of 8
The Scientific Impact of the
Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST)
for Solar System Science
Vera C. Rubin Observatory LSST Solar System Science Collaboration,
1
R. Lynne Jones,
2, 3,
∗
Michelle T. Bannister,
4
Bryce T. Bolin,
5
Colin Orion Chandler,
6
Steven R. Chesley,
7
Siegfried Eggl,
2, 3
Sarah Greenstreet,
2
Timothy R. Holt,
8, 9
Henry H. Hsieh,
10, 11
ˇ
Zeljko Ivezi
́
c,
2, 3
Mario Juri
́
c,
2, 3
Michael S. P. Kelley,
12
Matthew M. Knight,
13, 12
Renu Malhotra,
14
William J. Oldroyd,
6
Gal Sarid,
15
Megan E. Schwamb,
16
Colin Snodgrass,
17
Michael Solontoi,
18
and David E. Trilling
6, 19
1
Vera C. Rubin LSST Science Collaborations
2
DIRAC Institute and Department of Astronomy, University of Washington, Seattle, Washington, USA
3
Vera C. Rubin Observatory, Tucson, Arizona, USA
4
School of Physical and Chemical Sciences — Te Kura Mat
̄
u
, University of Canterbury, New Zealand
5
IPAC, California Institute of Technology, Pasadena, CA, USA
6
Department of Astronomy & Planetary Science, Northern Arizona University, Flagstaff, AZ, USA
7
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
8
Centre for Astrophysics, University of Southern Queensland, Queensland, Australia
9
Department of Space Studies, Southwest Research Institute, Boulder, Colorado, USA
10
Planetary Science Institute, Tucson, Arizona, USA
11
Academia Sinica Institute of Astronomy and Astrophysics, Taipei, Taiwan
12
University of Maryland, College Park, Maryland, USA
13
United States Naval Academy, Annapolis, Maryland, USA
14
Lunar and Planetary Laboratory, The University of Arizona, Tucson, USA
15
SETI Institute, Mountain View, CA, US
16
Queen’s University Belfast, Belfast, UK
17
Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK
18
Monmouth College, Monmouth, Illinois, USA
19
Lowell Observatory, Flagstaff, AZ, USA
(Dated: September 17, 2020)
∗
Corresponding author email: lynnej@uw.edu
arXiv:2009.07653v1 [astro-ph.IM] 14 Sep 2020
1.
INTRODUCTION
Vera C. Rubin Observatory will be a key facility for small body science in planetary astron-
omy over the next decade. It will carry out the Legacy Survey of Space and Time (LSST),
observing the sky repeatedly in
u
,
g
,
r
,
i
,
z
, and
y
over the course of ten years using a 6.5 m
effective diameter telescope with a 9.6 square degree field of view, reaching approximately
r
= 24.5 mag (5-
σ
depth) per visit. The resulting dataset will provide extraordinary oppor-
tunities for both discovery and characterization of large numbers (10–100 times more than
currently known) of small solar system bodies, furthering studies of planetary formation and
evolution. This white paper summarizes some of the expected science from the ten years of
LSST, and emphasizes that the planetary astronomy community should remain invested in
the path of Rubin Observatory once the LSST is complete.
2.
THE LSST AS A SMALL-BODY DISCOVERY MACHINE
Discovery and census of objects in the solar system is one of the four foundational science
cases for the LSST (Ivezi ́c et al. 2019). The suitability of the final LSST cadence for the
discovery and characterization of solar system objects will therefore be a high-priority con-
sideration in its selection. A solar system object observed by the LSST will be identified
as such with 95% efficiency (on average) if it was detected on at least three nights within
a window of 15 days, with a minimum of two visits per night (Ivezi ́c et al. 2019). Realistic
simulations performed using the Moving Object Processing System (MOPS; which was de-
veloped by the Pan-STARRS survey and adopted by LSST) show
>
99% linking efficiency
across all classes of solar system objects (Denneau et al. 2013), and at least 93% efficiency for
Near-Earth objects (NEOs; Vereˇs & Chesley 2017a,b) using this criterion. Newly discovered
objects will be reported to the Minor Planet Center, generally within 24 hours of discovery.
