by Rex Parker, PhD director@princetonastronomers

On with the Show.  As we move into mid-winter it gets harder to do astronomy outside.  Better to curl up with a good book or click the link to a seminar for your astronomy experience this month.  At the AAAP meeting on Feb 8 we offer a Princeton University guest speaker, Dr. Christopher Spalding, Postdoctoral Fellow in the Dept of Astrophysical Sciences.  For info about the talk and the Zoom link see the section below from Program Chair Victor Davis. 

Last month’s session with Dr Robert Williams of STScI was one of the best attended (61) since we’ve been Zooming. If you missed that meeting or other recent astrovideo live sessions, see the AAAP You Tube channel (link below).  Thanks to Victor Davis, Dave Skitt and Ira Polans for recordings and editing. https://www.youtube.com/channel/UCiJvXfK9DGCmGwiKK_Q6ieg. 

Visualizing Astronomy.  What would it be like to see in the infrared?  This question arises as we look forward to the commissioning of the James Webb Telescope, which unlike Hubble and most other telescopes was designed to see in the infrared and not the visible spectrum.  So why can’t we see outside the visible range (Figure below, electromagnetic spectrum with visible wavelengths in yellow)?  This is a circular question that shows that our perception of the universe has been completely shaped by the wavelengths we can see. 

The development of sight in animals is a deep topic in evolutionary biology, and in astronomy the limit of eyesight has an obvious major role.  As early as 1604 the Renaissance astronomer Johannes Kepler recognized this.  His treatise Astronomiae Pars Optica, explores the properties of light and in line drawings the structures of the eye of several animal species are depicted.  Two main reasons can be deduced for the limited spectral range of our eyes.  Firstly, the earth’s atmosphere absorbs most of the radiation outside this range (Figure above, cross-hatched areas). 

This opacity to radiation is due to fundamental properties of atoms and molecules in the gaseous state in the atmosphere and is essential for earth’s hospitality to life.  Water vapor strongly absorbs much of the infrared band, which is why infrared astronomy largely relies on orbiting telescopes, hopefully including the Webb.  The second major reason is that the molecular biochemistry of photoreception depends on chromophore-dependent biochemical processes which have been fine-tuned over the millennia to absorb light in the 400-700nm range of the spectrum only.  A photoreceptor that could detect outside this range was unlikely to be assembled from the proteins and accessory pigments available in evolution.  Such a photoreceptor would need a quite different molecular structure beyond the scope of molecules evolving in the history of life.  Our perception of the cosmos has earthly underpinnings only now being loosened by the advances of science and engineering.

Speed of an Asteroid.  At the meeting last month I tossed out an astro challenge for members with telescopes to observe a celestial object that few people ever see.  The challenge was to observe, and for extra credit take images, of the near-earth Asteroid 7482 1994 PC1 on or about the day of its closest approach to earth, ~6pm Jan 18.  It would be dim (magnitude ~10.3) but potentially observable from central NJ for only a couple of hours after twilight that night. I have heard that several AAAP members succeeded in the challenge.  Here I offer my own observations and a calculation of the speed of the asteroid based on the data collected, illustrating what amateur astronomers can do with today’s equipment.

The original 1994 discovery of this asteroid by R.H. McNaught in Sidings Springs Australia is written up in The Minor Planet Bulletin (vol 24, no.4, Oct-Dec 1997), accessible by internet search.  A little over 1 km in diameter, asteroid 7482 is one of about 1600 Apollo objects whose elliptical earth-crossing orbits make them top candidates for a possible collision in the future. In this case, with an orbital period of 1.65 years, one of its close approaches to earth happened on Jan 18, 2022.  Predicted to come within 0.013 AU (1.2 million miles) of the earth, 7482 sped by at a distance only slightly greater than the Webb Telescope orbital distance!

The days leading up to the event were cloudy, but amazingly the clouds over New Jersey dispersed around sunset on Jan 18.  With the near full moon rising at 5:40pm, observers had to work swiftly to catch the asteroid between twilight ending and full moon rising.  The astroimage below shows the path of the asteroid captured in a 10 min exposure with my 12.5” reflector telescope.  The brightest star in this image is 8.8 magnitude, the asteroid 10.3.  The speed can be better appreciated in an MP4 movie (click icon under the image below).  In the movie below, 58 frames of 12 seconds were combined for a total elapsed time of 11.6 minutes.  The asteroid blazed across the sky! 

Asteroid 7482 1994 PC1 at close approach on Jan 18, 6:15pm.  Image from central NJ using 12.5” telescope and ASI071 camera.  The 10 min exposure with tracking on stars reveals the movement of the asteroid as a streak.  Click on the icon below for video. Astrophotos by RAParker.

Just how fast is “blazed across the sky”?  With the tools of the modern amateur astronomer, we ought to be able to solve this.  I measured the distance in pixels movement in the image above using trigonometry (Pythagoras theorem) and used it to calculate angular velocity in radians per minute.  Then I used the formula for angular speed ω = θ/t (where ω is angular speed, θ is angle of rotation in radians, t is time);  the known scale of the optical system (0.46 arcsec per pixel);and the formula to convert angular to linear velocity:  v = r ω.  From the data and these formulae I calculated the approximate speed of the asteroid to be 42,500 miles per hour.  According to earthsky.org, professional astronomers calculated the speed at 43750 mile per hour, so my error was less than 3%.  To give some perspective, the moon in its orbit speeds along at 2288 miles/hr.  The asteroid flew by the earth at almost 19 times the speed of the moon!