OUTLINE

• The 1994 discovery (and related stories)

• The impact of this result in more modern research

A “missed” collision with the Sun

2a

a(1-e)

e<1e=1

Death in 3:1 resonance

Death in 6 resonance

Although the sample of NEOs studied in 1994 was limited, Paolo immediately understood that the collision with the Sun was the most frequent end-state of the NEO population, by far more probably than a collision with a planet or the ejection onto an hyperbolic orbit. This was going to change QUALITATIVELY and QUANTITATIVELY our view of the origin and orbital evolution of NEOs.

A little story……

H.F. Levison, reviewer of the 1994 Nature paper

Like St. Thomas, he would not believe it if he could not see it himself…….

Follow-up and modern implications of the 1994 result

Gladman et al.(1997):

70-80% of the asteroids coming from the 3:1 and 6 resonances eventually collidewith the Sun!

Because of these collisions the median lifetime of particles in these resonances is only ~2.5 My!!

Bottke et al., 2001, 2002:

Developed a NEO model by tracking particles coming out of the main resonances of the asteroid belt.

They found that to sustain a population of ~1200 NEOs with H<18, 220 asteroids have to pass through 3:1 and 6 every My! This is because most of them die quickly by colliding with the Sun!

Collisions could not inject so many asteroids in the resonances. This pushed forward the Yarkovsky idea (another great hit of Paolo)

Marchi et al. (2009):

Using the Bottke et al. model they found that many NEOsshould have passed very close to the Sun during their past dynamical history.

5% of the NEOs should have had q<0.15AU in their past life.

This is MUCH larger than what one would deduce just by looking at the current NEA orbital distribution.

Marchi et al. (2009):

Current (a,e) distribution of “hot” and “cool” asteroids.

“hot” asteroids are those which, with a probability of more than 50%, had q<0.1 AU in the past

“cool” asteroids are those which, with a probability of 90%, never had q<0.8 AU in the past.

q=0.1 AU is an important threshold because an asteroid with thisperihelion distance can exceed 1,000 K on its surface.

At this temperature, silicates can melt. Carbonates can have an explosive reaction, possibly triggering “cometary” activity (is this what happened to Phaeton when the Geminids were ejected?).

Research on spectral anomalies of “hot” asteroids is ongoing.

Interestingly, in 1994 Paolo already thought about this. Among the asteroids that he found to be likely to collide with the Sun is 3551 Verenia, a V-type (basaltic) body. Paolo proposed that, instead of coming from Vesta, Verenia could be a normal silicate asteroid which melted during a close approach to the Sun.

We are still waiting for a Sohoimage showing an asteroid falling onto the Sun

But nobody in the community now doubts that such asteroid-Sun collisions really occur

THANK YOU PAOLO!