The LSST alert stream will identify all transient objects in each image, including any
small body detections or trailed objects, and distribute alerts within 60 seconds of shutter
closure. In total, the LSST will obtain approximately 1.8 billion observations of 5.5 million
objects over 10 years. Data will be automatically reduced and delivered with systematic-
limited astrometric and photometric precisions of 10 mas and 0.01 mag, respectively. Rubin
Observatory will be by far the most prolific discovery and characterization machine of small
solar system bodies throughout the 2020s.
Table 1 summarizes estimated LSST discovery yields for several major solar system pop-
ulations. Most discoveries will occur during the first few years of the survey (Figure 1). A
significant fraction of these objects will receive multi-band observations suitable for measur-
ing colors and lightcurves. Beyond these major populations, the LSST will also discover an
order of magnitude more interstellar objects, active asteroids, Centaurs, planetary Trojans,
temporarily captured objects (i.e., “mini-moons”), Hildas, and inner Oort Cloud objects.
3.
SCIENCE ENABLED BY LSST
3.1.
Near-Earth objects
The LSST plays a crucial role in achieving the congressional target of discovering 90% of
all potentially hazardous asteroids (PHAs) with
H
≈
22 (Vereˇs & Chesley 2017a; Jones et al.
2018). Current predictions suggest that the LSST alone could be responsible for a five-fold
1
Table 1.
Small-body population numbers as of today (7/2020; JPL Small-Body Database) and
after LSST (approximate predicted LSST results based on simulations).
Population
Currently known (approximate) LSST discoveries (predicted)
Near-Earth objects
23 000
100 000
Main Belt asteroids
856 000
5 000 000
Jovian Trojans
8 000
280 000
Trans-Neptunian objects
3 500
40 000
Comets
4 000
10 000
Interstellar objects
2
>
10
0
2
4
6
8
10
Survey Time (years)
0.0
0.2
0.4
0.6
0.8
1.0
Cumulative Completeness
Completeness for various populations; baseline LSST simulations
PHA @H<20.00
PHA @H<22.00
NEO @H<20.00
NEO @H<22.00
MBA @H<18.00
MBA @H<21.00
Trojan @H<16.00
Trojan @H<18.00
TNO @H<6.00
TNO @H<8.00
Figure 1.
LSST discovery completeness as a function of time, for various modeled solar system
populations (Granvik et al. 2018; Grav et al. 2011; Petit et al. 2011; Kavelaars et al. 2009). Cu-
mulative completeness is reported for two different absolute magnitude (
H
) values; one near the
bright end of the population (solid lines) and one near the 50% completeness level (dashed lines).
These follow similar tracks over time, showing that most objects are discovered early in the survey.
increase in the total number of known NEOs by the end of the survey (Table 1), detecting
on the order of 100 000 NEOs at a range of sizes down to
H <
25 mag.
The LSST’s excellent astrometric precision (
σ
≈
10 mas) is expected to facilitate detections
of non-gravitational forces acting on orbits, including the Yarkovsky effect, solar radiation
pressure, outgassing, and collisions. Understanding how these effects influence NEO trajec-
tories is critical to our ability to precisely predict future impacts (Farnocchia et al. 2013).
The timely processing and publication of NEO observations allows impact monitoring
services to provide advance notice of close approaches and potential impacts. This facilitates
critical characterization efforts including radar, spectroscopic, and light curve observations.
Variations in the brightness of NEOs can also yield clues on NEO disruption mechanisms at
small perihelion distances (Granvik et al. 2016), which can then be utilized to probe NEO
internal structure and test dynamical models, both critical for effective planetary defense.
2
Yearly releases of NEO catalogs exclusively based on LSST observations enable unbiased
population predictions of unprecedented quality. Unbiased populations can be used to quan-
tify the long-term impact flux of NEOs as a function of size (Granvik et al. 2018). The orbital,
absolute magnitude, and taxonomy distributions within the NEO population derived from
the LSST data can help identify correlations between taxonomy and orbital properties for
NEOs, as well as determine the orbital distribution of objects down to ten meters in size.
3.2.
