Near Earth Objects (NEOs) are asteroids and comets whose orbits are in close proximity to Earth's orbit; specifically, they have perihelia less than 1.3 astronomical units. NEOs particularly near Earth asteroids (NEAs) are identified as potential destinations for future human exploration missions. In this presentation I provide an overview of the current state of knowledge regarding the astrodynamical accessibility of NEAs according to NASA's Near Earth Object Human Space Flight Accessible Targets Study (NHATS). I also investigate the extremes of NEA accessibility using case studies and illuminate the fact that a space-based survey for NEOs is essential to expanding the set of known accessible NEAs for future human exploration missions

Barbee, Brent W.

NASA Technical Reports Server

Accessible Near-Earth Objects (NEOs)
Presented to the 12th Meeting of the NASA Small Bodies Assessment Group
(SBAG)
Brent W. Barbee
NASA/GSFC
January 7th, 2015
https://ntrs.nasa.gov/search.jsp?R=20150000798 2019-08-31T14:03:03+00:00Z
Deﬁning NEO Accessibility Factors
 Astrodynamical
 Earth departure dates; mission Δv; mission duration; stay time; etc
 Physical
 NEO size(?); rotation rate; dust/satellites environment; chemistry; etc
 Architectural
 Launch vehicle(s); crew vehicle(s); habitat module(s); budget; etc
 Operational
 Operations experience; abort options/proﬁles; etc
Astrodynamical Accessibility is the starting point for understanding the
options and opportunities available to us.
Here we shall focus on Astrodynamical Accessibility.
Development of accessibility aspects may occur in parallel.
2
Astrodynamical Accessibility (NHATS)
 Earth departure date between 2015-01-01 and 2040-12-31
 Earth departure C3 ≤60 km2/s2
 Total mission Δv ≤12 km/s
 The total Δv includes (1) the Earth departure maneuver from a 400 km
altitude circular parking orbit, (2) the maneuver to match the NEA’s
velocity at arrival, (3) the maneuver to depart the NEA and, (4) if
necessary, a maneuver to control the atmospheric re-entry speed during
Earth return.
 Total round trip mission duration ≤450 days
 Stay time at the NEA ≥8 days
 Earth atmospheric entry speed ≤12 km/s at an altitude of 125 km
A near-Earth asteroid (NEA) that oﬀers at least one trajectory solution
meeting those criteria is classiﬁed as NHATS-compliant.
http://neo.jpl.nasa.gov/nhats/
3
Proﬁle of a Human Mission to an NEA
The purpose of NASA’s Near-Earth Object Human Space Flight Accessible Targets Study
(NHATS) (pron.: /næts/) is to identify known near-Earth objects (NEOs), particularly
near-Earth asteroids (NEAs), that may be accessible for future human space ﬂight missions.
The NHATS also identiﬁes low Δv robotic mission opportunities.
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Putting Accessibility Into Context
 What does “accessible NEO” mean? “Accessible” compared to what?
 Other solar system destinations:
Destination Total Δv (km/s) Round-Trip Mission Duration (days)
Lunar orbit ∼5 ∼One to several weeks
Lunar surface ∼9 ∼One to several weeks
Mars Surface 12.53 923 (500 day stay)
Elliptical Mars Orbit 6.29 923 (500 day stay)
Elliptical Mars Orbit 12.14 422 ( 7 day stay)
Elliptical Mars Orbit (w/ Venus ﬂyby) 12.81 485 ( 45 day stay)
Elliptical Mars Orbit (w/ Venus ﬂyby) 8.12 588 ( 45 day stay)
Mars ﬂyby 9.01 501 ( 0 day stay)
Mars ﬂyby (w/ Venus ﬂyby) 6.07 582 ( 0 day stay)
Phobos/Deimos Similar requirements to Mars
 Many Mars/Phobos/Deimos mission trajectories pass within Venus
distance (∼0.7 AU) of the Sun, or closer (thermal/radiation issues)
No round-trip mission to Mars (orbit, surface, or ﬂyby) or
Phobos/Deimos is possible with both Δv ≤12 km/s AND mission
duration ≤450 days.
