Tomorrow, Wednesday, January 13, 2010, a small asteroid called 2010 AL 30 (presumed diameter 10-15 meters) will buzz the Earth at just 1/3 of the lunar distance. The point of closest approach will be passed around 13:00 UTC (14:00 CET). +++ Please also note the update at the bottom of this post. +++
This announcement in the Minor Planet Electronic Circular lists a set of observations that were used for orbit determination. They start on January 10, 2010 - that's how short a warning we get with these little fellas.
At first sight the asteroid's orbital period appears remarkable: It's almost exactly one year, just a bit more. The orbit is inclined by around 4 degrees with respect to the ecliptic, and it's quite eccentric. The aphelion is larger than 1.3 AU, the perihelion less than 0.7 AU.
The orbital period led to speculation that the object might be man-made, in analogy to the object J002E3, which probably is a spent Saturn V stage from the Apollo missions that barely managed to escape into a heliocentric orbit and then was temporarily re-captured into an orbit around the Earth in 2002.
For various reasons, this scenario does not apply here. The 1-year orbital period probably is no more than a fluke.
Where does this thing come from?
Probably 2010 AL30 is of natural origin. However, the possibility that it is man-made cannot be completely excluded. If so, it might be the upper stage of a rocket used in an earlier planetary mission, possible to Venus. The current orbit would have been acquired through a Venus swingby and other orbital perturbations.
The perihelion radius is somewhat suspicious. It's almost identical with the orbital radius of Venus, as this diagram shows. I produced this based on the orbit determination results given in the MPEC for Jan. 4, 2010. I numerically propagated the given orbital state backwards in time, taking into account the gravitational attraction of all planets and the Sun. As can be seen, the asteroid orbit intersects the Venus orbit in two locations close to its perihelion (well, almost, because there also is a difference in inclination).
In such a scenario, it's very likely that at some point in time the object must get close to Venus and undergo some more or less significant orbital perturbations. My numerical simulation also showed that this was the case. But then, of course, the initial orbit determination uncertainties will tend to multiply in the propagation, especially if there are close planetary encounters. This is rocket science .... but still, I have to live with the inaccuracies in the data available.
When plotting the obtained distances from Earth and Venus over time, as in the diagram to the left (Note: logarithmic y-scale), you can see that there was a close Earth encounter almost exactly 1 year ago, on January 12, 2009. This makes sense: If the period is 1 year, and we have an encounter on January 13, 2010, then logically there must have been one also one year earlier. There are no Venus encounters or anything else that might have changed the orbit in 2009.
Then, it looks as if there was a close Venus encounter in spring 2006, preceded by an Earth encounter in late 2005. Again applying all due caution and pointing out the uncertainties, these results do not appear to be altogether inconsistent with the conjecture that we have here the Fregat upper stage of the Soyuz launch vehicle that launched the ESA spacecraft Venus Express on November 9, 2005, with Venus arrival on April 11, 2006.
There are indications that the object may be rotating quite fast, which also would not be surprising in the case of a spent upper stage. If this is in fact a Fregat, then it is clad in reflective metallized foil for thermal reasons, so its albedo would be a lot higher than that of a natural body. Despite the Fregat's smaller size, distinctly less than the 10-15 meters tentatively assumed for this body, the reflective material would make it appear bright.
I assume that the hypothesis I am putting on the table here will be easy to disprove or to confirm once we have more observations and also some spectroscopy and radar, which hopefully will be the case by tomorrow.
An Afterthought: Coincidences Galore ....
I just couldn't let it rest, so I decided to approach the problem from a different tack. Let's assume that 2010 AL30 is the Fregat upper stage from the Soyuz launcher used for Venus Express. Then I need to find out whether the gravitational perturbation of the stage during its close encounter with Venus could have led to the orbital parameters we see now. The answer to this analysis was clearer than I had expected.
The hyperbolic arrival velocity at Venus amounted to around 4.6 km/s, the date and time and the direction are known. I used this data to compute the post-encounter heliocentric orbital elements for a large set of sample points in the B-plane. The B-plane is a commonly used tool for mission analysis, it constitutes an imaginary plane that is centered in the target planet (here Venus) and is perpendicular to the hyperbolic arrival velocity vector.
The results are plotted in the attached diagrams. The white circle in the middle is the impact zone. If you try to target a flyby within this zone, the pericentre radius is too small, so the craft (a space probe or a spent stage) would enter the atmosphere and burn up.
