Opening a Window of Discovery on the Dynamic Universe


Two color composite images of asteroid 951 Gaspra taken by the Galileo spacecraft. The image on the left shows Gaspra in approximately true color. On the right the colors have been exaggerated to reveal possible differences in surface composition.

Why will the LSST look for NEOs?

December, 2005, Congress directed NASA to implement a near-Earth object (NEO) survey that would catalog 90% of potentially hazardous asteroids (PHAs). 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 that come within 1.3 astronomical units (AU) or less from the Sun at closest approach. 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 meters, 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. 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, an eight-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?

Surveying the whole observable sky at least once every three nights, with two observations per night, requires not only a large telescope, but a large camera. At the time of its completion, the 3.2 gigapixel LSST camera will be the largest astronomical camera in the world. With the joined resources of its mirror and camera, the LSST will be able to reach the mandated high-NEO completeness.

Why is a complex data processing system required?

With its 3.2 gigapixel 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 Management System will incorporate several million lines of state-of-the-art computer code.

How would the LSST find NEOs?

The LSST system will be sited at Cerro Pachon in northern Chile, with the first light anticipated by the end of the decade. 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 exceedingly robust because the near-Earth objects will be outnumbered one to one 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 the LSST baseline observing campaign demonstrate that it 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 the LSST the best option for finding 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 project reaches the threshold where different science drivers and different agencies (NSF, DOE and NASA) can work together to efficiently achieve seemingly disjointed but deeply connected goals. The main science themes that drive the LSST system design are 

  • Dark Energy and Matter
  • Inventorying the Solar System
  • The Transient Optical Sky and 
  • Mapping the Milky Way. 

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.

In particular, the Congressional mandate can be reached at only a fraction of the cost of a mission dedicated exclusively to NEO search.

Image Credit: 

Financial support for LSST comes from the National Science Foundation (NSF) through Cooperative Agreement No. 1258333, the Department of Energy (DOE) Office of Science under Contract No. DE-AC02-76SF00515, and private funding raised by the LSST Corporation. The NSF-funded LSST Project Office for construction was established as an operating center under management of the Association of Universities for Research in Astronomy (AURA).  The DOE-funded effort to build the LSST camera is managed by the SLAC National Accelerator Laboratory (SLAC).
The National Science Foundation (NSF) is an independent federal agency created by Congress in 1950 to promote the progress of science. NSF supports basic research and people to create knowledge that transforms the future.   

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