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NASA Confirms Asteroid ‘3200 Phaethon’ Will Almost Hit Earth

Tuesday, May 15, 2018 8:10
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Princeton astronomer Charles H. Young (1834-1908) used to tell his students that small asteroids were hardly more than “mountains broken loose,” compared to the solar system’s larger asteroids which are entire worlds unto themselves. This is probably an appropriate characterization of the asteroid 3200 Phaethon, which is an irregular chunk of rock about 3 miles (5 kilometers) wide — compared to the system’s largest asteroids which can have diameters more than 100 times that. 3200 Phaethon would be a quite insignificant member of the solar system were it not for the object’s remarkable orbit, which will carry it to within 6.4 million miles (10.3 million km) of Earth on May.

Since the asteroid’s discovery on Oct. 11, 1983, by NASA’s Infrared Astronomical Satellite (IRAS), this pocket space rock — catalogued as 1983 TB — has gone around the sun nearly 24 times. It follows a 523-day ellipse that takes the object deep within Mercury’s orbit during close approach to the sun and outside the orbit of Mars at furthest approach from the star. Phaethon is the third largest near-Earth asteroid classified as “potentially hazardous” by NASA, and as such, astronomers always keep a close watch on the rock’s path when it closely approaches our planet.

3200 Phaethon (1983 TB) was discovered on 1983 Oct. 11 by NASA’s Infrared Astronomical Satellite (IRAS). 
With a diameter of about 5 km, Phaethon is the third largest near-Earth asteroid classified as “Potentially Hazardous” after 53319 1999 JM8 (~7 km) and 4183 Cuno (~5.6 km).

Phaethon has an unusually high eccentricity of 0.890 and a perihelion of 0.140 au that is among the smallest known in the near-Earth asteroid population. Due to the close perihelion, Phaethon is named for the Greek mythological son of Helios (the Sun god). In Greek mythology, Phaethon  drove his father’s chariot for one day, lost control of its horses, and nearly set the Earth on fire.

Phaethon will approach within 0.069 au of Earth on 2018 May 17 when it will be a strong radar imaging target at Goldstone and Arecibo. This will be the best opportunity to date for radar observations of this asteroid and we hope to obtain detailed images with resolutions as fine as 75 m/pixel at Goldstone and 15 m/pixel at Arecibo. The images should be excellent for obtaining a detailed 3D model.

Extensive photometric observations by many observers have yielded a rotation period of 3.6 h, a lightcurve amplitude of up to ~0.4 mag, and a pole direction of (lambda, beta) = (85+- 13, -20+-10) deg (Ansdell et al. 2014).
Ansdell et al. also obtained axis ratios of x/y = 1.04 and x/z = 1.14, so Phaethon appears relatively unelongated
along its equator but somewhat flattened at its poles.

Spectroscopic observations by several observing teams strongly suggest that Phaethon is an optically-dark B-class 
object. However, thermal infrared observations dating back to the 1990s by Harris et al. give an optical albedo of 0.11 that seems somewhat bright for the B class. More recent observations by NASA’s NEOWISE mission have detected Phaethon and could provide an update on the diameter and albedo.

Phaethon was detected by radar at Arecibo in December 2007. Due to equipment problems, the 2007 data 
consist of a modest number of echo power spectra and a handful of delay-Doppler images from two days.
Echo power spectra obtained at Arecibo in 2007 were strong and provided a maximum bandwidth of 44 Hz.
Combined with the rotation period of 3.6 h, the bandwidth places a lower bound on the maximum pole-on dimension
of about 5.7 km. The 2007 delay-Doppler images show a rounded object, which is consistent with the shape of the echo power spectra, but were relatively weak and do not show detailed surface features. 
The 2007 Arecibo observations have a radar cross section of about 2.4 km^2, that, if we adopt a diameter of
5.1 km, imply a radar albedo of 0.12. As such, the SNRs in the tables below could be too low by about 20%.

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Phaethon is widely thought to be the parent body for the Geminids meteor stream due to similarities between
its orbit and that of the meteors (Whipple 1983; Williams and Wu 1993). Most meteor streams are associated with comets, so this raises the question of whether Phaethon could be an inactive comet nucleus.

Observations by Jewitt and colleagues have revealed episodic activity by Phaethon, but have not, to date,
shown the signature of particles as large as those in the Geminids meteors. The implication is that Phaethon
experiences occasional outbursts that produce particles far larger than any seen so far. As such, Phaethon is an unusual  object that appears asteroidal most of the time but occasionally shows low levels of activity when it is
near perihleion. Phaethon is classified as an asteroid, not as a comet, and despite the high eccentricity, the object’s Tisserand parameter is not cometary. 
The 2018 apparition is the closest to Earth since the asteroid’s discovery so it may be possible for optical observers to detect new activity. If Phaethon shows unexpectedly strong activity in 2018, then there is a small (perhaps very small) chance that CW radar observations might reveal echoes from a cloud of small particles similar to the “skirts” seen in radar observations of active comet nuclei.

The very low perihelion of Phaethon makes it a possible candidate for detecting general relativistic and/or solar oblateness effects in its orbital motion (Margot and Giorgini 2010), so one of our principal objectives is to obtain high-resolution radar ranging measurements to support this effort.

Phaethon will be brighter than 17th magnitude for about one month from May – June 2018. Phaethon is predicted to reach 11th magnitude in mid-May when it will be visible in small telescopes  for experienced observers at sites with dark skies.

Phaethon is potentially detectable at Goldstone for about three weeks and tracks are scheduled on ten days between May17-27. 
Due to its relatively rapid motion in declination, Phaethon is visible at Arecibo on only five days  from May. 17-22 and observations are scheduled on all of those dates.  We may also request time at Green Bank to receive Goldstone transmissions.

The 2018 encounter is the closest by this asteroid since 1974 and until 2093.Phaethon is classified as a “Potentially Hazardous Asteroid” by the Minor Planet Center.

But “down at the 140-meter size, which is our next threshold of interest, we have yet to find even half of them,” Chodas said. “And the percentage of the [asteroid] population we’ve found goes down exponentially as we go to smaller and smaller sizes, down to 100 meters, 80 meters, 50 meters, etc.”

These smaller meteors can cause significant damage. The 20-meter-wide asteroid that slammed into Chelyabinsk, Russia in 2013, for example, shattered windows and injured more than 1,000 people.

And bad as that was, it’s nothing compared to the damage that would be caused by the impact of an asteroid the size of 3200 Phaethon

If a space rock that big were to smash into Earth, the online Impact Earth asteroid damage calculator indicates that — depending upon factors including its speed and angle of impact — it could create a crater 65 kilometers (40 miles) wide and 1 kilometer (0.6 mile) deep.

Orbital and Physical Characteristics

Name Phaethon
Number 3200
Discoverer IRAS
Discovery date 1983 Oct 11
orbit type Apollo

Close approach date 2018 May 17
Close approach dist. 0.0689 au 
Close approach dist. 26.8 lunar distances

semimajor axis 1.271 au
eccentricity 0.890 
inclination 22.2 deg 
orbital period 1.433 y 
perihelion distance 0.140 au 
aphelion distance 2.402 au 
MOID 0.0206 au
Tisserand parameter 4.510 (asteroidal)

absolute magnitude (H) 14.6 
diameter 5.1 km
optical albedo 0.11
spectral class F, B 
rotation period 3.603 h
lightcurve amplitude 0.11 – 0.44 mag 
pole direction lambda = 85 deg, beta = -20 deg

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