Main Belt asteroids
Main Belt asteroids (MBAs) represent important compositional and dynamical tracers of
the solar system’s formation and evolution. Their orbital element distribution, size-frequency
distributions, and total mass provide key constraints for planetary migration models like the
Nice Model and the Grand Tack Model (Morbidelli et al. 2010; Walsh et al. 2011; Yoshida
et al. 2019), while taxonomic classifications provide additional constraints on those models
and also enable the tracing of more recent dynamical and physical evolution (e.g., DeMeo &
Carry 2014). The LSST will discover an order of magnitude more MBAs than are currently
known, providing new insights into solar system formation and evolution.
Beyond discovery, the multi-band, multi-epoch observations from the LSST (
∼
200–300
photometric measurements per object for over 5 million MBAs during its 10-year survey; Sec-
tion 5 in LSST Science Collaboration et al. 2009) will enable detailed physical studies. Precise
multi-band optical colors will enable robust and efficient taxonomic classification of MBAs
(Carvano et al. 2010), and detailed studies of asteroid families (e.g., Ivezi ́c et al. 2002; Parker
et al. 2008), where color precisions on the order of 0.01 mag for sufficiently bright asteroids
are expected to be achievable by the LSST. When combined with infrared data, optical
albedos can be estimated and used to improve taxonomic and family classifications (Mainzer
et al. 2012), and to derive physical size estimates (Moeyens et al. 2020; Ivezi ́c & Ivezi ́c 2020).
3.3.
Short-period comets and active asteroids
Active objects are small solar system bodies that exhibit any type of mass loss, whether
driven by ice sublimation, impact disruption, rotational destabilization, or other mecha-
nisms. These bodies inform us about the distribution of volatiles throughout the solar
system and hold clues about solar system formation. They include Jupiter-family comets
(JFCs), Oort Cloud comets (Section 3.5), active asteroids (Jewitt et al. 2015; Snodgrass
et al. 2017), active Centaurs (Jewitt 2009), and active interstellar objects (Section 3.6).
An order of magnitude increase in the known JFC population from the LSST will facilitate
improved modeling of their source regions and evolutionary processes (Brasser & Wang 2015)
and enable large-scale photometric and spectroscopic follow-up studies that will improve
our ability to identify and characterize distinct taxonomic classes (Cochran et al. 2015).
Meanwhile, only about 30 active asteroids are currently known, making them difficult to
characterize on a population-scale basis, given the diversity exhibited by the population
thus far (Jewitt et al. 2015). The identification of many more active asteroids by the LSST
will both enable population-level studies, such as efforts to ascertain the rates of rotational
and impact disruptions in the asteroid belt (Denneau et al. 2015; McLoughlin et al. 2015),
and increase the number of individual objects for targeted studies.
3
The LSST’s 10-year timescale is well-suited for the study of active asteroids and short-
period comets, as all active asteroids and about half of all known JFCs have orbital periods
of less than 10 years. LSST should produce data sets for a large number of these objects
around at least one full orbit, providing at least some of the following: constraints on the
initiation/cessation and seasonal variation of activity (A’Hearn et al. 1995), coarse sampling
of outbursts as functions of orbit position (Ishiguro et al. 2016), nucleus size estimates
(Fern ́andez et al. 2013), and rotational lightcurves to constrain rotation periods, axis ratios,
and pole orientations, and identify binarity or higher-order multiplicity (Lamy et al. 2004;
Kokotanekova et al. 2017). The LSST’s well-sampled data set will also enable systematic
studies of active object nuclei before and after active events, helping us to better under-
stand small body interiors, space weathering, and the effects of cometary outgassing (e.g.,
Bodewits et al. 2014). Extending the initial 10-year survey would permit many comets to
be observed over multiple orbits, investigating the secular evolution of cometary activity on
an unprecedented scale (e.g., Kelley et al. 2019; Nesvorn ́y et al. 2017).
3.4.
Trans-Neptunian Objects
Trans-Neptunian Objects (TNOs), Centaurs, and Scattered Disk Objects (SDOs) provide
key insights into planetary formation and evolution, recording the imprint of the formation
and migration of the giant planets in their orbital distributions and physical properties.
Decoding these imprints has been hindered by the lack of sufficiently large population samples
with high precision orbits and well understood discovery circumstances. The LSST will
discover on the order of tens of thousands of TNOs, SDOs and Centaurs, and will deliver these
objects with extremely precise orbits (due to high-precision astrometry spanning multiple
years). Many of the objects will have multi-color photometry suitable for calculating colors
and lightcurves for rough composition and rotation rate distributions.
