Sat-
(NEO) survey (see section 6.1.1). urn 1B and 16 Saturn V missions, but none of these concepts
imagined NEA missions. NEA human mission feasability
2.1. Apollo Applications Program/Post-Apollo concepts using Apollo hardware were analyzed in more
detail in 1971, but none of these concepts matured beyond
Arguably, the first realistic notions of human missions to the study phase (Hall, 1971).
asteroids were imagined in the 1960s by Cole (1963), Cole
and Cox (1964), and Smith (1966). While Cole was a futur- 2.2. Space Exploration Initiative
ist, he was also an aerospace engineer who suggested a
number of innovative scenarios for exploring the inner solar O’Leary (1977) discussed mining asteroids in the late
system by utilizing Apollo hardware to reach NEAs. For the 1970s, but it would be more than a decade later before NASA
most part, however, these ideas were general rather than examined the idea of visiting NEAs as part of the Space
specific. Smith (1966) examined the technical feasibility of Exploration Initiative (SEI) (Nash et al., 1989). Davis et al.
implementing a human flyby mission to (433) Eros. Smith (1993) pointed out that given the large investment, as part
suggested that the 1975 close approach of (433) Eros of the SEI, in a Mars landing mission, a program of robotic
(within 0.15 AU of Earth), coupled with the descending node and human exploration missions to NEAs would result in
of the asteroid’s orbit, would enable an opportunity for a additional return by (1) providing a precursor to a Mars mis-
relatively low-energy interplanetary mission (post-Earth sion (with flight testing of hardware and mission operations);
escape Dv = 4.2 km s–1). In the 1960s, Mars and even Venus (2) providing a highly visible program milestone and thus
were considered primary targets for initial human missions. sustaining momentum for the SEI in the interval between a
Smith pointed out that missions to asteroids would comple- return to the Moon and a Mars mission; (3) providing a large
ment the as-yet only conceptual missions to Mars and be- science return via in situ observations and macroscopic
yond, and that a flyby mission of (433) Eros could be ac- sample return; and (4) carrying out an assessment of the
complished by a single, upgraded Saturn V along with resources available in the NEA population.
derivatives of the Manned Orbital Research Laboratory None of the ancillary SEI studies on asteroids and aster-
(MORL) and the Apollo capsule. The MORL was a concept oid exploration specified or even suggested a particular ar-
for a 5-metric-ton space station lofted by a Saturn 1B, with chitecture toward achieving the human exploration of these
Gemini or Apollo capsules utilized for crew transport. For primitive bodies. However, the variety of overview studies
the Eros mission concept, Smith surmised that an adapted did begin to search the asteroid database for potential low
MORL could serve as a habitation module, while the propulsive change in velocity (low Dv) targets in order to
Apollo capsule would be used as a launch and Earth reentry develop preliminary “suggestive” lists of possible targets,
vehicle for the crew. MORL was never launched, as NASA including but not limited to 1989 ML, 1989 UQ, 1991 VG,
had more ambitious plans with the Apollo Applications and 1991 JW (Davis et al., 1993; Jones et al., 1994).
Orbital Workshop, which later became Skylab. By 1992, SEI had largely faded from view, suffering from
The mission architecture considered by Smith (1966) in a flawed, defective decision-making process (Hogan, 2007).
his analysis began with injecting an S-IVB/Interstage Unit As NASA acknowledged budgetary realities, notions of
(with the crew and habitation module) into a two-day geo- human exploration beyond LEO were abandoned. A new
centric elliptical orbit. The S-IVB evolved from the upper emphasis was placed on “faster, better, cheaper” robotic
stage of the Saturn 1, and was the third stage of the Saturn V missions while maintaining the status quo on human space-
rocket, which for lunar missions was ignited twice: once to flight activities; this mantra referred to the Discovery pro-
place the combined Apollo Command and Service Modules gram, begun in 1992, that was intended to spur proposals,
and Lunar Lander (the Apollo “stack”) into LEO after second- subject to a competitive peer-review process, for low-cost,
stage cutoff, and again for translunar injection (TLI). Upon high-impact robotic science missions to explore the solar
completing spacecraft and systems checkout, the spacecraft system. The Near Earth Asteroid Rendezvous (NEAR) mis-
“stack” would depart with a second impulse on a 270-day sion to (433) Eros (later renamed NEAR-Shoemaker in
journey outbound toward Eros. Then, 257 days after the honor of the late Eugene “Gene” Shoemaker) was the first
closest approach to the asteroid, the crew would return to Discovery mission launched. In addition, by 1996, the Na-
Earth. A 500+-day round-trip mission with upgraded Apollo tional Space Policy had removed human spaceflight explo-
and Saturn V hardware was an audacious concept at best. ration from the U.S. national agenda. As a result, it would
However, this was the first feasibility study of its kind that be 15 years before NASA would again consider piloted
examined what might be possible for missions beyond the missions to asteroids.858 Asteroids IV
2.3. NASA Near-Earth Object Observations Program In the first CxP study related to NEAs, Mazanek et al.
(2005) looked at relatively short round-trip missions (45–
In 1998, NASA established a goal to discover 90% of the 90 days) and the hardware that might enable such missions.
