HUT Observatory

High School Students Measure Fast-Rotating Asteroid

Guided by instructor Caroline E. Odden at Phillips Academy in Andover, Massachusetts, high school astronomy students Joey Verhaegh and David McCullough have analyzed 730 images recorded from one night in 2012 at HUT Observatory.  The pictures show a recent near-Earth asteroid that attracted world-wide attention as it passed by.  The analysis has revealed a rotation period for the house-sized object of about 12.2 minutes — rather fast as run-of-the-mill asteroids go, as they more typically turn in hours, not minutes.  The results are being prepared for submission to Minor Planet Bulletin edited by Professor Richard P. Binzel of MIT.  A preliminary graph of the object’s changing brightness, which repeats in a Asteroid Graph from Andovercycle, is shown here.  We might take this opportunity to explain some about the science, art, and adventure of projects like this, which are among the many excellent opportunities nowadays for small observatories.

When explaining to curious friends how a current focus at HUT Observatory is measuring the rotation of asteroids, we can’t help but approach the subject a bit apologetically, it sounding so ridiculously arcane.  To begin with, asteroids are typically so small and distant that they almost never appear more than star-like, even in the world’s largest optical telescopes.   Their distinguishing characteristic is that they appear to move.  But this alone has attracted the attention of astronomers since the discovery of the first one, named Ceres, on January 1, 1801.  They were “minor planets,” orbiting the Sun mainly between Mars and Jupiter, and for centuries, astronomers could only speculate, in mind’s eye, what they were really like.

Eventually it was noticed that some of them changed brightness in a curious way.  Change was expected, given that they shone by reflected sunlight, like our Moon.  Varying angles and distances between the Earth, asteroid, and Sun made for varying brightness.  But the odd changes were ones occurring too quickly to be caused by the slow pace of orbital geometry.  The conclusions were thus that asteroids are irregularly shaped and that they change apparent brightness as they spin in space.  It’s simple:  When a larger profile is facing us, there is more reflected sunlight.  Given that so little other information could be gleaned from the tiny, stellar appearance of these objects, determining rotation was a breakthrough.  The first rotation recognized was for asteroid number 433, discovered in Germany in 1898.  It displayed such a great range of brightness it was concluded to be strikingly oblong and was consequently named Eros, for the Greek god of love.

Eros

Six views of asteroid (433) Eros recorded by passing spacecraft NEAR-Shoemaker in 2001.

To this day there are many asteroids with undetermined rotations, and one can’t help but feel a bit like an explorer when, using nothing more than a backyard telescope, one charts the changing brightness, finds the length of the repeating cycle, and thus infers the rotation of a distant, untouchable, and generally unresolvable space rock.  Perhaps it requires someone already hearing the “call” of astronomy to wax enthusiastic about an achievement like this.  The thing is, however, it’s amazing how much one can learn about an asteroid from its rotation.  For example, we now know that some asteroids are not monolithic structures, strong as iron.  A good number of asteroids are more like loose conglomerations of gravel, barely holding together due to gravity.  If an object like that is spinning fast — say, once in a few hours — it fattens-out at its equator and looks something like the flying saucer from the old show, Lost in Space.   If by some reason it comes to spin too fast, it can break apart into a double asteroid – forming a system with an asteroid moon orbiting around a larger central body.  That situation is weird enough to be cool, even to the mind’s eye of a non-astronomer.  It is now believed that about 15% of asteroids are in fact in double systems, or in even more complicated ones.  A first step in recognizing potential double asteroids is to notice ones that are spinning fast.

Some asteroids are spinning really fast, at least compared to run-of-the-mill objects.  A full rotation of these objects is measured in minutes, not hours.  It’s simply fun, at least to an asteroid astronomer, to nail a fast rotator.  The fastest objects tend to be small and solid.  Such proved to be the case recently with the near-Earth object designated 2012 TC4 that swung by Earth in October, 2012, its closest approach to us being about 25% of the distance to the Moon.

Astronomers using radar can actually image the surface of some near-Earth objects like 2012 TC4, and they benefit from the world-wide community of asteroid observers.  On October 8, the day the object was discovered, Lance Benner of NASA’s Jet Propulsion Laboratory wrote to MPML, the world-wide asteroid e-mail forum, “Greetings MPML.  2012 TC4 is a potential radar target later this week, but the plane-of-sky pointing uncertainties around the closest approach are too large for us, so we are requesting astrometry.  Please report it to the Minor Planet Center as usual.  The nominal close approach will be at 0.000635 AU (0.25 Earth-Moon distances) on October 12.  With an absolute magnitude of 26.5, this object could be roughly 16 meters in diameter.”

In this jargon was a challenge to world-wide observers that, working together, it might be possible to see the object, by radar, much better than just via old mind’s eye.  Shortly after Benner’s post, asteroid authority Alan W. Harris added to the forum, “For anyone doing photometry on this object, the spin period is almost certain to be under one hour, possibly much under an hour, so short exposures will be necessary to do a lightcurve.”  Harris was essentially upping-the-ante of observational interest.  He recognized that such a small, essentially house-sized object was likely to be a very fast rotator.