The discovery, orbital classification, and physical characterization of large numbers of
TNOs will help address the origin of the cold classical Kuiper belt, determining if these
objects are primordial or were implanted into these orbits early in the history of the solar
system, and ascertain its connections with other populations, such as Oort Cloud comets,
Centaurs, and planetary Trojans (e.g., Nesvorn ́y 2018).
Due to its depth and wide sky coverage, the LSST will discover many more high perihelia,
large semi-major axis TNOs, such as inner Oort cloud objects (
i.e.
Sedna-like objects) and
“extreme” TNOs (with
q >
40 AU and
a >
150-250 AU), than the
<
20 that are currently
known. A larger sample with well understood discovery circumstances will help ascertain
whether their source is related to a hypothesized distant planet (Trujillo & Sheppard 2014;
Batygin et al. 2019), Neptune migration, stellar perturbations, or some other mechanism
(e.g., Kavelaars et al. 2020), and also whether their observed orbital alignment is real (pos-
sibly indicating the presence of a distant planet) or the result of observational bias (e.g.,
Trujillo 2020; Brown & Batygin 2019; Shankman et al. 2017). Additionally, the LSST may
directly detect a distant planet if it is sufficiently bright and within the survey footprint,
otherwise ruling out its presence in 61% of the sky (Trilling et al. 2018). A potential survey
extension using deeper (10 minute,
r
≈
26) exposures would increase the survey sensitivity
for more distant and smaller TNOs.
4
3.5.
Oort cloud comets
As the most distant solar system population, the Oort Cloud and its structure reflect the
early evolution of the solar system and that of the local Galactic environment (Heisler &
Tremaine 1986; Kaib et al. 2011). Oort Cloud objects are beyond current observational
limits in-situ, however, and thus Oort Cloud comets (OCCs) provide our only opportunities
to probe the physical and dynamical properties of these objects. The LSST is expected
to discover thousands of OCCs (Solontoi et al. 2010), with generous photometric orbital
coverage thanks to the survey’s exceptional single-visit sensitivity limit.
The discovery of distant OCCs, with perihelion distances,
q
,
>
10 au, enables follow-up
characterization of a population that has likely never been closer to the Sun than Jupiter,
and therefore in an earlier evolutionary phase than OCCs with
q <
5 au. At this stage,
comet orbits are less altered by planetary perturbations and non-gravitational forces, and
offer a rare opportunity to place constraints on the Oort cloud population. Furthermore, the
discovery of comets with perihelia beyond 15 au directly tests the existence of the inner Oort
cloud (Vokrouhlick ́y et al. 2019). Currently, no comets have been discovered with
q >
12 au
(as of July 2020; Giorgini et al. 1996), making this an important discovery space for LSST.
Early detection of inbound OCCs provides us with the opportunity to study the active be-
havior of this population. A long standing problem with our current knowledge of the OCC
population is the apparent paucity of long-period objects, the so-called long-period comet
fading problem. Ad-hoc models of OCC fading (e.g., splitting, disintegration, depletion
of volatiles) have succeeded in addressing the missing comets (Wiegert & Tremaine 1999),
but physically-motivated explanations are still needed (Vokrouhlick ́y et al. 2019). With an
increased discovery rate and broad orbital coverage of OCCs, LSST will enable the study of
key aspects of OCC evolution, such as a statistical description of when comets begin phases
of activity, undergo fragmentation or disintegration, and the follow-up study of these events.
3.6.
Interstellar objects
Detections of the first two known interstellar objects, 1I/‘Oumuamua in 2017 (e.g., Meech
et al. 2017) and 2I/Borisov (e.g., Fitzsimmons et al. 2019) in 2019, have given the first
glimpses of macroscopic material from other solar systems and revealed that the range of
properties of small bodies is broader than previously known. Based on the discoveries of 1I
and 2I, it now seems reasonable to estimate that the LSST will discover on the order of ten
interstellar objects (ISOs) during its first ten years. While each newly discovered ISO will
undoubtedly be studied in great detail, collective statistics for the population must reach
critical mass to enable meaningful tests of planet formation mechanisms and planetesimal
ejection rates in other star systems. Continued surveying by Rubin Observatory beyond
the initial 10 year LSST would be highly beneficial to the study of ISOs by improving
population statistics. A future survey extension with deeper imaging would increase the
survey volume to allow more robust investigation of the ISO size distribution and reveal
whether it is reflective of these objects’ formation or evolution, insight critically needed in
order to understand the context for 1I and 2I (cf. ‘Oumuamua ISSI Team et al. 2019).