The first diagram shows that a post-encounter orbit with a semi-major axis of 1 AU (and therefore a period of 1 year) is easy to reach. The B-plane target parameters must then be on the dotted white line.
The second diagram shows the achieved post-encounter perihelion. No problem with around 0.7 AU.
Finally, the aphelion radius, shown in the third diagram. The orbit of object 2010 AL30 has an aphelion of just over 1.3 AU. No problem there, as the dotted white line shows. And here's what's strange: For the B-plane target area delimited by the little red circle in all three plots, you get exactly the combination of all three: a period of 1 year, a perihelion of just under 0.7 AU and an aphelion of 1.3 AU.
And there is more: This marked zone happens to be exactly the zone that you would target at if a Venus orbit insertion over the planet's north pole is wanted (It appears to be on the bottom side in the graphs, but that is just due to the way the B-plane axes are defined. Anyway, astronomers are used to seeing things represented upside down). Incidentally: Venus Express was targeted for orbit insertion over the north pole.
So, let's summarize briefly:
Numerical backwards propagation of the orbital state of object 2010 AL30, as determined in January 2010, turns out to appear to lead to a close Venus encounter in spring 2006 and an Earth encounter in late 2005. These epochs are consistent with the Venus arrival and Earth launch dates, respectively, of ESA's Venus Express probe.
From the known approach conditions of Venus Express to Venus one can derive that a target point close above the planet's north pole, as was chosen in that mission, would lead to a deflection of an inert body on the same trajectory, such as the spent Fregat stage, that has a period of 1 year with perihelion and aphelion distances of 0.7 and 1.3 AU, respectively. These orbital parameters closely coincide with those of object 2010 AL30.
Obviously this does not constitute proof. But I would say that we have a surprisingly long chain of unusual coincidences here. Wouldn't you agree?
It is now Thursday, January 14, 2010. Object 2010 AL30 has come and gone. Improved orbit determination results are now available. The backwards propagation I performed on January 12 was based on the "old" set of orbital data. In fact, though there is some change in the orbital parameters, this change is a lot smaller than I expected, in view of the relatively short arc spanned by optical observations.
As you can see from the description of part of my analysis above, where I propagated the orbit backwards from the date for which the January 12, 2010 orbit determination was provided, I obtained a Venus encounter in early spring 2006. Date and distance were not entirely consistent with the Venus Express case, but close enough to warrant a closer look, and I expected that the uncertainties in the orbit determinaton, together with a magnification effect of the 2009 Earth encounter, would lead to some change here, moving the encounter date to April 11 and reducing the encounter distance.
However, as the post-encounter orbit determination does not differ significantly from the pre-encounter ones, there is no alternative but to conclude that 2010 AL30 in fact did pass no closer than 0.011 AU to Venus in 2006 and that this happened on February 25, some 6 weeks prior to the Venus Express arrival (see the updated distance plot). This is not consistent with the conjecture that 2010 AL30 might be the Fregat upper stage.
What does this mean? As stated earlier, probably 2010 AL30 is of natural origin. This hasn't changed. Even though the initial conjecture that the timeline of planetary encounters of object 2010 AL30 might indicate that it is the upper stage of the rocket that launched the Venus Express spacecraft did not survive the ongoing process of orbit determination - well, that's the way science works, in the end it's the data that counts - my other set of calculations also holds.
Anyone with a computer and some experience with celestial mechanics can check my results: if you send a spacecraft to a planet, it is not unlikely that the expended upper stage will pass close to the target planet and be deflected into an orbit that differs significantly from the original one. There is no reason to assume that man-made objects, even uncontrolled ones such as spent rocket stages, can only encounter the Earth with a relative velocity and direction that are akin to those it was launched to.
And to clarify another piont that was frequently made in reference to the 2010 AL30 issue: There is considerable confusion related to the significance of the fact that the orbital period amounts to almost exactly 1 year. This point is only of relevance if the object in question escaped the Earth at a very low velocity and remained in an orbit very similar to that of the Earth (either a "leading" or a "trailing" orbit.). On such an orbit there is no chance of meeting another massive body, so the object's orbit around the sun will not change significantly, and there is a high probability it will come back to the Earth, again at low relative velocity. But for rocket stages from interplanetary missions, this does not apply. These very likely did enconter a massive body along the way and one should not assume that their orbit must have remained essentially unchanged or that their period is close to one year.
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