[meteorite-list] Computing Paths to Asteroids Helps Find Future Exploration Opportunities

[Ron Baalke]

http://www.nasa.gov/content/goddard/computing-paths-to-asteroids-helps-find-future-exploration-opportunities/

Computing Paths to Asteroids Helps Find Future Exploration Opportunities
June 30, 2014

As left over building blocks of the solar system's formation, asteroids 
are of significant interest to scientists. Resources, especially water, 
embedded within asteroids could be of use to astronauts traveling through 
deep space. Likewise, asteroids could continue to be destinations for 
robotic and human missions as NASA pioneers deeper into the solar system, 
to Mars and beyond.

NASA is developing the capabilities needed for astronauts to reach Mars 
in the 2030s.

To test these new technologies, the agency is planning a mission to identify, 
capture and redirect an asteroid to a stable orbit around the moon in 
the 2020s, which astronauts will visit. NASA is studying candidate asteroids 
for the Asteroid Redirect Mission (ARM). One of the systems that helps 
to identify such an asteroid is the Near-Earth Object Human Space Flight 
Accessible Targets Study (NHATS) developed and maintained at NASA's Goddard 
Space Flight Center in Greenbelt, Maryland.

NHATS is an automated system that uses specialized computer algorithms 
to compute spacecraft trajectories for possible round-trip mission opportunities 
to visit a Near-Earth Asteroid (NEA). It is the first study to perform 
a thorough investigation of NEA accessibility for human space flight and 
the only automated accessibility monitoring system of its kind in the 
world. After two years of operation, NHATS has identified more than 1,000 
NEAs that could be destinations for future robotic or human missions, 
enabled by future technology. In the near-term, some of them could be 
potential candidates for the ARM mission.

"We didn't know what the NEA-accessibility landscape for human spaceflight 
really looked like until the NHATS was created," said Brent Barbee of 
NASA Goddard, NHATS project lead. "As of 1 July, 2014, there are now 1,217 
NEAs identified by our project that require less flight time and energy 
to visit and return from than does a Mars mission."

[Graphic]
This chart shows the human-crewed mission opportunities to NEAs that have 
been identified as of June 7, 2014. Blue and green asterisks are missions 
that require less time (horizontal axis) and energy (vertical axis) than 
a Mars mission.
Image Credit: Brent Barbee

This chart is updated every few months

Asteroids have a wide range of sizes, from about the size of a car to 
objects resembling small moons hundreds of miles across. Their gravity 
is relatively weak, making them interesting targets for exploration missions. 
Most asteroids are found in the Main Asteroid Belt between the orbits 
of Mars and Jupiter, but there is a substantial population whose orbits 
come close to Earth's. Small asteroids are much more numerous than big 
ones - astronomers estimate near-Earth space likely contains millions 
of NEAs a few yards (meters) across, nearly 16,000 NEAs between 100 and 
300 yards across, and nearly 5,000 NEAs between 300 and 1,000 yards in 
size. To be classified as a NEA, the asteroid's orbit must come within 
1.3 times the average distance of Earth's orbit about the Sun.

[Graphic]
This diagram shows various NEA orbits. The yellow dot is the Sun, the 
blue-green dot is Earth, and the thick black line is Earth's orbit. The 
thin black line is the NEA's orbit. AU is Astronomical Unit, Earth's approximate 
distance from the Sun, about 93 million miles (almost 150 million km).
Image Credit: Brent Barbee

Because their orbits take them close to Earth’s orbit, some NEAs are potential 
Earth impact threats. NASA has a program to detect NEAs, estimate their 
orbits, and assess whether they pose an impact risk. The automated Sentry 
system identifies potentially hazardous Near-Earth Objects (NEOs - "objects" 
includes comets as well as asteroids) using observations from telescopes 
at observatories around the world and in space. Sentry was designed and 
implemented, and is managed, by NASA's NEO Program Office at the Jet Propulsion 
Laboratory (JPL) in Pasadena, California.

All telescopic observations of NEOs (professional and amateur) to determine 
their position and orbit are transmitted to the Minor Planet Center (MPC), 
which is the International Astronomical Union (IAU) sanctioned global 
clearinghouse for all such observational data. Once an initial orbit is 
determined, the MPC delivers the observational data for NEOs to JPL, which 
then computes a higher precision orbit for the NEOs based on the observational 
data. The orbit data for each NEO can be accessed through JPL's Small-Body 
Database , and the JPL Horizons system provides an interface through which 
ephemeris data (position and velocity versus time) can be accessed for 
each of the NEOs.