4.
CONNECTIONS WITH MISSIONS AND OTHER FACILITIES
5
Previous studies combining SDSS asteroid data with data from other ground and space-
based sources forecast strong synergistic value of the LSST dataset (e.g., Parker et al. 2008;
Carvano et al. 2010; Mainzer et al. 2012). Both the multi-color nature and well-sampled light
curves for an unprecedentedly large sample of small bodies will be exceedingly valuable for
other missions. Follow-up of rare objects discovered by the LSST will be a high priority for
facilities as the James Webb Space Telescope and 30m-class telescopes like the Thirty-Meter
Telescope, Giant Magellan Telescope, and Extremely Large Telescope. Deep coordinated
searches with missions such as the Roman Space Telescope would open new ways to probe
small and cold solar system objects. Meanwhile, combining data from the LSST and the
upcoming NEO Surveillance Mission spacecraft will allow determination of diameters and
albedos for an unprecedented number of solar system objects (cf. Mainzer et al. 2019), while
LSST lightcurves will be essential for assembling data collected over several days by NASA’s
SPHEREx mission, which is expected to collect 0.75-5.0
μ
m spectral data for
∼
200,000
asteroids, into self-consistent spectra for each object.
Discoveries from the LSST will help to enhance the upcoming NASA Discovery missions
Lucy (launch date 2021), targeting seven Jovian Trojans, and Psyche (launch date 2022),
targeting potentially metallic asteroid (16) Psyche. These can offer opportunities to extend
the Lucy mission with new targets of opportunity (Schwamb et al. 2018b), while observations
of larger populations of similar asteroids will add context to the results of the missions.
ESA’s Comet Interceptor mission (Snodgrass & Jones 2019) is scheduled to launch in 2028
and wait at the Earth-Sun Lagrange point L2 for up to
∼
3–5 years until a suitable long-period
comet or ISO is identified for a flyby, potentially making the first ever in situ observations of
a comet entering the inner solar system for the first time. The LSST’s survey power makes
it very likely to be the initial discoverer of the eventual target, and the expected discovery
at large heliocentric distance will be critical for mission optimization.
5.
CONCLUSIONS AND PROPOSED ACTIONS
Rubin Observatory and LSST can be transformative for planetary astronomy. An exciting,
but non-comprehensive, list of science impacts has been summarized here; additional impacts
are outlined in the LSST Solar System Science Collaboration roadmap (Schwamb et al.
2018a) and the LSST Science Book (LSST Science Collaboration et al. 2009). The LSST
will significantly increase the discovery rates of new objects in small-body populations from
NEOs to TNOs and active objects, and enable large-scale investigations of the physical and
dynamical properties of small bodies. In addition, the LSST can support future spacecraft
missions by helping to discover primary, flyby, and extended mission targets and providing
context to the results of targeted missions.
Effective use of Rubin Observatory for solar system studies is contingent on
preparedness now, as well as strong grant support once data begin to flow. At
present, no focused programs exist specifically for analysis of LSST solar system
data
. Unlike NASA or DOE programs which typically come with a science team, funding,
and key projects to be executed, Rubin Observatory is considered a
facility
, with no estab-
lished funding for solar system science. This is in marked difference to some international
partners who already have preparatory LSST science funding and are gearing up for first
6
science. A real risk exists that – having fully funded its M$600+ construction – the U.S. as-
tronomy community may miss out on key LSST discoveries.
The Decadal Survey should
encourage NSF, NASA, and DOE to develop new Rubin-specific grant programs
to ensure this facility fulfils its full potential as a solar system exploration ma-
chine.
Given that most LSST discoveries will occur during the first few years of the survey,
preparedness and early investment will be crucial.
Upon the completion of the 10-year LSST survey, many important additional solar system
science investigations will be possible with an extended or “Phase 2” survey. As such,
planetary science should be a high-priority consideration in planning the post-
LSST future of Rubin Observatory.
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