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Putting Accessibility Into Context
 As of 2014-12-06, 1317 NHATS-compliant NEAs have been discovered
 Of those,
 49 can be visited and returned from for less Δv than Lunar orbit
 556 can be visited and returned from for less Δv than the lunar surface
 All 1317 are more accessible than Mars, Phobos, or Deimos
 More and more NHATS-compliant NEAs are being discovered and
identiﬁed
 The NHATS data processing is automated, observers are automatically
notiﬁed, web-site is updated daily
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Comparisons to ARM
0 50 100 150 200 250 300 350
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Round -Trip F l ight T ime , d
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All NEA Mission Earth Return Entry Speeds 11.101 − 11.962 km/s
ARM Crewed (5d stay)
2000 SG344 (8d stay), 2029 launch
2006 RH120 (8d stay), 2028 launch
2010 UE51 (8d stay), 2023 launch
2009 BD (8d stay), 2021/22 launch
2011 MD (8d stay), 2023 launch
2008 HU4 (8d stay), 2025 launch
Itokawa (8d stay), 2035 launch
Bennu (8d stay), 2036 launch
2008 EV5 (64d stay), 2024 launch
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Putting it all together ...
(Available Online at: http://www.lpi.usra.edu/sbag/science/)
Communicating all of this to
the public, and even to
technical folks who are
non-specialists, is very
challenging.
NRC’s “Pathways to Exploration” Report (draft)
BEFORE Dr. Richard Binzel’s 8 July 2014 Letter
to the NRC and NAC pointing out the existence of NHATS data ...
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Adding The Missing NHATS Data
The missing NHATS data overlaid on the original NRC ﬁgure,
from Dr. Richard Binzel’s 8 July 2014 Letter to the NRC and NAC
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NRC’s “Pathways to Exploration” Report (ﬁnal)
AFTER Dr. Richard Binzel’s 8 July 2014 Letter to the NRC and NAC
Figure 1.9 in:
http://www.nap.edu/catalog/18801/pathways-to-exploration-rationales-and-approaches-for-a-us-program
13
Recent Nature Article
Binzel, R., “Find asteroids to get to Mars,” Nature, Volume 514, 30 October 2014, pages 559–561
http://www.nature.com/news/human-spaceflight-find-asteroids-to-get-to-mars-1.16216?WT.ec_id=NATURE-20141030
PDF: http://www.nature.com/polopoly_fs/1.16216!/menu/main/topColumns/topLeftColumn/pdf/514559a.pdf
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NAC Findings/Recommendations
Excerpt from page 4 of the 4 August 2014 letter to the NASA Administrator from the
NASA Advisory Council (NAC); note that (1) the date is after Dr. Richard Binzel’s 8 July
2014 Letter to the NRC and NAC, and (2) the NAC letter uses the same references as the 8
July 2014 letter
(http://www.nasa.gov/offices/nac/meetings/JULY-30-31-2014_presentations.html)
(http://www.nasa.gov/sites/default/files/files/SquyresLetterToBolden_tagged.pdf)
(http://www.nasa.gov/sites/default/files/files/SquyresLetterToBolden.pdf)
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How Accessible Can NEOs Be?
 How many accessible NEOs are out there waiting for us to ﬁnd them?
 And, how accessible are they?
 In future studies we may apply the NHATS algorithms to simulated
NEOs predicted by modern NEO population models
 That will at least tell us what additional accessible NEOs are predicted
by our population models
 But we won’t really know until we deploy a space-based NEO survey
telescope
 In the meantime, we can look at some historical NEO accessibility data
to gain a sense of just how accessible NEOs in their natural orbits can be
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Analysis of 2006 RH120 and 2009 BD
 The NHATS system monitors NEA accessibility for missions departing
Earth 2015–2040
 However, some NEAs oﬀered their best mission opportunities during time
frame surrounding when they were discovered
 To illustrate this, during July of 2014 Paul Chodas and I investigated the
mission accesibility of two NEAs: 2006 RH120 (∼2–3 m in size) and 2009
BD (∼4 m in size)
 2006 RH120 was temporarily captured by the Earth from about
September 2006 to June 2007
 But was not given a minor planet designation until February 18, 2008
 We believe objects the size of 2006 RH120 are captured
by the Earth about once per decade
17
Mission Trajectories Comparison
2006 RH120
Δv ≤ 12 km/s, Dur ≤ 450 d Δv ≤ 4.5 km/s, Dur ≤ 150 d
2015–2040 2006–2007 2015–2040 2006–2007
Min. Δv Min. Dur. Min. Δv Min. Dur. Min. Δv Min. Dur. Min. Δv Min. Dur.
