Accessible NEAs

Observability

• The observability of each NEA is analyzed by generating its geocentric ephemeris through to the year 2045.

• Optical observing constraints vary widely from observatory to observatory. The constraints used for the NHATS table were purposely chosen to represent programs with access to large-aperture telescopes, in order to include even the difficult observational opportunities. The optical constraints are as follows:

  • The magnitude has to reach 24.0 or brighter
  • The angular distance from the Sun (solar elongation) has to exceed 60 degrees
  • The 3-sigma plane-of-sky uncertainty must be less than 1.5 degrees over a 3-day period
  • The object must be at least 5 degrees away from the galactic equator
  • Observations are excluded for the 4-day period around each full moon

The peak visual magnitude (Vp) during the tracking opportunity is shown as a guide to observers.

• Many asteroids with poorly-determined orbits violate the plane-of-sky uncertainty constraint soon after discovery; these objects are considered lost. A secondary filter is then applied to simulate a serendipitous re-discovery of such an object by one of two asteroid survey programs. The survey programs are simulated by removing the plane-of-sky uncertainty constraint and imposing the following survey constraints:

  • To simulate current programs, a limiting magnitude of 21.5 and minimum solar elongation of 70 degrees are used
  • To simulate the proposed LSST survey, a limiting magnitude of 24.0 is used, starting in the year 2021, and sky coordinates must be within the LSST survey region

The dates of possible survey recoveries are shown in the table with leading and trailing ‘?’ in order to indicate that these are far from certain.

• Radar tracking opportunities for Goldstone are determined by calculating the daily signal-to-noise ratio (SNR) values using the best known physical parameters for the asteroid (primarily size and rotation period), as well as the actual parameters for this antenna. The radar constraints are as follows:

  • The SNR must be at least 10
  • Either the 3-sigma plane-of-sky uncertainty must be less than 0.75 arc-min, or
  • There must be an optical observing opportunity shortly before, with magnitude brighter than 21.5 and plane-of-sky uncertainty less than 3 degrees, 3-sigma. This is meant to simulate the optical astrometry often requested to lower the pointing uncertainty and make the radar experiment possible.

The entry in the table shows the date of the peak SNR, and the estimated SNR value follows in square brackets.