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Near-Earth Object Human Space Flight Accessible Targets Study (NHATS)
Introduction

Observers, mission planners, and other interested users are invited to use the following tool to identify future observing opportunities for those near-Earth objects that may be well-suited to future human space flight round trip rendezvous missions. Please consider the assumptions and caveats (listed below) related to these data to assist in their proper interpretation.

NHATS Table

A table showing NHATS-compliant NEAs that can be filtered using various constraints and sorted by various fields is available at the link above.

The Near-Earth Object Human Space Flight Accessible Targets Study (NHATS) began in September 2010 under the auspices of NASA Headquarters Planetary Science Division of the Science Mission Directorate in cooperation with the Advanced Exploration Systems Division of the Human Exploration and Operations Mission Directorate. Its purpose was to identify any known NEOs, particularly Near-Earth Asteroids (NEAs) that might be accessible by future human space flight missions. The Goddard Space Flight Center (GSFC) and the Jet Propulsion Laboratory (JPL) independently performed the first phase of the NHATS study in parallel to validate the results.

NEAs are discovered almost daily, and often the time just after discovery is also the optimal time to provide follow-up observations to secure their orbits and characterize their physical nature. These follow-up observations are particularly important for those NEAs that could become potential future mission targets. Hence, it is prudent to monitor these NEA discoveries daily and run an analysis to determine if any among them warrant additional study as they might become attractive mission targets.

Brent Barbee (GSFC) has developed a process that automatically downloads orbital information on newly discovered NEAs from the JPL Small Bodies Database (SBDB) on a daily basis. He then performs trajectory calculations using the method of patched conics for the spacecraft and with full precision ephemerides for the Earth and NEOs obtained from JPL's Horizons system to determine which among them may meet the NHATS accessibility filters (see below for subscription option). The results of this daily analysis are then immediately uploaded to the NHATS table.

A process generated by Paul Chodas (JPL) then provides, for each NHATS-compliant NEA, the details of future observation opportunities that might allow the NEA orbit to be improved with follow-up optical astrometric data. Some of these observing opportunities would also allow the NEA's physical nature to be characterized using photometric and spectroscopic observations. In cases where there are future close Earth approaches, radar astrometric and physical characterization observations may be possible; these opportunities are listed as well.

Alan Chamberlin (JPL) was largely responsible for creating this NHATS website.


Assumptions and Caveats for NHATS-Compliant NEAs
  1. The list of potential mission targets should not be interpreted as a complete list of viable NEAs for an actual human exploration mission. As the NEA orbits are updated, the viable mission targets and their mission parameters will change. To select an actual target and mission scenario, additional constraints must be applied including astronaut health and safety considerations, human space flight architecture elements, their performances and readiness, the physical nature of the target NEA, and mission schedule constraints.

  2. The target NEAs in these tables were identified using a Lambert solution technique. Since no mid course maneuvers, gravity assists or continuous thrust options (e.g. solar electric propulsion) were considered, there are certain to be additional mission options that do not appear within this table.

  3. The trajectory filter parameters were purposely chosen to be highly inclusive in order to provide a broad spectrum of mission possibilities. To pass the trajectory filter and be characterized as NHATS-compliant, a NEA must offer at least one round trip trajectory solution that satisfies the following constraints:

    1. Earth departure date between 2015-01-01 and 2040-12-31
    2. Earth departure C3 less than or equal to 60 km2/sec2
    3. Total mission delta-V (dV) less than or equal to 12 km/s. The total delta-V includes the Earth departure maneuver from a 400 km altitude circular parking orbit, the maneuver to match the NEA's velocity at arrival, the maneuver to depart the NEA and, if necessary, a maneuver to control the atmospheric re-entry speed during Earth return.
    4. Total round trip mission duration less than or equal to 450 days
    5. Minimum stay time at the NEA of 8 days
    6. Earth atmospheric entry speed less than or equal to 12 km/s at an altitude of 125 km


  4. Trajectories are computed using the method of embedded trajectory grids. This provides a comprehensive analysis by stepping through, at 8-day intervals, all combinations of departure dates, outbound flight times, stay times, and inbound flight times. The trajectory calculations are performed using patched conics with Lambert solutions for the spacecraft and with full precision ephemerides for the Earth and NEAs obtained from JPL's Horizons system.

NEA Observability

  1. The observability of each NEA is analyzed by generating its geocentric ephemeris through to the year 2040.

  2. 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:

    1. The magnitude has to reach 24.0 or brighter
    2. The angular distance from the Sun (solar elongation) has to exceed 60 degrees
    3. The 3-sigma plane-of-sky uncertainty must be less than 1.5 degrees over a 3-day period
    4. The object must be at least 5 degrees away from the galactic equator
    5. 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.

  3. 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:

    1. To simulate current programs, a limiting magnitude of 21.5 and minimum solar elongation of 70 degrees are used
    2. 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.

  4. Radar tracking opportunities for Arecibo and 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 these antennas. The radar constraints are as follows:

    1. The SNR must be at least 10
    2. Either the 3-sigma plane-of-sky uncertainty must be less than 0.75 arc-min, or
    3. 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.


Subscribe to NHATS Notices

If you'd like to subscribe to the NHATS email daily notification service, visit the following link to signup.
https://lists.nasa.gov/mailman/listinfo/nhats

If you'd like to subscribe to notifications of specific NEAs that are of interest for observations, visit the following link to signup.
https://lists.nasa.gov/mailman/listinfo/small-bodies-observations

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