An existential comet, Mars "sea" level

Q:  I've heard that a comet of naked-eye brightness will appear in May — if it still exists! What does that mean — "still exists"? (Tilly, Lyme Regis, Dorset, England)

A:  Just that.  Comet 73P/Schwassmann-Wachmann 3 is dying.  By May, she may exist only as a diffuse dust tail, or she may be a bright glowing orb. 

Comet 73P breaking up in December 1995; at least two nuclei then present. Her long, glowing tail trails away from the Sun.  Courtesy of Jim V. Scotti.

In September 1995, as expected, 73P cruised along her greatly elongated 5-year path about the Sun, compelled by gravity and nearing, once more, the main attractor — about a month from her closest point to the Sun (perihelion).  She shone reflected sunlight dimly at 13th magnitude.  Then, over the course of the month, her dirty snowball core (the nucleus, perhaps a half mile (1 km) across) shattered into five pieces.  We don't know why, but it meant death. 

Early that month, she showed a "tremendous outburst", writes Guy Ottewell in the 2006 Astronomical Calendar.   Apparently, she released huge quantities of water, and continued to do so through September.  By early October, her brightness had soared a thousandfold to 6th magnitude.  In December, observatories imaged five separate small gas and dust clouds emanating from four separate pieces of the nucleus.  The comet had broken up.  Why?

Comet 73P on Jan. 31, 1996 after it had broken into three pieces. The New Technology Telescope at La Silla, Chile made this false-image photo. Courtesy of European Southern Observatory.

Perhaps, as she neared the Sun's warmth, heat stress shattered her like an ice cube dropped into steaming hot coffee.  Perhaps, a drifting rock hit and broke her into pieces.  Or, tidal forces from the Sun may have pulled her apart.  In any case, a comet averages about a thousand orbits before decaying away.  So, Comet 73P may be about 50,000 years old, but going fast.

We have a rare chance to see the last bits of a comet, if her big pieces remain large enough for us to see when she comes by in May.  If she "still exists."  By April 11, she had disintegrated from five pieces into 40 hunks, which we are tracking. 

Hubble gives the details: The "B" piece of Comet 73P and its fragments, 18 April. Courtesy of NASA, ESA, H. Weaver (JHU/APL), M. Mutchler and A. Levay (STScI).

In the Northern Hemisphere, start watching early in May.   Country folk may see a bright fuzz ball; city folk may glimpse just the central core.  Wait until the Moon sets, which on May 1, is about midnight Eastern Daylight Time.  On May 8 — our last best chance to see her — the Moon sets close to dawn, and the comet is close to the Ring Nebula (M57) in the constellation Lyra.  Binoculars help.  After that, the comet gets closer to Earth but the Moon waxes brighter and nears the comet, making viewing such a (maybe) dim object nearly impossible with the naked eye. 

By May 22, the comet gets even closer, and people in the Southern Hemisphere can see her, if she still exists.

"As each piece of comet fragments, we get a look at its interior and its pristine material," says astronomer Robert Massey of the Royal Observatory Greenwich in London.  The fragments of this comet will be unusually close to Earth, and will travel across the sky in widely separated paths. If they fragment and flare (as they just did on April 24), this could be a magnificent event — "imagine three Hale Bopps at once!"

Further Reading:

Astronomical Calendar 2006 by Guy Ottewell; see pages 60 through 63, Sky & Telescope

The comet with a broken heart, 25 April 2005 press release, European Southern Observatory

Mini-comets approaching Earth, Science@NASA

Update on Comet Schwassmann-Wachmann 3 by Alan M. MacRobert and Greg Bryant, Australian Sky & Telescope

Comet 73P/Schwassmann-Wachmann 3--possibly the brightest comet in nearly a decade (The sky this month) by Martin Ratcliffe and Alister Ling, Astronomy May 2006 v34 i5 p61

Comet, Wikipedia

73P/Schwassmann-Wachmann 3, Wikipedia

New Technology Telescope, Wikipedia

Q:  What does zero elevation mean on the Mars map that Google recently published?  Is it sea level?  (Somewhere, USA)

A:  Mars has no seas or oceans so the 'mean sea level' that we use on Earth to define a zero elevation won't work on Mars.  Instead, at Mars' equator, we have measured its mean radius with the Mars Orbiter Laser Altimeter (MOLA).  It is 2100 miles (3396.2 km).  This value is the "sea" level of Mars.  Technical details.

"Zero elevation (or 'sea level') on Mars is essentially the planet's average elevation," says Carol Hughes, spokeswoman for Arizona State University.

A detail of the Mars map showing where the Mars rover, Opportunity, landed on the Meridiani plain in January 2004.  Courtesy of NASA/JPL/Arizona State University.

By the way, the difference between Mars' highest point (the top of Olympus Mons) and lowest point (Hellas impact basin) is nearly 19 miles (31 km).  Whereas, the difference between Earth's highest (the top of Mt. Everest) and lowest (Mariana Trench) is only 12 miles (20 km). Mars is about three times rougher than Earth (taking into account Mars' smaller radius and, therefore, greater curvature).  Also, the shield volcano, Olympus Mons, is the highest known mountain in the solar system.

Further Reading:

Zero level of MOLA elevations by D.E. Smith and M.T. Zuber, NASA GSFC and MIT.

Mars Orbiter Laser Altimeter (MOLA) Science Investigation web page, Goddard Space Flight Center, NASA

Mars Odyssey Mission, Arizona State University

Mars geodetic parameters from MOLA, Goddard Space Flight Center, NASA

Definition of geoid, National Geodetic Survey, NOAA

I invite my readers to answer a question a month.  Here's a question for anyone to answer:  Why do birds sleep on one foot?  I'll publish the best answers, and you get credit.  So include your full name and where you live:  the city, state and country.  Also, include your email address, which I won't publish, so I can contact you.  Please email your answers to contact@wonderquest.com.  I may edit the answers for brevity and clarity. Thanks!  April

(Answered May 2, 2006)