NEAs larger than 1 km in diameter and in 2005, Congress This study conducted a top-level mission analysis of a
extended that goal to include 90% of the NEAs larger than “2.5 launch Earth-orbit rendezvous architecture,” which
140 m. There are thought to be approximately 1000 NEAs required two Ares 5 upper-stage Earth-departure stages
larger than 1 km and roughly 15,000 larger than 140 m. The (EDS) for Earth departure and arrival at the asteroid, and
progress toward meeting these goals can be monitored on one Ares 1 vehicle for the crew. As CxP was rolled out, the
the NEO Discovery page at http://neo.jpl.nasa.gov/stats/ nominal architecture for the return to the Moon was the
(accessed October 6, 2015). The NEO Observations program “1.5 launch” scenario, or one Ares 5 and the Ares 1 vehicle
has functioned as a key foundational source for the asteroid for the crew. The 1.5 launch architecture was intended to
identification and characterization activities of NASA. initially place its payload into LEO (parking orbit) and to
Since NASA’s initiation of the NEO Observations pro‑ be reignited to depart Earth on a translunar trajectory, very
gram in 1998, NEO surveys have been extremely success- much akin to the S-IVB stage of the Saturn V. The Mazanek
ful, reaching the goal of finding more than 90% of the et al. (2005) concept involved a double rendezvous in LEO
estimated NEAs larger than 1 km and a significant fraction with the first EDS providing the departure Dv from the
of the estimated NEAs larger than 140 m. The vast majority Earth-Moon system, while the second EDS, along with the
of these discoveries have been made by NASA-supported lunar lander’s descent stage, would provide the braking Dv
groundbased telescopic surveys, including the Catalina Sky for rendezvous, proximity, and surface operations at the
Survey (CSS) and Spacewatch near Tucson, Arizona (http:// asteroid. Following surface exploration operations, including
www.lpl.arizona.edu/css/ and http://spacewatch.lpl.arizona. collection of samples, the Orion service module would
edu/, both accessed October 6, 2015); the Lincoln Near Earth perform the Dv maneuver (trans-Earth injection) to return
Asteroid Research (LINEAR) project near Socorro, New to Earth.
Mexico (http://www.ll.mit.edu/mission/space/linear/, accessed The second CxP study also focused on utilizing hardware
October 6, 2015); the Panoramic Survey Telescope and Rapid developed for the return to the Moon, but with a slight dif-
Response System (Pans-STARRS) 1 on Haleakala, Maui, ference. It examined the feasibility of adapting that hardware
Hawai‘i (http://pan-starrs.ifa.hawaii.edu/public/, accessed within the then-planned launch vehicle infrastructure with
October 6, 2015); Lowell Observatory Near-Earth-Object minimal modifications to the Orion vehicle [e.g., only flying
Search (LONEOS) near Flagstaff, Arizona (http://asteroid. two or three astronauts, inclusion of a science instrument
lowell.edu/asteroid/loneos/loneos.html, accessed October 6, module (SIM) bay on the service module section of the
2015); and the Near-Earth Asteroid Tracking (NEAT) project Orion, etc.]. Four launch options were assessed: (1) Ares 1/
run by NASA’s Jet Propulsion Laboratory (JPL) (http://neo. Orion and an evolved expendable launch vehicle (EELV)
jpl.nasa.gov/programs/neat.html, accessed October 6, 2015). such as the Atlas V and Delta IV rockets; (2) Orion atop an
The Near-Earth Object Wide-field Infrared Survey Explorer Ares 4 rocket; (3) Orion atop an Ares 5 rocket; and (4) Ares 1/
(NEOWISE), a near-infrared space telescope in an Earth po- Orion and Ares 5 (with a modified Altair lunar lander).
lar orbit, discovered and characterized NEOs for 10 months These feasibility studies were based upon anticipated CxP
in 2010 before its cryogens were exhausted, and is operating hardware performance and primarily addressed the orbital
now in a post-cryogenic mission (http://neowise.ipac.caltech. mechanics (i.e., based on low Dv targets and their accessibil-
edu/, accessed October 6, 2015). The LINEAR survey has ity). Coincident to and following the CxP studies, a more
transitioned to a new facility, LONEOS and NEAT have exhaustive examination of asteroid exploration via the Orion
been discontinued, and Spacewatch now operates primarily spacecraft was led by Hopkins (2009) and Hopkins and Dis-
in a follow-up capacity. sel (2010) at Lockheed Martin Space Systems; the latter
study was called “Plymouth Rock.”
2.4. Constellation Program Based on the expected launch vehicle development sched-
ules (ca. 2009) and the assumption of a long-duration test
NASA’s Constellation Program (CxP) was formulated as flight, the Plymouth Rock concept looked at several candidate
a response to the goals laid out in the Vision for Space Ex- NEA targets before settling on 2008 EA9 and 2000 SG344 for
ploration (VSE) (http://www.nasa.gov/pdf/55583main_ possible missions in the 2019–2030 timeframe (Fig. 2).
vision_space_exploration2.pdf, accessed October 6, 2015). The Plymouth Rock concept went to greater depth than
The milestones for the CxP were to (1) complete the Inter- previous studies. It proposed utilization of Orion capsules;
national Space Station (ISS); (2) return to the Moon by 2020; mission durations of 4–7 months; sufficient habitable volume
and (3) place humans on Mars as the end goal. During the and life support consumables for at least two and preferably
CxP’s existence from 2005 to 2009, two studies examined three crew members over this time span; a spacecraft propul-
the feasibility of reaching a number of NEAs utilizing CxP sion system with at least 1.5 km s–1 of Dv capability for
hardware elements (i.e., Ares 5 and Ares 4 launch vehicles; major course maneuvers in deep space; extravehicular ac-
Ares 4 with an Atlas V and Centaur upper stage; Ares 5 with tivities (EVAs) or at least minimal telerobotic capability to
a modified Altair lunar lander as a habitation module; etc.). collect macroscopic samples in geological context; and aAbell et al.: Human Exploration of Near-Earth Asteroids 859
2029 opportunity to asteroid 2000 SG344
Total mission DV = 1.732 km s–1
145-day mission duration Atmospheric entry
11.16 km s–1
Earth departure
× 105 • Return transfer
3.322 km s–1 73 days 2000 SG344
2 + +
+ at landing
K 0 Moon orbit + +
–2
+ +
0 Outbound +
transfer + + Trans-Earth
–1 67 days + injection
–2
+ + 0.988 km s–1
+
+
–3 2000 SG344 +
× 106 + + 3
–4 at launch + +
2
–5 +
1
Distances in km × 106 –6 0
–1 Asteroid arrival × 106
–7
–2 maneuver
–8 –3 0.744 km s–1
Geocentric-Ecliptic Coordinate System
Trajectory tick marks in 10-day increments
Fig. 2. Plymouth Rock mission to 2000 SG344 (Hopkins and Dissel, 2010).
reentry thermal protection system to withstand ~12 km s–1 Human Space Flight Accessible Targets Study (NHATS).