Total Δv (km/s) 3.972 11.942 3.501 9.147 4.711 4.993 3.843 4.451
Total Duration (days) 450 34 386 18 146 122 146 58
Earth Dep Date 18-Aug-2027 4-Aug-2028 18-Jun-2006 9-Mar-2007 3-Jul-2028 3-Jul-2028 1-Mar-2007 12-Jan-2007
Return Entry Speed (km/s) 11.083 12.000 11.085 11.811 11.101 11.112 11.075 11.091
2009 BD
Δv ≤ 12 km/s, Dur ≤ 450 d Δv ≤ 6.0 km/s, Dur ≤ 270 d
2015–2040 2008–2012 2015–2040 2008–2012
Min. Δv Min. Dur. Min. Δv Min. Dur. Min. Δv Min. Dur. Min. Δv Min. Dur.
Total Δv (km/s) 4.978 11.876 3.464 11.054 5.876 5.964 3.843 5.998
Total Duration (days) 370 114 354 18 266 258 258 50
Earth Dep Date 30-Nov-2033 25-May-2034 15-Jun-2010 17-May-2011 10-Feb-2034 10-Feb-2034 8-Sep-2009 15-Apr-2011
Return Entry Speed (km/s) 11.131 11.909 11.138 11.871 11.181 11.204 11.123 11.141
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Comparisons to ARM
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58 Day Mission to 2006 RH120
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2006RH120
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50 Day Mission to 2009 BD
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2009BD
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Comments on Mission Trajectories
 January 2007 is only a few months after the discovery of 2006 RH120,
and a year before the object received its minor planet designation;
suﬃcient time (a long enough arc of observations) is needed to ascertain
whether an object is artiﬁcial or natural
 Such considerations will generally be important to mission analysis for
small NEAs in any context
 On the other hand, April 2011 is a full 2 years after the discovery of 2009
BD and so would likely be a feasible launch date for a mission, at least
from the perspective of having a suﬃciently long observation arc
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Observability and Accessibility Coinciding
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Observability and Accessibility Coinciding
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Remarks
 2006 RH120 was most accessible when near Earth, around the time of its
discovery
 When it was a temporarily captured object (“mini-moon”), 2006 RH120
oﬀered round-trip mission accessibility approaching that of an object in a
lunar DRO
 Same Δv, but ∼2 month round trip rather than ∼1 month round trip
 Subject to the aforementioned caveats and additional considerations
 Though it was not a temporarily captured object, 2009 BD was also
most accessible when near Earth, around the time of its discovery
 Both objects oﬀered long accessibility seasons surrounding the times
when they were discovered
 Enhanced NEO survey capabilities (e.g., a space-based NEO survey
telescope) might have the potential to discover highly accessible NEAs
such as these years in advance of their peak mission accessibility seasons,
aﬀording us the opportunity to prepare missions to visit them in their
native orbits
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Conclusions and Findings
 Many accessible NEOs have been discovered and identiﬁed.
 We have an automated system to monitor the accessibility of the NEA
population (NHATS).
 It is likely that many more accessible NEOs are waiting to be
found.
 Further study is required to learn what modern NEO population models
have to say on this point.
 Findings: Current survey capabilities tend to discover NEOs very
close to the times of their optimal mission opportunities.
 A space-based NEO survey telescope is needed to discover NEOs with
implementable mission opportunities (far enough in advance of their
mission opportunities).
 Such an asset would simultaneously beneﬁt human exploration, planetary
defense, and science.
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Survey Beneﬁts Human Exploration
27
Appendices
PCC Comparison: 2006 RH120
Standard NHATS Analysis 2015–2040 NHATS-like Analysis 2006–2007
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PCC Comparison: 2009 BD
Standard NHATS Analysis 2015–2040 NHATS-like Analysis 2008–2012
30
Motion Relative to Earth: 2006 RH120
Image Credit: Paul Chodas/JPL
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Motion Relative to Earth: 2006 RH120
−0.4 −0.2 0 0.2 0.4 0.6 0.8 1 1.2
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2006-01-01 to 2007-12-31
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Motion Relative to Earth: 2009 BD
Image Credit: Paul Chodas/JPL
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Motion Relative to Earth: 2009 BD
−0.4 −0.2 0 0.2 0.4 0.6 0.8 1 1.2
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2008-01-01 to 2014-12-31
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Accessible Near-Earth Objects (NEOs)

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