Looking for Lurkers: Co-orbiters as SETI Observables

Brief

The paper surveys eight known Earth co-orbital objects (quasi-satellites, horseshoe orbits, tadpole orbits) and contends that their orbital stability (mean resonance lifetime 0.33 Myr per Morais & Morbidelli 2002), proximity, sustained solar flux, available raw materials, and concealment potential make them superior lurker sites compared to the lunar surface, Lagrangian dust clouds, or the main asteroid belt. The primary candidate is 2016 HO3, 40-100m diameter, minimum approach 0.0348 AU, stable for roughly a century and projected to remain so for centuries, which has never been imaged by planetary radar. Benford estimates radar detection of 2016 HO3 at SNR > 100 and lays out five search strategies: passive radio and optical monitoring, microwave/IR/optical SETI observations, active planetary radar imaging, simultaneous radar painting with SETI listening (a localized form of METI the paper argues is ethically uncontroversial), and eventual robotic or crewed inspection missions. The paper acknowledges Breakthrough Foundation funding and notes China's April 2019 announcement of a planned probe mission to 2016 HO3.

Metadata

Category

Search

Venue

The Astronomical Journal

Type

Peer-reviewed

Year

2019

Authors

James Benford

DOI

10.3847/1538-3881/ab3e35

arXiv

1903.09582

Access

Open access

Length

530.7 K

Programs

Breakthrough Listen, Breakthrough Foundation

Instruments

planetary radar, Lick Observatory, optical telescopes

Tags

SETI, technosignature, near-Earth objects, co-orbital, METI, planetary radar, Bracewell probe, astrobiology

Key points

• Co-orbital objects share Earth's orbital period and approach Earth annually at distances far shorter than any body except the Moon, making them energetically accessible and persistently close for observation purposes.p.2
• 2016 HO3 (Kamo'oalewa) is 40-100m in diameter with a minimum approach distance of 0.0348 AU (5.2 Mkm), the smallest, closest, and most stable known Earth quasi-satellite, discovered in 2016.p.5
• Morais & Morbidelli (2002) found a mean resonance lifetime of 0.33 million years for Earth co-orbitals, establishing that these objects are stable on timescales relevant to long-duration ETI probes.p.8
• 2006 FV35 has held its horseshoe orbit state for approximately 100,000 years and will remain in it for roughly 800 more years, illustrating the multi-millennium stability available at some co-orbital sites.p.8
• Benford estimates 2016 HO3 is detectable by planetary radar with SNR > 100; 'glinty' radar returns with large rapid changes in signal-to-noise ratio would be a diagnostic indicator of an artificial surface.p.9
• The Moon is ruled out as a lurker site: orbital imagery covers nearly the full surface at ~1-meter resolution, and any probe capable of responding would already have triggered on the many signals directed at the Moon during crewed and robotic missions.p.10
• A probe launched at 0.1c could travel from 30 light-years to Earth in 300 years; approximately 3,000 main-sequence stars lie within 100 light-years, meaning origin civilizations need not be near neighbors.p.12
• 2016 HO3 requires a delta-v of approximately 4.5 km/sec from Earth orbit and approaches annually in October; China announced a mission to it in April 2019.p.12

Verbatim

• “A recently discovered group of nearby co-orbital objects is an attractive location for extraterrestrial intelligence (ETI) to locate a probe to observe Earth while not being easily seen.”

  p.1

• “2016 HO3 has been a stable quasi-satellite of Earth for almost a century, and it will continue to follow this pattern as Earth's closest companion for centuries to come. It never wanders farther away than about 100 times the distance of the moon or comes closer than 38 times that.”

  p.8

• “I estimate that 2016 HO3 is detectable with a signal-to-noise ratio>100.”

  p.9

• “The 'glinty' reflections from spacecraft, large rapid changes in signal-to-noise ratio, would be an indicator of an artificial object [18].”

  p.9

• “One straightforward message to send would be a photograph of the object we are sending the message to. Taking the highest resolution pictures of it as it rotates would simply say "We see you."”

  p.10

• “Co-orbitals are attractive targets for SETI searches because of their proximity. We should move forthrightly toward observing them, both by observing them in the electromagnetic spectrum and planetary radar, as well as visiting them with probes.”

  p.12

Most interesting

• 2015 SO2 and 2016 HO3 have maintained nearly identical orbits for years and approach Earth simultaneously each year, an unusual coincidence that Benford notes without a natural explanation.
• None of the known Earth co-orbital objects had been imaged or 'pinged' by any planetary radar as of the paper's 2019 publication, despite some being known for decades.
• The paper proposes that transmitting a high-resolution radar image of a co-orbital object back to itself, 'We see you', is the minimal meaningful message that might solicit a response from a passive probe.
• Benford reframes near-Earth METI as ethically unproblematic compared to interstellar transmission: any lurker in the co-orbitals already knows humanity exists, so 'drawing attention to ourselves' objections do not apply.
• Cruithne (3753), the famous 'second moon of Earth,' experienced a close encounter with Mars only 2,500 years ago and may leave its current orbit within 5,000 years.
• The paper was funded by the Breakthrough Foundation, the same organization behind Breakthrough Listen, suggesting institutional interest in near-Earth probe searches as a complement to stellar SETI.