(Hopkins and Dissel, 2010). NHATS is an automated system designed to monitor the
Plymouth Rock considered a dual-Orion launch by an accessibility of, and particular mission opportunities offered
unmanned Ares 5 with the supplemental Orion “deep-space by, the NEA population. This is analogous to systems that
vehicle” and EDS on a Delta IV heavy (or Ares 1) rocket automatically monitor the impact risk posed to Earth by the
with the crew. After rendezvous in LEO, the EDS would place NEA population. The NHATS system identifies NEAs that
the combined “stack” on an Earth departure trajectory to the are potentially accessible for future round-trip human space
asteroid. Hopkins and Dissel (2010) evaluated life support flight missions and provides rapid notification to asteroid
consumables (i.e., food, water, CO2 removal, O2 generation, observers so that crucial follow-up observations can be ob-
etc.) and acknowledged that even with the quasi-dual Orion tained following discovery of accessible NEAs. NHATS was
(a standard Orion crew cabin capable of reentry and the en- developed in 2010 and was automated by early 2012. NHATS
hanced Orion deep-space vehicle, not capable of reentry, with data are provided via an interactive website (http://neo.jpl.
a stretched cabin), this would be a minimalistic mission. nasa.gov/nhats/, accessed June 9, 2015), and daily NHATS
The majority of the mission concept studies that have notification emails are transmitted to a mailing list (https://
been conducted to date [i.e., CxP, NASA’s “Human Explo- lists.nasa.gov/mailman/listinfo/nhats, accessed June 9, 2015);
ration Framework Team” (HEFT)] have not considered the both resources are available to the public.
benefit that NEO surveys would provide by finding more ac- Automation of the NHATS processing was motivated by
cessible targets to enable mission durations on par with ISS the fact that newly discovered NEAs are often only detect-
expeditions (i.e., ~180 days). The current low discovery level able for several days or weeks surrounding their discovery
of viable candidate NEAs as potential human destinations is epochs because of their faintness in the night sky and ten-
largely due to the fact that the world’s NEO observing assets dency to both approach and depart Earth’s vicinity rela-
are currently confined to the vicinity of Earth. Analyses of tively quickly. The brief window of time surrounding dis-
past trajectory opportunities to known NEOs have shown covery is therefore critical to obtain follow-up observations
that some were highly accessible during the timeframes of that can provide information about the NEA’s physical
their discovery, because they had to closely approach Earth characteristics and improve estimates of its orbit. The auto-
in order to be detected. Many of these have synodic periods mated NHATS system supports those efforts by rapidly
of several decades and longer, and would be more easily de- identifying those NEAs that are particularly accessible and
tected by a deep-space telescopic NEO infrared (IR) survey quickly notifying observers.
mission at, for instance, the Sun-Earth Lagrange L1 point An NEA is classified as NHATS-compliant if there exists
(SEL1), where such a telescope could focus on the so-called at least one round-trip trajectory solution to the NEA that
“sweet spots” in order to find human-accessible NEOs lead- satisfies NHATS key analysis constraints: (1) the total
ing and trailing Earth’s orbit (Mainzer et al., 2015). propulsive change in velocity (Dv) required for the round-
trip mission must be ≤12 km s–1; (2) the total round-trip
3. TARGET POPULATION DYNAMICS mission duration must be ≤450 days; (3) the stay time at
the NEA must be ≥8 days; and (4) Earth departure must
The population of NEAs that may be accessible for human occur sometime during the years 2015 through 2040. These
space flight missions is defined by the Near-Earth Object constraints are discussed in greater detail later in section 3.4.860 Asteroids IV
The discussions presented hereinafter regarding NEA popu- periods of time between the comparatively brief seasons of
lation estimates and dynamical considerations are largely observability and mission accessibility.
driven and framed by NHATS data, because NHATS pro-
vides a comprehensive accessibility assessment for the entire 3.2. Near-Earth Asteroid Population Estimates
NEA population.
As of June 9, 2015, the number of known NEAs is 12,697,
3.1. Astrodynamics Background and the current rate of NEA discovery is approximately 1000
to 1500 per year. NEAs are further classified according to
The accessibility of an NEA is largely framed in terms both composition and orbit (see the chapters by Binzel et al.
of the total Dv and round-trip mission duration required to and Mainzer et al. in this volume). The four NEA orbital
visit it. The other related consideration is what range of Earth groups are Amors (exterior to Earth’s orbit), Apollos (Earth-
departure dates offer feasible (e.g., low Dv/low mission dura- crossing, semimajor axis >1 AU), Atens (Earth-crossing,
tion) missions to the NEA. Mission Dv is largely influenced semimajor axisAbell et al.: Human Exploration of Near-Earth Asteroids 861
Semilatus Rectum [p = a(1–e2)] (AU)
0.45 1.5
Atens
0.4 Atens 1.4 Apollos
Apollos Amors
0.35 Amors 1.3
Eccentricity
0.3 1.2
0.25 1.1
0.2 1
0.15 0.9
0.1 0.8
0.05 0.7
(a) (b)
0
0.8 1 1.2 1.4 1.6 1.8 2 0 2 4 6 8 10 12 14 16 18
Semimajor Axis (AU) Inclination (°)
Fig. 3. Semimajor axis vs. eccentricity and inclination vs. semilatus rectum for the NHATS-compliant
NEAs by orbit group.
ellipse focus, running perpendicular to the line of apsides. TABLE 2. Uncorrelated statistics for NHATS-compliant
Because it is a function of semimajor axis and eccentricity, NEA orbital elements.
the semilatus rectum also describes the energy of the orbit.
