arXiv:1808.00609 - The excitation of a primordial cold asteroid belt as an outcome of the planetary instability

Paper: The excitation of a primordial cold asteroid belt as an outcome of the planetary instability
Authors: Rogerio Deienno, Andre Izidoro, Alessandro Morbidelli, Rodney S. Gomes, David Nesvorny, Sean N. Raymond

Abstract: The main asteroid belt (MB) is low in mass but dynamically excited. Here we propose a new mechanism to excite the MB during the giant planet ('Nice model') instability, which is expected to have featured repeated close encounters between Jupiter and one or more ice giants ('Jumping Jupiter' -- JJ). We show that, when Jupiter temporarily reaches a high enough level of excitation, both in eccentricity and inclination it induces strong forced vectors of eccentricity and inclination across the MB region. Because during the JJ instability Jupiter's orbit `jumps' around, the forced vectors keep changing both in magnitude and phase throughout the whole MB region. The entire cold primordial MB is thus excited as a natural outcome of the JJ instability. The level of such an excitation, however, is typically larger than the current orbital excitation observed in the MB. We show that the subsequent evolution of the Solar System is capable of reshaping the resultant over-excited MB to its present day orbital state, and that a strong mass depletion (∼90%) is associated to the JJ instability phase and its subsequent evolution throughout the age of the Solar System

My Comment: Solar System dynamics are what drew me into astronomy in the first place. More than any results of this paper, I find it absolutely stellar as it clearly explains what is being simulated, and why, and points the reader at additional sources of information on the particulars. In sense it manages to not only present the science that was done, but also how that science was accomplished.

My Scrawling Notes:

arXiv:1807.08322 -- Pre-airburst Orbital Evolution of Earth's Impactor 2018 LA: An Update

Paper: Pre-airburst Orbital Evolution of Earth's Impactor 2018 LA: An Update
Authors:C. de la Fuente Marcos, R. de la Fuente Marcos 

Abstract: Apollo meteoroid 2018 LA has become only the third natural object ever to be discovered prior to causing a meteor airburst and just the second one to have its meteorites recovered (at Botswana's Central Kalahari Game Reserve). Here, we use the latest orbit determination of 2018 LA (solution date 18-July-2018) to search for minor bodies moving in paths comparable to that of 2018 LA using the D-criteria, which are metrics to study orbit similarity, and N-body simulations. Our results further confirm the existence of a dynamical grouping of asteroids that might be related to 2018 LA and show that the impactor could be a recent fragment spawned by a larger object, the 550-m wide, potentially hazardous asteroid (454100) 2013 BO73. Spectroscopic observations of 454100 during its next flyby with our planet (brightest at an apparent visual magnitude of 18.4 on 2018 mid-November) may confirm or deny a putative similar chemical composition to that of the recovered meteorites of 2018 LA.

My Comment: A second very short letter (busy week this), but it does something very remarkable, very hard, and very needed -- attempting to link the space rocks on the ground to the rocks in space. For the most part all we can do is try to match spectra of meteorite samples in the lab to the reflectance spectra of asteroids in space. It is filled with many pitfalls. This is really neat as it gives a second diagnostic tool to work with - the dynamics of the impactor's orbit.

My Scrawling Notes:

arXiv:1807.08728 -- P/2017 S5: Another Active Asteroid Associated with the Theobalda Family

Paper: P/2017 S5: Another Active Asteroid Associated with the Theobalda Family
Authors: Bojan Novakovic

Abstract: In this note we have shown that a newly discovered comet P/2017 S5 (ATLAS), that moves around the Sun in an asteroid-like orbit, is a member of the Theobalda asteroid family.

My Comment: Super short letter, but packed full of references giving a clear "how you do this science" outline. Love it. One of my favorite experiences with an undergrad research student was when she serendipitously found a candidate active asteroid while working with me on a comet project. Turns out it (most likely) wasn't but working out that puzzle involved her getting time on a 3.5m telescope(!), and having her work praised by Jocelyn Bell Burnell(!!).

My Scrawling Notes:

Killer rocks from space!

I had an excellent topic raised today at the Adler Planetarium's Space Visualization Lab about everyone's favorite killer asteroid, 99942 Apophis (the asteroid formerly known as 2004 MN₄). Indeed there is a small chance of Apophis having a rather intimate encounter with our home planet on April 13, 2036.  By slim, I really mean slim.  As of earlier this year the "odds" of Apophis hitting us were being cited at about 1:250,000.  For perspective, that's about the same odds as rolling all 6's on 7 playing dice.

It all comes down to how Apophis passes the Earth on it’s next close passage (no real chance of impact associated with this one!) in 2029.  If Apophis misses the Earth just right it could pass through an exceedingly narrow gravitation keyhole, which would alter the orbit of the asteroid just enough to make a possible impact in 2036.  That said, it isn’t anything to get worked up about, the odds of the asteroid missing us are probably even better than what are being cited due to how incredibly hard it is to pin down the exact positions of the Earth and Apophis with enough accuracy to really predict the impact.

Now how can that be?  I mean it is just gravity after all.  It is true that just about any second year physics major in the country could take on the problem of the Sun and Apophis, and solve the system such that they could predict at all times where it would be in its orbit.  It is also true that adding just a single additional body to that system (say Jupiter, or Earth) makes it such that no single person can exactly solve the resulting system!  This “n-body” problem is at the heart of the very complex calculations needed to predict if an impact can take place.  Since it can’t be worked out analytically, the orbits of asteroids in the solar system must be solved numerically, through simulating the orbits.  Work out all the forces involved, let the solar system move for a very short period of time, work out the changed forces, evolve the system some more, and on and on.  The resulting solution is only as good as the computer and code that is trying to solve it!

What type of accuracy is needed?  A whole lot.  It takes just 7 minutes for the Earth to travel a distance equal to its diameter.  Every 7 minutes that go by Earth is in a completely new portion of space.  One needs to find out if the asteroid shares that space within that 7 minute window.  To complicate matters even more, to get that level of precision, much more than just point-particle Newtonian gravity must be taken into account.  For example, the Sun isn’t a perfect sphere, and is slowly losing mass.  The masses of the Planets and other asteroids are only known to a finite amount, and our own Milky Way Galaxy exerts a tidal force on the solar system.  Largest of all the uncertainties is the way in which the asteroid itself absorbs and reradiates sunlight (the Yarkovsky effect).  Depending on the thermal make-up of the asteroid and how it is spinning this force can significantly alter its orbit, even of fairly short time scales.

In short, it is really hard to nail down exactly where an asteroid will be at a particular time, which creates enough uncertainty to be unable to rule out an impact for objects like Apophis.  As time goes on (and the projected time errors grow smaller) I fully expect the impact chance to continue to drop, although you’ll never see that on cover of the New York Times, as “Asteroid to miss the Earth – everything is fine” probably won’t move many copies.

Worse case, Apophis is only about 270 meters across, enough to cause some significant damage, but not any sort of Doomsday Scenario.

This figure by J. Giorgini (JPL) shows the results of evolving the orbit of Apophis under dynamical conditions, and assumed properties of the asteroid.  The "Nominal" solution shown in red is the "most probable" position of the asteroid on April 13, 2036 - about 0.3 AU from the Earth.  The blue shows the 3-sigma range of the position uncertainty which does encounter the Earth, indicating the simulation produces a small possibility of impact on this date.  The unknown physical properties of the asteroid introduce even more uncertainty, sliding the whole line of probabilities along the asteroid's orbit.  For more details on this dynamical model check out JPL's Apophis Webpage.