Transcript Slide 1

Summary of Issues Relating to
the Orbits of Near-Earth Objects
Don Yeomans and
Jon Giorgini (NASA
NEO Program
Office)
September 2007
Potentially hazardous asteroid Itokawa
Terminology
• “Near Earth Objects (NEOs)”- any small body
(comet or asteroid) passing within ~ 45 million km
of Earth’s orbit
• “Potentially Hazardous Asteroids (PHAs)” – an
asteroid that has a potential risk of impacting the
Earth at some point in the near future. (Very few
comets get close to Earth)
– Asteroids passing within ~ 7.5 million km of
Earth’s orbit (about 20 times the distance to the
Moon)
– PHAs about 20% of all discovered near-Earth
asteroids
NEO Search and Mitigation Study Milestones
1992 - House Com. on Science requests a NEO Detection Workshop (NASA) & a NEO
Interception Workshop (DOE).
•
NASA recommends six 2.5 m telescopes with limiting magnitude = 22 to
enable the discovery of 90% of NEOs larger than 1 km within 25 yrs.
•
NEO Interception Workshop (Los Alamos, NM) summary report issued
noting that technically credible approaches exist to prevent most impact
disasters (e.g., nuclear stand off blasts favored)
1995 – NASA sponsored “Shoemaker Report,” which
recommends the discovery of 90% of NEOs (D > 1 km)
within 15 years.
1998 - NASA HQ states goal to the House Subcom. on
Space & Aeronautics (i.e., by 2008, find & track 90% of
NEOs whose D > 1 km).
2003 - NASA NEO Science Definition Team report
recommends extending search down to D~140 m
Currently, the NASA goal is to discover 90% of the near-Earth asteroids,
larger than 1 kilometer, by the end of 2008.
In Dec. 2005, Congress asked that NASA plan, develop, and implement a
Near-Earth Object Survey Program to detect, track, catalogue, and
characterize the physical characteristics of near-Earth objects equal to, or
greater than, 140 meters in diameter in order to assess the threat of such
near-Earth objects to the Earth. It shall be the goal of the Survey program to
achieve 90% completion of its near-Earth object catalogue (by the end of
2020).
The Coming Tidal Wave of PHA Discoveries
• NASA’s report (3/2007) to Congress outlined several
search techniques (optical & space-based IR) that could
carry out the next generation of search. Assuming only
LSST and PanSTARRS are operational, by 2020 the
coming surveys will generate:
 ~50 times the current data flow
 ~17,000 PHA discoveries
D>140 m (83% complete)
 ~80,000 PHA discoveries
D>50 m (~40% complete)
 ≥10 times the current rate
for Earth impactor warnings
Warning Forecast for SG & SDT
Surveys (after S. Chesley)
Current Survey
Future Survey
(90% D>1000m)
(90% D>140m)
TS=1
Several dozen
Hundreds
TS=2
Several
A few dozen
TS=3
Perhaps one
Several dozen
TS=4
Perhaps one
Several
TS=5
~5% chance
Perhaps one
TS=6
~1% chance
~1% chance
TS=7
~1% chance
~1% chance
PS=0
Perhaps one
Several
PS=1
Perhaps one
Probably one
10000
1000
100
Minimum
Nominal
Maximum
Extreme
10
1
0.1
0.01
0.001
TS=1
TS=2
TS=3
TS=4
TS=5
TS=6
TS=7
PS=0
PS=1
The Benefits of Radar Data for
NEO Trajectory Predictions
• Radar astrometric data are complementary to plane-of-sky optical
observations and far more precise.
• Radar data increase the average interval of predictability (relative to
optical only orbit solutions) from 80 to 370 years (factor of 4.6), shrinks
the next apparition plane-of-sky position uncertainty by an average
factor of ~300 and typically provides statistically significant warnings of
impact during the discovery apparition – instead of the 2nd apparition
usually required by optical-only orbits.
• Radar data can quickly knock down impact false alarms and provide
checks on optical data biases and dynamic models.
• While radar cannot observe all PHAs at each apparition, it is most useful
for the most worrisome objects – those recently discovered PHAs with
relatively short warning times.
99942 Apophis (2004 MN4)
The Poster Child for Potentially Hazardous
Asteroids
• Discovered at Kitt Peak by
Tucker, Tholen & Bernardi
on June 19, 2004
• Lost due to poor observing
circumstances and
astrometric difficulties
• Recovered Dec. 18 by G.
Garradd at Siding Spring
• Estimated diameter
– 270±60 m (pv = 0.33,
Delbo et al. 2007)
2029 Impact Threat
• For a brief period (Dec. 23-27, 2004) Apophis indicated a
substantial probability for a 2029 impact
– IP as high as 2.7% on Dec. 27, 2004 (“Roulette odds”)
– Unprecedented Torino Scale = 4, Palermo Scale = +1
Apophis
270 meter-sized, near-Earth asteroid
Apophis discovered in June 2004.
Optical and radar data ruled out an
Earth or moon impact on April 13,
2029 but it will pass about 5 Earth
radii above Earth’s surface with the
perturbed motion allowing a remote
(i.e., 1/45,000) chance of an Earth
impact in 2036
Continuing astronomical observations (radar + optical) through 20122013 will almost certainly rule out an Earth encounter in 2036.
However, on April 13, 2029, asteroid Apophis will briefly become a
naked eye object (~3.5 mag) as it passes near Earth traveling through
the constellation of Cancer at some 42 degrees per hour.
Apophis:
• Good fit prior to radar
• … but Arecibo acquired target
4.5σ (Doppler) and 2.8 σ (delay)
off predictions -- very unusual.
• Delay-Doppler in fit then showed
March 2004 Spacewatch data to be
biased 1.4-arcsec; incompatible
with radar.
• Delay-Doppler corrected solution
moved 2029 encounter much closer
to Earth - where the preexperiment solution was 99.9%
certain it wouldn’t go.
Radar Observations of Apophis
• Radar observations in 2005 & 2006, when combined with optical data,
reduced 2029 uncertainties in Apophis’ position by 98%.
 Radar data allowed identification of optical biases (1.4”) in 2004 prediscovery data and the refined orbit (optical+ radar) moved approach 4.4
Earth radii closer to Earth. It is now at 4.96 ± 0.08 Earth radii above
Earth’s surface on Apr. 13, 2029.
 2029 close Earth approach will multiply existing position uncertainties
thereafter with the most uncertain effect (up to ~4700 Earth radii in 2036)
being due to unmodeled thermal re-radiation (Yarkovsky effect) and light
pressure. Radar observations in 2013 could constrain these effects by
helping to measure Apophis’ spin vector, shape and surface
characteristics.
 Radar and optical data in 2011-2013 will most likely remove slight
possibility of an Earth impact in 2036 (> 95% likely).
 Similar observations of PHA 1950 DA in 2032 will likely rule out an Earth
impact possibility in 2880.
– Again, radar is most helpful for most worrisome objects.
Summary Points
Near-Earth asteroids are critically important members of the Earth's
neighborhood. They:
Are key to understanding the origin of the solar system
Brought much of the water & organics to the early earth - allowing life to form
Their Earth collisions punctuated evolution - allowing only the most adaptable
species to evolve further (i.e., us)
Could be valuable raw materials (water, metals, minerals) for future interplanetary
exploration and stepping stones for Mars exploration
Represent a threat - one that we can do something about
Radar investigations of many NEAs are roughly equivalent, in their science content,
to space flyby missions.
Although Goldstone and Arecibo are complementary systems, Arecibo observations
have been especially important due to their superior sensitivity
65% of all radar experiments to characterize NEAs were done at Arecibo
47% of all binary NEAs were discovered as a result of Arecibo observations
85% of the NEAs with radar astrometry have Arecibo data that allow long, accurate
extrapolations of their motions during their first apparitions
Summary Points
Arecibo was instrumental in clarifying the Earth close approach of Apophis
in 2029 and it will be critical to get Arecibo observations of this object in
2013 to understand its surface characteristics and rotation state to define
the Yarkovsky effect and allow a definitive statement as to whether or not
this object will strike the Earth in 2036.
Likewise, future Arecibo observations will likely rule out the possibility of an
Earth collision of asteroid 1950 DA in 2880.
In the unlikely event that an Earth threatening object is discovered, Arecibo
radar observations could facilitate mitigation by defining, or constraining, its
size, shape, mass, spin state, composition and orbit.
Arecibo observations are critical for eliminating the threats of the most
worrisome potentially hazardous objects.
History of Known NEO
Population
1990
1950
1900
1800
1999
2007
Earth
Crossing
Outside
Earth’s
Orbit
The Inner Solar System in 2006
Known
• ~400,000
minor planets
• ~4600 NEOs
• ~845 PHOs
New Survey Will
Likely Find
• 100,000+
NEOs
(> 140m)
• 20,000+ PHOs
Scott
Manley
Armagh
Observatory
Inner …
Outer …
Tectonics, weathering & sedimentation erase craters on Earth, BUT
• 195 confirmed impact craters (2006)
• 512 other probable impact craters
• Two adequately sized & powered RADAR transmit systems
exist:
• Goldstone (DSS-14): 70 m, 430 kw (X-band)
• Arecibo: 305 meter, 900 kw (S-band)
• Other receivers: Greenbank, VLA, DSS-13, + others
• Historically, < 5% of system time available for asteroids