Note that asteroids with Earth-like orbits will have a value Orbital Element Minimum Mean Maximum
of p near 1 AU. Semimajor axis (AU) 0.764 1.163 1.819
The majority of the currently known NHATS-compliant Eccentricity 0.012 0.224 0.447
NEAs (60%) are Apollos, yet only 12% of the currently Inclination 0.021° 5.180° 16.253°
known Apollos are NHATS-compliant. Furthermore, 33% of
Atens are NHATS-compliant, yet Atens are currently a minor-
ity among NEAs at ~8% of the known NEA population. This than other NEAs, as expected, the extent to which NHATS-
phenomenon is investigated in Adamo and Barbee (2011) compliant NEA orbits can deviate from being Earth-like is
and may lead to an improved understanding of where the notable. Table 2 summarizes the uncorrelated minimum,
most accessible NEAs tend to reside in orbital element space. mean, and maximum values for NHATS-compliant NEA
Figure 4 presents the relationships between semimajor orbit semimajor axes, eccentricities, and inclinations.
axis, eccentricity, and inclination for the NHATS-compliant Table 2 clearly indicates that the mean NHATS-compliant
NEAs in comparison to all known NEAs. Note that seven NEA orbit is rather Earth-like, although the mean orbital
NEAs are excluded from Figs. 4a,b because their orbit semi- inclination of 5.180° is displaced from the ecliptic plane
major axis is >5 AU, and/or their orbit inclination is >80°, where Earth’s mean orbit plane resides. Orbit inclination is
which would adversely affect the scales of the plots. While a significant driver of the total Dv required for a spacecraft
the NHATS-compliant NEAs have more Earth-like orbits to visit an NEA, so the maximum NHATS-compliant NEA
1 2.5
All NEAs (a ≤ 5 AU) All NEAs (i ≤ 80°)
0.9
NHATS NEAs NHATS NEAs
0.8 2
Semilatus Rectum
[p = a(1–e2)] (AU)
0.7
Eccentricity
0.6 1.5
0.5
0.4 1
0.3
0.2 0.5
0.1
(a) (b)
0 0
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 10 20 30 40 50 60 70 80
Semimajor Axis (AU) Inclination (°)
Fig. 4. Semimajor axis vs. eccentricity and inclination vs. semilatus rectum for the NHATS-compliant
NEAs and all known NEAs.862 Asteroids IV inclination of 16.253° is noteworthy. However, this must be of the total NHATS-compliant NEA population (including considered in the context of Figs. 3b and 4b, which show objects not yet discovered) from the currently available that NHATS-compliant NEAs with high inclination have data is limited because the data are likely influenced by semilatus rectum values near 1 AU; this value is commensu- observational bias; accessible NEAs will tend to closely rate with the energy level of Earth’s orbit (as determined by approach Earth, and smaller (less bright) NEAs are often semimajor axis and eccentricity). Thus, the higher NEA orbit only detectable when near Earth. inclinations can only be tolerated by the Dv budget when the NEA’s semimajor axis and eccentricity are quite Earth-like. 3.3. Dynamical Considerations for the Entire Figure 5 presents histograms showing the distributions of Near-Earth Asteroid Population orbit semimajor axis, orbit eccentricity, orbit inclination, and absolute magnitude for the known NHATS-compliant NEAs The aforementioned role of observational bias may also and all known NEAs; these distributions add context to the be evident in the relationship between minimum round-trip abbreviated statistics presented in Table 2. Dv for the NHATS-compliant NEAs and their absolute Figure 5d shows the distribution of absolute magnitudes, magnitudes, shown in Fig. 6a, which indicates that the more H, of the known NHATS-compliant NEAs vs. that for all accessible (lower Dv) NEAs also tend to be smaller (i.e., have known NEAs. The mean value of H is 21.823 for all known larger absolute magnitudes). Nevertheless, Fig. 6a also shows NEAs and 24.796 for the known NHATS-compliant NEAs. that there are some rather bright (~100-m-class) NEAs for Since H can be used as a proxy for asteroid size, the known which the round-trip Dv requirements are modest. NHATS-compliant NEAs tend to be smaller, on average, A consequence of accessible NEAs occupying Earth-like than the overall known NEA population, assuming that orbits is that the synodic period between such NEAs and the distribution of geometric albedo is similar in the two Earth can be large. This is shown in Fig. 6b, which presents populations. If the geometric albedo is assumed to be 0.14 the minimum round-trip Dv for the NHATS-compliant NEAs (considered representative for the average NEA), the mean vs. their synodic periods relative to Earth. Figure 6b shows diameter of the known NHATS-compliant population is only synodic periods
Abell et al.: Human Exploration of Near-Earth Asteroids 863
Synodic Period Relative to Earth (yr)
34 30
32
25
Absolute Magnitude (H)
30
28
20
26
24 15
22
10
20
18
5
16
(a) (b)
14 0
3 4 5 6 7 8 9 10 11 12 3 4 5 6 7 8 9 10 11 12
Minimum Round-trip ∆v (km s )–1
Minimum Round-trip ∆v (km s )
–1
Fig. 6. NHATS-compliant NEA minimum round-trip Dv vs. (a) absolute magnitude and (b) synodic
period relative to Earth.
synodic periods >30 years; approximately one-third of those Thus, only 9% of the currently known NHATS-compliant
have a synodic period between one and several centuries, NEAs are also classified as PHAs, but this is largely based on
although the synodic period calculation can be misleading their H values rather than their Earth MOID values, meaning
for such cases (Adamo, 2013). Care must be exercised when these asteroids are too small to be formally classified as PHAs.
