Starship Asterisk*

http://www.planetary.org/blog/article/00002303/ wrote:
2010 AL30: Watch out for low-flying asteroids
By Emily Lakdawalla Jan. 12, 2010 | 10:52 PST | 18:52 UTC
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<<In less than 24 hours, a newly discovered asteroid known as 2010 AL30 will be zipping past Earth at an altitude of approximately a third the Earth-Moon distance. There's no chance it'll hit us, but it's generating a lot of excitement in the community of amateur and professional near-Earth asteroid observers. People around the world are pointing telescopes at it, small and large. Even that grandest of telescopes, the Arecibo radio telescope in Puerto Rico, will be pointing at it and could determine its shape and rotation rate. There's active discussion going on through the Minor Planets Mailing List. A good place to watch for images of the asteroid is spaceweather.com.

What do we know about 2010 AL30 so far? Its orbital period is about exactly one Earth year -- common enough for Earth-crossing asteroids -- and it's relatively bright, suggesting it is a decently large space rock of approximately 10 meters in diameter. At this size, if 2010 AL30 were on an Earth impact trajectory, it would likely explode in the atmosphere, and rain rocks onto the ground, but it wouldn't create a crater. It'd boom, but wouldn't cause an air blast like the one at Tunguska. So it'd be unpleasant to be underneath it, but fatalities would have to be caused by direct hits, unlike for a big blast or a crater, where an expanding shock wave would harm everything within a certain radius. I'd warn people to stay indoors -- and then run outside to find fragments when it's over! (I based the preceding discussion on information output from the Earth Impact Effects simulator at the University of Arizona.)

Path of near-Earth asteroid 2010 AL30

This chart shows the predicted trajectory of near-Earth asteroid 2010 AL30 from January 12 at 00:00 to January 14 January at 06:00 UTC. Tick marks cross the trajectory at each hour. The tick marks bunch up on approach and departure because the asteroid approaches Earth almost directly opposite the Sun, and departs it almost directly toward the Sun. The asteroid is predicted to pass within a third of the Earth-Moon distance. Credit: Created with Bill Gray's Guide 8 software by Gerhard Dangl

I thought that this map of the predicted position of 2010 AL30 in the sky was interesting. Tick-marks are on the map at one-hour intervals. See how the tick marks are all bunched up, then spread out wide, then are all bunched up again? That's because the asteroid is coming at us from the direction almost exactly opposite to the Sun. One consequence of this approach direction is that it's actually pretty hard to refine the orbit; very tiny inaccuracies in the measured position of 2010 AL30 are magnified into large uncertainties in its eventual trajectory. This is not a serious problem for 2010 AL30 observations because what we do know about its orbit is enough to tell us that it's not going to hit us. But if this were a real potential Earth impactor, those uncertainties would translate into similar uncertainties about where exactly on the ground the asteroid would hit.>>

http://spaceweather.com/ wrote:
<<ASTEROID FLYBY: Mystery object 2010 AL30 is flying past Earth today only 1/3rd the distance to the Moon, and telescopes around the world are watching. In Colombia, amateur astronomer Alberto Quijano Vodniza used a 14-inch Meade LX200 to record the close approach:

• 
  "2010 AL30 is the faint streak moving among the stars," says Vodniza.
  "The full-length animation reveals a second much brighter object.
  That's a satellite that happened to be passing by at the same time."

Discovered barely three days ago, 2010 AL30 is catalogued as a 10m-class asteroid. Curiously, however, its elliptical orbit has a period of almost exactly one year, the same as Earth. NASA's Goldstone radar in the Mojave desert was scheduled to ping 2010 AL30 between 2:20 and 4:40 UTC on Jan. 13th. The echoes should reveal the nature of this interesting passerby. Stay tuned for updates.>>

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 	Near-Earth Asteroids  	

	Potentially Hazardous Asteroids (PHAs) are space rocks larger than approximately 100m that can come closer to Earth than 0.05 AU. None of the known PHAs is on a collision course with our planet, although astronomers are finding new ones all the time. On January 13, 2010 there were 1092 potentially hazardous asteroids.

Jan. 2010 Earth-asteroid encounters:

Asteroid 	Date(UT)  Miss Distance Mag.    Size

2010 AL2 	 Jan. 11       11.5 LD	20	  23 m
24761 Ahau   Jan. 11       70.8 LD   16	1400 m
2000 YH66 	Jan. 12       69.5 LD   17	1100 m
2010 AL30 	Jan. 13        0.3 LD	14	  18 m

Notes: LD means "Lunar Distance."
AU. MAG is the visual magnitude of the asteroid on the date of closest approach.

http://www.planetary.org/blog/article/00002304/ wrote:
Goldstone detects "STRONG" radar echoes from 2010 AL30
By Emily Lakdawalla Jan. 12, 2010 | 21:05 PST | Jan. 13 05:05 UTC
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<<Radio scientist Lance Benner posted to the Minor Planets Mailing list this evening the following message:

We have detected STRONG radar echoes from 2010 AL30 at Goldstone. The bandwidth is consistent with the object's expected size and the ~9 minute rotation period found by Bill Ryan and Richard Miles. We hope to obtain a precise size estimate soon.

The Doppler correction came in at about the 1-sigma level relative to our a priori estimate. We're updating the ephemeris now. Thank you again for all your help!

Nine-minute rotation? That is fast!! The observations of 2010 AL30 from Goldstone took place from 2:20 to 4:40 UTC on January 13.

When Benner says "thank you for all your help," his message is addressed to amateur astronomers around the world who pointed their telescopes at 2010 AL30 upon its approach, gathering information that helped to refine its orbit. In an earlier message to the group, Benner said "The pointing uncertainties have shrunk from about 523 arcseconds to about 11 arcsec at the start of tonight's track. Without all the astrometry you have reported, our observations would not be possible."

Although no backyard 'scope can equal the capability of Goldstone to measure the shape, size, and physical properties of an asteroid, Goldstone depends upon accurate positional information for its pointing. I'm sure they could have detected 2010 AL30 with less precise information; but to point in exactly the right direction results in much stronger echoes from the asteroid, improving the quality of their data by leaps and bounds. And being able to point so precisely might have been what allowed Benner to get time on such an important instrument on such short notice -- I'm not sure.

That's my favorite aspect of the study of near-Earth objects: the way that the science benefits from the participation of people at extremes of the observing spectrum, from the serious but amateur backyard astronomer to the scientists who run the world's largest telescopes.>>

http://www.planetary.org/blog/article/00002305/ wrote:
ESA mission analyst suggests 2010 AL30 might be Venus Express rocket
Jan. 13, 2010 | By Emily Lakdawalla
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<<Well, now, this is interesting. 2010 AL30 zipped past us harmlessly about five hours ago. Because of its one-year orbital period, many people speculated it might be a manmade object, but as I posted last night, NASA issued a release discounting this possibility. Overnight, though, an ESA mission analyst named Michael Khan performed some analysis (that's his job, I guess!) that have led him to suggest that 2010 AL30 might, in fact, be artificial -- specifically, there's a distinct possibility it could be the Fregat upper stage of the rocket that launched Venus Express to Venus. At the end he says:

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?

Sadly I do not have the expertise I would need to evaluate the merits of his argument; an astronomer on the Minor Planets Mailing List posted this morning that Khan's analysis used outdated emphemerides. The Goldstone observations should be able to settle this issue.>>

• Artist impression of the Venus Express spacecraft separating
  from the Soyuz Fregat upper stage rocket after launch (ESA)