Barbee developed the NHATS system to find easily accessible asteroid mission 
opportunities based on the JPL/Horizons data. "In a sense, the NHATS system 
complements hazard tracking," said Barbee. "The NHATS system monitors 
the opportunities offered by NEAs, while the JPL Sentry system monitors 
the hazards NEAs may pose to Earth."

Each day the NHATS system downloads the list of the known NEAs, figures 
out which ones are newly discovered and which ones have updated orbit 
data available, and then downloads the orbit data files for those NEAs 
from Horizons. The NHATS system then applies the NHATS algorithms to each 
of those NEA orbit data files to compute all the possible round-trip trajectories 
to those NEAs using a method of embedded trajectory grids that Barbee 
developed. Embedded trajectory grids are used to calculate the various 
possible spacecraft paths, or trajectories, to a target NEA based on mission 
criteria.

In order for a NEA to be identified as a potential human mission destination, 
it must meet several criteria. "The NHATS criteria were developed by a 
human exploration committee in September of 2010," said Barbee. "The idea 
was for the criteria to mean that round-trip missions to the NHATS-compliant 
NEAs would be less demanding than even the least demanding round-trip 
missions to Mars." The criteria include departure dates not too far in 
the future (no later than 2040), a reasonable amount of time at the asteroid 
to explore (at least 8 days), a round-trip flight time of 450 days or 
less, and a lower fuel requirement than a Mars mission.

[Graphic]
This diagram illustrates the parts of a conceptual human-crewed mission 
to an asteroid. The blue oval represents Earth's orbit, the green oval 
is the asteroid's orbit, and the red arcs are the spacecraft's trajectory 
to and from the asteroid.
Image Credit: Brent Barbee

Barbee maintains a mailing list to which the GSFC NHATS computer automatically 
transmits each day's processing results.

"Anyone can sign up for the mailing list, but the intent is for astronomers 
and NEO scientists to sign up so that they receive rapid notification 
when a NEA is discovered that is particularly accessible. This helps ensure 
that follow-up observations are obtained in a timely manner," said Barbee.

"I check the daily NHATS results message as soon as it arrives to see 
what 'the night's catch' brought in for newly discovered and updated NEAs," 
said Lindley Johnson, NASA's NEO Programs Executive. "The information 
is crucial because it's our first look at opportunities to observe smaller 
NEAs when they are very close to Earth. Most often we have only a few 
days after discovery to make follow-up observations, so rapid notification 
is critical. Follow-up observations are important because they allow us 
to establish the NEA's orbit around the Sun more accurately, and to learn 
about the NEO's spin state, size, and composition. All of that information 
is vital for mission planning."

NHATS began in September of 2010 but was not fully automated until March 
20, 2012. Barbee expects the project to continue indefinitely, as there 
are many more mission opportunities to be found. "At present we have discovered 
11,180 NEOs of all sizes, and we estimate that there are at least 10,000 
NEOs larger than 100 yards in size that we haven't found yet," said Barbee. 
Many will also provide opportunities for longer-duration robotic spacecraft 
missions.

An example of a long-duration robotic asteroid sample return mission is 
the Origins Spectral Interpretation Resource Identification Security -- 
Regolith Explorer (OSIRIS-REx) mission managed by NASA's Goddard Space 
Flight Center, which will investigate and return a sample from a NEA named 
Bennu. Scheduled for launch in late 2016, the spacecraft will reach Bennu 
in 2018 and return a sample to Earth in 2023.

NASA's asteroid initiative is underway to support the agency's efforts 
to understand the population of potentially hazardous NEOs and characterize 
a subset of interest, including those suitable for future asteroid exploration 
missions. The initiative brings together the best of NASA's science, technology 
and human exploration efforts to achieve President Obama's goal of sending 
humans to an asteroid by 2025.

NHATS is funded by NASA's Near-Earth Objects Observations (NEOO) program. 
NASA JPL manages the Near-Earth Object Program Office for NASA's Planetary 
Science Division, Science Mission Directorate in Washington. JPL is a 
division of the California Institute of Technology in Pasadena.

For more information about NHATS, visit:

http://neo.jpl.nasa.gov/nhats/

Bill Steigerwald
NASA Goddard Space Flight Center
william.a.steigerwald at nasa.gov