computing the synodic period relative to Earth for NEAs with Additionally, although the NHATS analysis is specifically
very Earth-like orbits. For example, NEAs on “horseshoe” designed to identify NEAs that are particularly accessible for
orbits, which are oscillating orbits as seen from Earth, will round-trip human space flight missions, it also has a more
tend to have effective synodic periods (i.e., the elapsed time general use. NHATS-compliant NEAs will tend to be generally
between close encounters with Earth) that are much smaller accessible for other types of missions, including one-way or
than what the typical synodic period calculation reports us- round-trip robotic missions involving longer mission durations
ing the standard equation based on Keplerian orbit dynamics. and more complicated flight plans (e.g., incorporating addi-
Figure 6b shows that the most accessible NEAs have relatively tional deep-space maneuvers, gravity assist planetary flybys,
large synodic periods, on the order of one to several decades. etc.). For example, almost every NEA that has been visited, is
However, Fig. 6b also shows that there are a number of currently scheduled to be visited, or has been seriously consid-
NEAs with low to modest round-trip Dv requirements and ered as a potential target for funded robotic science missions
relatively small synodic periods approaching five years or has turned out to be NHATS-compliant. Examples include
less. Note that the NEAs with large synodic periods and large (25143) Itokawa (1998 SF36), (101955) Bennu (1999 RQ36),
Dv requirements are NEAs with very Earth-like semimajor (162173) Ryugu (1999 JU3), and (341843) 2008 EV5, all of
axes (near 1 AU), but high orbit inclinations and/or orbit which are also classified as PHAs. Itokawa was visited by
eccentricities. Large synodic periods mean that accessible JAXA’s Hayabusa (samples returned in 2010), Bennu will be
NEAs discovered when they are near Earth will not return visited by NASA’s OSIRIS-REx (launch in 2016 with samples
to Earth’s vicinity for a considerable amount of time after to be returned in 2023), and Ryugu will be visited by the
departing; this clearly complicates efforts to deploy missions Japan Aerospace Exploration Agency’s (JAXA) Hayabusa2
to such NEAs. A spacebased survey telescope stationed far (launched on December 3, 2014, with samples to be returned
from Earth would be capable of discovering accessible NEAs in 2020). In addition, 2008 EV5 was the target of the Euro-
before they closely approach Earth, potentially affording pean Space Agency’s (ESA) proposed MarcoPolo-R sample
enough advance notice to plan and execute human and/or return mission, but the mission was not selected for funding.
robotic missions to those NEAs. In summary, the subpopulation of NHATS-compliant NEAs
The tendency of known NHATS-compliant NEAs to provides an excellent starting point for identifying low-Dv
closely approach Earth is further illustrated by comparing them candidate targets for a wide variety of possible mission types.
to known PHAs. An NEA is classified as a PHA if its MOID Table 3 summarizes the properties of selected accessible
with Earth’s orbit is ≤0.05 AU and its absolute magnitude H NEAs. These NEAs were selected by querying NASA’s
is ≤22. Currently, 1034 of the known NHATS-compliant NEAs NHATS website for NEAs offering mission opportunities
(83%) have Earth MOID ≤ 0.05 AU and compose 21% of all that meet the following criteria: Dv ≤ 6 km s–1, total mission
known NEAs with Earth MOID ≤ 0.05 AU. However, only duration ≤360 days, stay time at NEA ≥8 days, and Earth de-
178 known NHATS-compliant NEAs (14%) have H ≤ 22. The parture dates between the years 2020 and 2030. These criteria
number of known NHATS-compliant NEAs that are classified are considered notionally representative of an accessible NEA.
as PHAs (both Earth MOID ≤ 0.05 AU and H ≤ 22) is 115. As expected, these NEAs all occupy relatively Earth-like orbits864 Asteroids IV
TABLE 3. Selected accessible NEAs and their physical, orbital, and accessibility properties.
Minimum Minimum
Est. a Dv Duration Spectral Rot. Radar
NEA H Diameter OCC (AU) e i Solution Solution Type Period Observed
(m) Dv Duration Dv Duration (h)
(km s–1) (d) (km s–1) (days)
2007 YF 24.8 19–85 5 0.953 0.120 1.652° 5.89 258 5.965 250 ? ? No
2013 WA44 23.7 32–142 3 1.100 0.060 2.302° 5.936 354 5.936 354 ? ? No
2012 BB14 25.0 17–78 3 1.064 0.099 2.645° 5.453 354 5.985 338 ? ? No
2009 HC 24.7 20–89 4 1.039 0.126 3.778° 4.627 354 5.975 306 ? ? No
2001 CQ36 22.5 68 0 0.938 0.178 1.261° 5.824 354 5.993 338 ? ? Yes
2012 PB20 24.9 18–81 4 1.054 0.095 5.839° 5.82 354 5.969 346 ? ? No
2000 SG344 24.7 20–89 2 0.978 0.067 0.111° 3.556 354 5.973 114 ? ? No
2015 JD3 25.7 12–55 4 1.058 0.009 2.730° 5.048 354 5.983 306 ? ? No
2014 KF39 25.3 15–68 2 1.041 0.079 3.644° 5.634 354 5.978 338 ? ? No
2011 UX275 25.8 12–54 6 1.035 0.076 4.541° 5.903 354 5.903 354 ? ? No
2011 BP40 25.4 14–65 7 1.121 0.151 0.913° 5.848 330 5.964 306 ? ? No
2015 BG92 25.1 17–74 3 1.050 0.057 7.744° 5.218 178 5.911 162 ? ? Yes
2014 JV54 25.8 12–54 2 1.043 0.060 7.746° 5.978 354 5.978 354 ? ? No
and their optimal mission opportunities tend to involve flight Figure 7 illustrates the concepts described above by com‑
times of approximately six months to one year. Note that only paring the round-trip mission accessibility of selected NHATS-
two of these NEAs have been observed by radar, and none compliant NEAs to the accessibility of an object in LDRO
of them have a known spectral type or rotation period; addi- (see section 5 for a discussion of ARM robotic mission op-
tional characterization of these and other accessible NEAs is tions). For reference, the Dv and round-trip duration require-
greatly needed, though challenging to accomplish using only ments for visiting the LDRO are approximately 4.5 km s–1
groundbased observing assets. That said, the diligence of the and 25 days, respectively, where 5 of the 25 days are spent
current groundbased surveys is evidenced by the fact that most at the NEA. The data in Fig. 7 demonstrate that the most
of these accessible NEAs have an orbit condition code (OCC) accessible of the NHATS-compliant NEAs have mission ac-
≤5. The OCC is a 0 to 9 integer scale measuring how well an cessibility approaching that of an NEA in LDRO. However,
NEA’s orbit is known, and numerous high-quality observations this comparison can be extended and tied to the discussion
are needed to provide good knowledge of an NEA’s orbit. An of NEA accessibility during discovery epochs by applying
OCC ≤5 is accepted as a notional threshold for an NEA not the NHATS algorithm to selected NHATS-compliant NEAs
being “lost” (not locatable in the sky at what should be its next during the timeframe surrounding their discovery epochs.