DSC_0019

The HUT Observatory control room is below the dome and contains computer control for the telescope, rotating dome, and electronic cameras. While images are being recorded, it’s possible, and sometimes necessary, to strategize with astronomers observing the same target from other facilities. Modern technology makes all this surprisingly practical as the adjacent text describes.

Later that day it proved clear at HUT Observatory, so we jumped on the target with a long series of 2-minute exposures.  As they accumulated, observer John W. Briggs assembled them into a makeshift time-lapse movie.  The result was something that he had never witnessed before.  As he wrote to Harris, “Alan, you are certainly correct regarding the fast rotation of this object!  I am tracking it tonight at HUT Observatory in Colorado doing 2-minute exposures through an R filter.  About 30 minutes into the exposure series, I decided to blink what I had, in part just to enjoy the time-lapse-movie effect of the good tracking based on the RA and dec rates I got from the JPL website.  But lo and behold, the darn thing is blinking practically on-and-off before my eyes, in the 2-minute exposures!  Very, very neat!”  After this, Harris and Briggs connected in a phone call, and the advice from Harris was to use shorter exposures to better resolve the fast rotation.  All this was going on in the middle of the night, as exposures were being taken and recorded under computer control.

As the Earth turned, other astronomers followed the target.  Briggs wrote to the MPML forum, “The amplitude is certainly large with 2012 TC4.  I kept on it for 330 1-minute exposures last night, and when I blink these frames, the changing brightness of the object is delightfully obvious.”  Richard Miles of the British Astronomical Association had observed it using a remote-controlled 2-meter telescope in Australia and wrote, “10-second exposures were used to exclude the possibility of rotation periods of the order of 1 minute.  Folding the lightcurve data appears to show a period of 0.0084 +/- 0.0002 days (726 +/-18 sec) and an amplitude of 1.2 +/- 0.2 magniute.  However, as Alan has pointed out, this object may be a tumbler.  Certainly the data show one or two small discrepancies, so a multi-period solution may ultimately yield a better fit to the data.”  In other words, the fast spin of the object was confirmed.

Lance Benner, hoping to be successful with his colleagues using the radar system on the giant Goldstone dish in California, encouraged observers.  Writing to Briggs, “Thank you very much for your help with this.  The rotation period information is really helpful.”  Meanwhile, the brightness changes were so strong that some observers were able to estimate them simply by eye through a telescope.  Bob King in Duluth, Minnesota, wrote, “Hi everyone.  It was fascinating to observe 2012 TC4 through my 37-cm scope this evening (Oct. 12 1-1:30 UT).  Regular, repeating changes in magnitude from approximately 13.6 to 14.6 were seen.  I could only roughly estimate a period using eyeballs only.  The time between maxima was approximately 6-7 minutes.”

Asteroid expert Brian Warner, writing from Palmer Divide Observatory in Colorado, replied, “Hi Bob.  The 6-7 minute interval between maxima fits with a rotation period of about 12 minutes that several photometry efforts have reported since, in this case, there are going to be two maximums per rotation (think a spinning potato).  Unfortunately, all I saw tonight were cloud bottoms.”

It was soon relayed on the MPML forum that astronomers in Italy found a period of 0.204 hours, or 12.2 minutes.  The graph of HUT data above, made by the students at Phillips, confirms this period exactly.  The graph shows many cycles folded over each other to reveal the true nature of an average cycle, which runs from phase 0 to 1 along the bottom of the graph.  A double-hump nature is revealed, making it clear why the earliest observers, including Briggs, suspected a period more like six minutes.

70meter

The 70-meter radio telescope at the Goldstone facility in California is used regularly to image near-Earth asteroids using a radar technique. 2011 photograph by Emily Lakdawalla.

Unfortunately, Lance eventually relayed, “We’re having serious trouble at Goldstone and have yet to detect a radar echo.  We’ve checked everything we can think of… and have less than 30 minutes left.  Not looking good.”

Alan Harris wrote in overview of the rotation discussion, “I think you are all ‘in violent agreement.’  Richard’s six minute period was likely the simple (half) period, the time between successive minima.  Too bad about the radar….”  And Brian Warner concluded, “Given the general scatter, the close agreement bodes well for the solution, though tumbling may be hard to establish.  Yes, a real shame about radar.  Murphy can be very mean.”  [“Murphy’s Law” being that anything that can go wrong, will go wrong.]

While the radar wasn’t successful on this pass of object 2012 TC4, it was spectacularly so on, for example, object 2005 YU55, one of the first near-Earth objects followed at HUT Observatory.  The excitement among specialists, all contributing what they can to the effort, is renewed with each of these celestial encounters.  They are in fact common.  The unusually close March, 2013, passage of  object 2013 ET, for example, drew international attention far in excess of either 2012 TC4 or 2005 YU55.

Non-astronomers would be surprised how relatively straightforward it is enter into these observations.  We are gratified that some leading schools and educators are coming to recognize the potential of involving younger students with actual science, beyond mere exercises.  As of March 29, 2013, HUT Observatory has coauthored five papers with teachers and students in Minor Planet Bulletin, with two more in preparation and an expectation of many more to come.  The enthusiasm and response of interested students to activities like these can not be overemphasized.  –JWB.