Comprehensive NEO Detection, Characterization, and Orbits

December 2005 Congress directed NASA to implement a near-Earth object (NEO) survey that would catalog 90% of Potentially Hazardous Asteroids (PHAs) larger than 140 meters in 15 years. In order to fulfill the Congressional mandate, a 10-meter class telescope with a 3000 Megapixel camera and a sophisticated and robust data processing system is required. These desiderata are met by the Large Synoptic Survey Telescope (LSST). It is fortunate that the same hardware and software requirements for an NEO survey are driven by science unrelated to NEOs: LSST reaches the threshold where different science drivers and different agencies (NSF, DOE and NASA) can work together to efficiently achieve seemingly disjoint, but deeply connected, goals.

Why is Finding NEOs Hard?

The objectives of the George E. Brown, Jr. NEO Survey Act (Public Law No. 109-155) are to detect, track, catalog, and characterize the physical characteristics of PHAs equal to or larger than 140 meters in diameter with a perihelion distance of less than 1.3 AU (Astronomical Units) from the Sun, achieving 90 percent completion of the survey within 15 years. The Act was signed into law by President Bush on December 30, 2005. Ground-based optical surveys are the most efficient tool for comprehensive NEO detection, determination of their orbits and subsequent tracking. A survey capable of extending these tasks to PHAs with diameters as small as 140 m, as mandated by Congress, requires a large telescope, a large camera, and a sophisticated data acquisition, processing and dissemination system.

Why is a large telescope required?

A typical 140-meter object positioned in the main asteroid belt (between Mars and Jupiter, at a distance of 2.5 AU from the Sun) appears very faint (a visual magnitude of 25.5). Despite their name, NEOs are typically found far from Earth. In principle, very faint objects can be detected using long exposures, but for objects moving as fast as typical NEOs, the so-called trailing losses limit the exposure time to about 30 seconds. In order to detect 140-meter NEOs in the main asteroid belt in 30 seconds, a 8-meter class telescope is required. In fact, some of these asteroids move so fast on the sky that 15 seconds is the maximum exposure; LSST will take pairs of 15 seconds exposures at each sky position.

Why is a large camera required?

At the time of its completion, the 3000 Megapixel LSST camera will be the largest astronomical camera in the world. This large size comes from a requirement that the whole observable sky should be observed at least every three nights, with two observations per night. With the implied 10 square degree large field of view, LSST will be able to reach the mandated high NEO completeness.

Why is a complex data processing system required?

With its 3000 Megapixel camera obtaining images every 30 seconds, the data rate will be about 20 terabytes (equivalent to the entire Congressional Library) per night. Not only that this is a huge data rate, but the data have to be processed and disseminated in real time, and with exquisite accuracy. It is estimated that the LSST data system will incorporate several million lines of state-of-the-art computer code.

How would LSST Find NEOs?

The LSST system is the only proposed astronomical facility that can detect 140-meter objects in the main asteroid belt in less than a minute. The LSST system will be sited at Cerro Pachon in northern Chile, with the first light scheduled for 2014. In a continuous observing campaign, LSST will cover the entire available sky every three nights, with two observations per night. Over the proposed survey lifetime of 10 years, each sky location would be observed about 1000 times.

Two NEO detections in a single night are required to estimate its motion, so that its future, or past, detections can be linked together. This linkage has to be done exceedingly robustly because the near-Earth objects will be outnumbered one to hundred by main-belt asteroids which present no threat to Earth. By reliably linking detections on multiple nights, the NEO's orbit can be reconstructed and used to compute its impact probability with Earth.

The high-fidelity simulations of LSST baseline observing campaign demonstrate that LSST will discover and catalog 80-90% of potentially hazardous asteroids larger than 140 meters, with a median of 40 nights of observations per object. Simulations strongly suggest that with an achievable optimization of baseline strategy, LSST will be able to reach the goal mandated by Congress.

Why is LSST a Superb Choice for finding NEOs ?

The LSST is currently by far the most ambitious proposed survey of the sky. With initial funding from the US National Science Foundation (NSF), Department of Energy (DOE) laboratories and private sponsors, the design and development efforts are well underway at many institutions, including top universities and leading national laboratories. The main science themes that drive the LSST system design are Dark Energy and Matter, the Solar System Inventory, Transient Optical Sky and the Milky Way Mapping. It is this diverse array of science goals that has generated the widespread excitement of scientists ranging from high-energy physicists to astronomers and planetary scientists, and earned it endorsement of a number of committees commissioned by the National Academy of Sciences.

Fortunately, the same hardware and software requirements are driven by science unrelated to NEOs: LSST reaches the threshold where different science drivers and different agencies (NSF, DOE and NASA) can work together to efficiently achieve seemingly disjoint, but deeply connected, goals. In particular, the Congressional mandate can be reached at only a fraction of the cost of a mission dedicated exclusively to NEO search.

The added costs to achieve this mission in two optimization scenarios

The following plot shows the completeness fraction vs time for two scenarios, two of which incur added costs associated with reliably meeting this mission in a timely way: