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

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,” whichrecommends 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

NEO Search and Mitigation Study Milestones

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(90% D>1000m)

Future Survey(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

0.001

0.01

0.1

1

10

100

1000

10000

TS=1 TS=2 TS=3 TS=4 TS=5 TS=6 TS=7 PS=0 PS=1

Minimum

Nominal

Maximum

Extreme

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

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 2012-2013 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

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 pre-experiment 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 pre-

discovery 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 systemBrought much of the water & organics to the early earth - allowing life to formTheir 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 explorationRepresent 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 sensitivity65% of all radar experiments to characterize NEAs were done at Arecibo47% of all binary NEAs were discovered as a result of Arecibo observations85% of the NEAs with radar astrometry have Arecibo data that allow long, accurate extrapolations of their motions during their first apparitions

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.

Summary Points

1800180019001950

History of Known NEO Population

ArmaghObservatory

19901999

OutsideEarth’s Orbit

EarthCrossing

ScottManley

2007Known• ~400,000 minor planets• ~4600 NEOs• ~845 PHOs

New Survey Will Likely Find• 100,000+ NEOs (> 140m)• 20,000+ PHOs

The Inner Solar System in 2006

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