apparition). Note that it is generally required that OCC = 0 That analysis was performed for 2006 RH120 and 2009 BD,
before launching a spacecraft to an NEA. Finally, not shown in the former of which was discovered when it was temporar-
Table 3 are the departure declinations associated with the mis- ily captured by Earth from approximately September 2006
sion trajectory solutions. For the trajectories given in Table 3, through June 2007.
the associated Earth departure declinations vary between 8° The results show that 2006 RH120 could have been visited
and 62°, with an average of approximately 33°. The departure using a total Dv of only 4.451 km s–1 and a round-trip mission
declination is a significant mission parameter because if the duration of only 58 days in mid-January 2007, while 2009 BD
absolute value of the declination is larger than the launch site could have been visited for a total Dv of 5.998 km s–1 and a
latitude, then additional out-of-plane Dv will be required to round-trip mission duration of 50 days in mid-April 2011.
reach the Earth departure hyperbola, thus reducing the mass Thus, both 2006 RH120 and 2009 BD were at their most
performance of the launch vehicle. For reference, the latitude accessible locations near the times when they were discov-
of NASA’s Kennedy Space Center (KSC) is 28.5°. ered, and their accessibilities during those times, especially
Even though the NHATS analysis system was designed that of 2006 RH120, rival that of an NEA in LDRO. Note that
and implemented prior to the conception of NASA’s proposed the mid-April 2011 launch for 2009 BD would have come
Asteroid Redirect Mission (ARM), which seeks to capture about two years after its discovery and so a sufficiently long
and return a multi-ton boulder from a large NEA and place observation arc could have been obtained to characterize
it in a lunar distant retrograde orbit (LDRO) for human its orbit and ascertain that it was not an artificial object.
visitation, the mission accessibility of NEAs classified as However, 2006 RH120 did not even receive its minor planet
NHATS-compliant is sufficiently general that candidate ARM designation until February 2008, so January 2007 would
targets also tend to be NHATS-compliant. have been much too early to launch a mission to the NEA.Abell et al.: Human Exploration of Near-Earth Asteroids 865
All NEA Mission Earth Return Entry Speeds 11.101–11.962 km s–1
10
9
Total Round-Trip ∆v (from LEO) (km s–1)
8
7
6
5
4
ARM Crewed (5-d stay)
3 2000 SG344 (8-d stay), 2029 launch
2006 RH120 (8-d stay), 2028 launch
2010 UE51 (8-d stay), 2023 launch
2 2009 BD (8-d stay), 2021/2022 launch
2011 MD (8-d stay), 2023 launch
2008 HU4 (8-d stay), 2025 launch
1 Itokawa (8-d stay), 2035 launch
Bennu (8-d stay), 2036
2008 EV5 (64-d stay), 2024 launch
0
0 50 100 150 200 250 300 350
Round-Trip Flight Time (d)
Fig. 7. Comparison of round-trip mission accessibility for selected NHATS-compli-
ant NEAs and an entire small NEA or captured boulder in LDRO.
These results serve to emphasize (1) how accessible NEAs such that any mission meeting the constraints would be less
can be in their natural orbits, and (2) that a spacebased tele- demanding (e.g., in terms of Dv and/or duration) than any
scope stationed far from Earth would be needed to discover round-trip mission to Mars (e.g., conjunction class, opposi-
and characterize NEAs sufficiently far in advance of their tion class, short stay, long stay, or Mars free-return flyby; all
peak mission accessibility seasons to enable missions for with or without Venus gravity assist).
which we would otherwise not have enough advance notice. The Earth departure C3 is twice the specific (per unit
mass) energy of the spacecraft with respect to Earth. The
3.4. Near-Earth Object Human Spaceflight Accessible total mission Dv is the sum of the following Dv maneuvers:
Targets Study Algorithm (1) departure from a circular 400-km-altitude LEO; (2) NEA
arrival (matching NEA’s heliocentric velocity at the time
The NHATS algorithm uses the method of embedded tra- of arrival); (3) NEA departure; and (4) reduction of Earth
jectory grids (Barbee et al., 2010, 2011, 2013) to perform a atmospheric entry speed if necessary (the atmospheric entry
comprehensive analysis of available round-trip trajectories to speed reduction Dv is modeled as being performed at Earth’s
a particular NEA. The notional mission sequence is depicted gravitational sphere of influence for conservatism; some tra-
in Fig. 8, which illustrates the four key parameters that must jectories will naturally have Earth atmospheric entry speed
be varied in order to identify optimal round-trip trajectories to ≤12 km s–1). These calculations use a value of 3.986004415 ×
an NEA: (1) Earth departure epoch, (2) time of flight to reach 105 km3 s–2 for Earth’s gravitational parameter and a value of
the NEA, (3) stay time at the NEA, and (4) duration of return 6378.136 km for Earth’s radius. The total round-trip mission
flight to Earth. The method of embedded trajectory grids is duration is the sum of (1) the time of flight required to reach
utilized to efficiently compute solutions for all combinations the NEA from Earth, (2) the stay time at the NEA, and (3) the
of those parameters within a four-dimensional parameter space time of flight required to return to Earth from the NEA.
