Development of Discovery space after the LSST survey

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(Luzius) #1

Hi all,

It is frequently said that with the start of normal operation of the LSST there will be no small solar system objects left for smaller telescopes to discover, in the meaning of getting to know their pure existence as compared to their exploration - at least for the sky that is accessible for the LSST.
Their discovery depends on an object’ brightness, which is a function of heliocentric distance, size, albedo, and filter being used. For a given filter and brightness I assume the LSST to have a spherical geocentric discovery-space. During the decade-long survey - with probable 4 year extension ref. - the discovery-space radius remains roughly constant and is synonymous to detection-space with discovered objects leaving it outwards and new objects entering the sphere inwards on their orbital way towards the Sun.
Are there studies about the discovery/detection-space radius in Astronomical Units for a given filter and object brightness, thus size and albedo? I could not find any.

After the LSST survey is finished the discovery-space decreases to the radius accessible by smaller survey telescopes. But it will take some time until new undiscovered objects will have crossed the LSST’s discovery-sphere radius and reach the e.g. 1 or 2 m telescope’s discovery-sphere.
Are there studies as to how long - years or decades - it will take until smaller telescopes will be able to discover genuinely new objects flying inwards?

(Rob Seaman) #2

The small bodies community extends from near-sun to comets and KBOs. Somebody else will have to address the latter (or even main belt asteroids). You might look at early NEO community papers, e.g., from the big Asteroids volumes, for geometry-based NEO discovery arguments. Usually the survey volume was modeled more like an onion with enhanced detectability from the opposition phase effect. “Spherical” would also tend to imply that such a survey telescope operates at the center of the survey volume, which will never be the case for a ground-based facility due to limitations in operating near-sun.

The mega-constellations may limit LSST operations at low solar elongations, challenging the proposed twilight near-sun survey. Even small telescopes should continue to have access to a significant survey volume near-sun, e.g., comet hunters, large IEOs and small mini-moons. NEO Surveyor is anticipated to dominate at low solar elongations, including efficiently looking for Earth Trojans, but that is not the same thing as reaching a 100% success rate.

Numerous factors govern success with moving object discovery and both the Vera Rubin Observatory and NEO Surveyor project teams have addressed these issues through detailed object-by-object simulations. Both teams (with overlapping participation) have found a continuing need to include the contributions of the northern surveys to reach the 90% level for 140-m NEOs in an efficient manner. The LSST covers the south, NEO Surveyor alternately to the east and west, leaving significant areas of sky on any given night in the north for the ongoing surveys (or perhaps a new large-aperture NEO-optimized survey).

In your terms, the survey volume for the currently operating 1.5-1.8-m NEO projects reaches out past the orbit of Mars for 140-m objects. Apparent magnitude decreases steeply with distance, and the search radius will increase from that 1.5-1.6 AUs to about 2.0 AU (for 140-m objects of nominal albedo). The IR is different, but the numbers work out roughly similar for the 0.5-m NEO Surveyor which will have significant detectability to 140-m objects of all albedos out to again around 2.0 AU heliocentric distance. Less than that for smaller objects.

As you imply there is a different shell (the area swept out by the frustum of a cone might be better to model than a sphere) surveyed by the Rubin Observatory, NEO Surveyor, Catalina Sky Survey, and Pan-STARRS, as well as the smaller aperture telescopes like ZTF and ATLAS. Both the angular extent as well as the distance will vary between given contemporaneous observing sessions, and the ongoing NEO survey telescopes compare quite well with the Rubin field-of view. NEOs will enter the various survey volumes from many directions and at different rates (a result derivable from Granvik, The sensitivity to small close-approachers and impactors will differ from 140-m objects, and for that matter there aren’t significant numbers of NEOs larger than 1-km remaining to be discovered by Rubin and NEO Surveyor.

The survey strategies are quite different as well as the cadence. LSST will require multiple visits over a week or two or more to link its candidate tracklets into detections. And NEO Surveyor will require a roughly similar interval before returning to a survey field on the same side of the sun to link significant discovery arcs. In the meantime, NEOCP-based rapid follow-up will continue to deliver near-real-time confirmation of targets accessible to the northern surveys. LSST and NEO Surveyor cadences will also adversely affect their sensitivity to rapidly moving objects. How all these factors interact is a complex convolution. To answer your final question, the various LSST / NEOCam simulations say the ongoing northern surveys will discover objects throughout, including 140-m objects. Discovery is a community activity and the surveys and follow-up facilities will benefit from coordinating operational strategies.

(Luzius) #3

Sure, the LSST discovery/detection space is a sphere only to first approximation, if taking into account the season-dependent sky access of the LSST on Earth’s orbit around the Sun, heliocentric and geocentric geometry aspects for NEOs, hemispheric coverage limitations etc. So due to the last mentioned aspect alone the LSST has about 20’000 square degrees of sky coverage (footnote 11 ref.), thus roughly a half-sphere.
I also assumed that for the discovery half-sphere for a given object brightness/filter/exposure time e.g. in 600 AU heliocentric distance the difference between heliocentric and geocentric geometries are negligible. This sure isn’t the case for NEO observations.

Who should I probably contact concerning my question applied to the extreme outer solar system?