and locate optimal solutions (e.g., minimum total mission Dv The trajectories to/from the NEAs are computed by solv-
or minimum total mission duration). Each of the four key ing Lambert’s problem with precise ephemeris files for Earth
parameters are stepped through at eight-day intervals. and NEAs obtained from the JPL Horizons system (http://
The NHATS trajectory constraints are shown in Table 4. ssd.jpl.nasa.gov/?horizons, accessed June 9, 2015), and us-
These constraints were developed in 2010 by a human ex- ing a value of 1.32712440018 × 1011 km3 s–2 for the Sun’s
ploration committee, and the constraint values were selected gravitational parameter. The Lambert trajectory solutions866 Asteroids IV
Asteroid Orbit Arc
During Which Spacecraft Asteroid Arrival Maneuver: ∆vARRA
Stays at Asteroid for Some Spacecraft Trajectory
Amount of Time: TSTAY Asteroid
to Asteroid with Some
at Arrival
Asteroid at Asteroid Departure Time of Flight: TOFEA
Earth Departure
Asteroid Departure Maneuver: ∆vDEPA Maneuver: ∆vDEPE
Spacecraft Trajectory
to Return to Earth with Some 2
Time of Flight: TOFAE 3
Earth at Earth Departure
Spacecraft has Some
1
Velocity Relative to Earth
Upon Earth Return: vS/ERET
4
Earth at Return
Sun
Earth Orbit
Asteroid Orbit
Fig. 8. Round-trip mission profile assumed in NHATS.
TABLE 4. NHATS trajectory analysis parameter constraints. 12 km s–1 and round-trip duration ≤450 days. Furthermore,
the most aggressive Mars mission options are only sparsely
Parameter Constraint available during particular Earth departure years and entail
Earth departure date 2015-01-01 to close approaches to the Sun (Venus orbit distance or less).
2040-12-31 It is important to recognize that while the NHATS cri-
Earth departure C3 (km2 s–2) ≤60 terion for Dv extends up to 12 km s–1, the most desirable
Total mission Dv (km s–1) ≤12 NEA mission opportunities involve total mission Dv of only
Total round-trip mission duration (d) ≤450 6–7 km s–1. Thus, the fact that >1000 NHATS-compliant
Stay time at NEA (d) ≥8
NEAs are currently known should not be misconstrued to
Earth atmospheric entry speed (km s–1) ≤12
imply that an enhanced survey for additional target NEAs
is unnecessary. On the contrary, additional enhanced NEA
survey efforts are very much needed.
omit mid-course maneuvers and gravity assists, but those Thus far a number of NEAs have been discovered that
trajectory design techniques are unlikely to be useful when offer very low-Dv and short-duration mission opportuni-
total round-trip mission duration is limited to no more than ties. This point must be kept in mind when considering the
450 days. Additionally, the Lambert solutions have been found NHATS analysis criteria in the context of Table 5. Although
to be quite accurate when compared to precision trajectory these criteria extend up to Dv of 12 km s–1 and mission du-
solutions obtained via differential corrections and high-fidelity ration of 450 days, some NEAs offer mission opportunities
force models for the spacecraft (Barbee et al., 2011). requiring much less Dv and shorter mission duration. This is
The NHATS criteria are defined such that any NHATS- seen in Fig. 9, which depicts the 62,138 round-trip trajectory
compliant NEA is more dynamically accessible than Mars solutions available in the NHATS website database for the
(and, therefore, the martian moons Phobos and Deimos). 1245 known NHATS-compliant NEAs. The grid-like nature
Table 5 illustrates this by providing a summary compari- of the data points is simply an artifact of the binning scheme
son of mission parameters for NHATS NEAs and Mars in used within the NHATS website database to keep the data
various modalities: round-trip visits to the surface of Mars, volume manageable (many millions of trajectory solutions are
round-trip visits to a highly elliptical orbit around Mars (no in fact computed and stored outside of the website database).
landing) with and without a Venus flyby, and free-return The data points in Fig. 9 represent optimal (minimum Dv and
round-trip missions to Mars that neither enter orbit around minimum mission duration) round-trip solutions that satisfy
Mars nor land on the surface of Mars (with and without NHATS analysis constraints. The solid line in Fig. 9 indicates
a Venus flyby). Table 5 shows that no possible Mars mis- the minimum Dv from LEO to barely reach Earth escape
sion opportunity of any kind requires both round-trip Dv ≤ conditions; the Dv requirement for any viable NEA missionAbell et al.: Human Exploration of Near-Earth Asteroids 867
TABLE 5. Comparison of mission parameters for NHATS NEAs and Mars.
Round-Trip Round-Trip Stay Time Closest Earth
Destination Dv Duration at Destination Approach Departure Notes
(km s–1) (d) (d) to Sun Year(s)
NHATS NEAs ≤12 ≤450 ≥8 Usually near 2015–2040
1 AU
Lowest Dv Mars 12.530 923 500 Usually near 2035 Round-trip to Mars surface
surface visit 1 AU
Lowest Dv visit 6.290 923 500 Usually near 2035 No landing; stay in elliptical
to Mars orbit 1 AU Mars orbit
Very short visit 12.136 422 7 ~0.72 2034 No landing; stay in elliptical
to Mars orbit Mars orbit
Short visit to Mars 12.813 485 45 0.70 2035 No landing; stay in elliptical
orbit (with Venus flyby) Mars orbit
Short visit to Mars 8.120 588 45 0.62 2033 No landing; stay in elliptical
orbit (with Venus flyby) Mars orbit
Mars free-return flyby 9.010 501 0 0.73 2018 No landing; no stay in Mars
orbit
Mars free-return 6.065 582 0 0.70 2021 No landing; no stay in Mars
(with Venus flyby) orbit
Mars mission data taken from NHATS summary chart available at http://www.lpi.usra.edu/sbag/science/NHATS_Accessible_NEAs_Summary.png
(accessed September 12, 2014).
15 For added context, when examining Fig. 9, consider that
Minimum Escape
Total Round-Trip ∆v (from LEO) (km s–1)
14 All NHATS round-trip missions to a low-altitude circular lunar orbit or
13 2015–2030 the lunar surface require a total mission Dv of 5 or 9 km s–1,
12
respectively, and one to several weeks of mission duration.
11
Some of the NHATS mission solutions have durations that
10
9
begin to approach lunar mission duration, but a substantial
8 number of NHATS mission solutions require less Dv than a
7 lunar mission. Specifically, as of September 12, 2014, there
6 are 522 NHATS-compliant NEAs that can be visited round-
5 trip for less total Dv than a round-trip mission to the lunar
4 surface (Dv ≤ 9 km s–1), and 44 NHATS-compliant NEAs that
3 can be visited round-trip for less total Dv than a round-trip
2
mission to a low-altitude circular lunar orbit (Dv ≤ 5 km s–1).
1
Finally, it is also useful to compare the data points shown in
0
0 50 100 150 200 250 300 350 400 450 500 550 600 Fig. 9 to the Mars mission data contained in Table 5.
It is illustrative to further examine a particular mission op-
Round-Trip Flight Time (d) portunity to an exemplar NEA. Consider NEA 2000 SG344,
which is currently regarded as the most accessible of the
Fig. 9. Round-trip Dv and mission duration for selected NHATS-compliant NEAs because it offers the overall lowest
mission opportunities to the NHATS-compliant NEAs. mission Dv and the largest number of mission trajectory solu-
tions within the NHATS analysis interval. A round-trip mission
opportunity to spend 16 days at 2000 SG344 is available with
trajectory will always be at least slightly higher. Note that an Earth departure date of July 14, 2029, a total mission Dv
several mission opportunities identified by NHATS closely of 4.989 km s–1, and a total mission duration of 154 days.
approach this threshold. Also, the data markers in Fig. 9 The Earth atmospheric entry speed upon return would be
indicate when particular mission opportunities are available 11.157 km s–1 (nearly as low as the fastest atmospheric entry
with Earth departure dates between 2025 and 2030; that par- ever logged by humans: 11.069 km s–1 during Apollo 10),
ticular range of years may be programmatically desirable, and the minimum distance to the Sun would be 0.976 AU, the
marking the data in that way illustrates the concept that NEA maximum distance from the Sun would be 1.027 AU, and
missions must launch during certain Earth departure seasons. the maximum distance from Earth would be only 0.055 AU868 Asteroids IV
1 Earth
SC Trajectory
0.8 2000 SG344
Departure 1
Arrival 1
0.6
Departure 2
Arrival 2
0.4
0.2
Y – HCI (AU)
0
–0.2
–0.4
–0.6
–0.8
–1
–0.8 –0.6 –0.4 –0.2 0 0.2 0.4 0.6 0.8 1
X – HCI (AU)
Fig. 10. Heliocentric inertial view of a 154-day round-trip mission to NEA 2000 SG344.
(21.226 lunar distances). Figure 10 depicts a heliocentric A few years before the return of the Hayabusa spacecraft,
inertial view of the mission trajectory. JAXA began planning its next asteroid sample return mis-
sion, Hayabusa2. Hayabusa was considered an engineering
4. INTERNATIONAL EFFORTS demonstration mission, but Hayabusa2 will primarily focus
AND CONTRIBUTIONS on science, with a goal of furthering understanding of the ori-
gin and evolution of the solar system, particularly as relates
4.1. Japanese Perspective to water and organic matter. It is generally understood that
C-type asteroids contain more organic matter and hydrated
4.1.1. Hayabusa and Hayabusa2 missions. Asteroid minerals than S-type asteroids such as Itokawa. Therefore,
sample return missions are regarded as one of the most C-type asteroid (162173) Ryugu (1999 JU3) was selected
important space goals for space-related science and engi- as Hayabusa2’s target. From a technological point of view,
neering in Japan. The successful Hayabusa mission (see the the purpose of Hayabusa2 is to develop more reliable and
chapter by Yoshikawa et al. in this volume), conducted by robust systems for sample return exploration. The scale of
JAXA, was the world’s first attempted asteroid sample return the spacecraft is similar to Hayabusa, but many parts have
mission. The second such mission, Hayabusa2, is currently been modified or redesigned to prevent most of the difficul-
underway (Yoshikawa et al., 2014). ties experienced by the Hayabusa spacecraft. The Hayabusa2
Hayabusa was launched in May 2003, arrived at its target spacecraft will carry new instrumentation as well, such as
asteroid (25143) Itokawa in September 2005, and returned to an impactor that will create a small crater on the surface of
Earth in June 2010. After its seven-year voyage, the Hayabusa the asteroid. The spacecraft launched in December 2014 and
spacecraft successfully returned asteroidal surface material is scheduled to arrive at Ryugu in June 2018. It will remain
from Itokawa, although much less than was originally hoped at the asteroid for approximately 18 months, departing in
for. The curation facility, located at JAXA’s Sagamihara December 2019 and arriving at Earth in December 2020.
Campus, has retrieved approximately 400 particles from the 4.1.2. Solar sail missions. Hayabusa and Hayabusa2 are
spacecraft’s sample catcher to date. In 2012 and 2013, two both considered challenging missions, but JAXA is currently
international Announcements of Opportunity (AOs) related to considering an even more challenging mission in which a
Itokawa particles were issued, and the samples were distrib- spacecraft powered by a solar power sail will return a sample
uted to approximately 30 research groups all over the world. from a Jupiter Trojan asteroid (e.g., an asteroid located in
Many sample analysis results have already been reported and the L4 or L5 Lagrange points that precede or follow Jupiter
the analyses are still ongoing (e.g., Yurimoto et al., 2011). in its orbit) (Kawaguchi, 2004). A solar sail is a large mem-You can also read
