I thought the first STEREO-B pictures of C/2011 L4 PANSTARRS joined by planet Earth were sweet enough enough. No, no. This one – and the accompanying video – are far nicer. They were made on March 13 when PANSTARRS had more fully entered the telescope’s field of view.
Relish the many plumes striping the comet’s broad dust tail. Astronomers call them “striae” (STRY-eye or STRY-ee), and they result from the release of massive amounts of dust from vaporizing cometary ices. As they’re blown back by the pressure of sunlight to form the tail, the particles get sorted out into individual plumes according to their varying sizes. Let’s hope that once PANSTARRS gets high enough to see in a darker sky, the beautiful striae will reveal themselves.
STEREO-B video of Comet PANSTARRS and a blast from the sun
The video was compiled using multiple single frames from STEREO-B. The field of view is about 20 degrees (about as wide as the constellation Orion is tall) and the sun is out of the field of field. STEREO-B orbits on the opposite side of the sun from Earth and looks back toward the comet which is fortuitously lined up with the Earth. Amping up the excitement another notch, a coronal mass ejection from the sun appears to be headed for the comet. This is a trick of perspective – the blast missed PANSTARRS by some distance.
Another animation of the comet from March 13 STEREO-B images. Sun is off to the left.
Speaking of distance, you’ll see in the lower left of the orbital diagram that STEREO-B is 1.9 A.U. from Earth. An A.U. or astronomical unit is equal to Earth’s distance from the sun or 93 million miles, so the spacecraft is currently about 177 million miles from our planet. PANSTARRS meanwhile is 1.1 A.U.s from Earth (102 million miles) and very approximately 75 million miles from STEREO-B.
I only toss these figures out so you can appreciate the enormity of the comet’s dust tail. The Earth barely registers as more than a blip in the picture. Even accounting for PANSTARRS being some 40% closer to STEREO’s camera than Earth, the tail measures at least a million miles long. Compare that to Earth’s 8,000-mile-diameter and you can begin to appreciate its size.
Since comet tails aren’t solid objects, the amount of mass they contain is very small despite appearances. Indeed, they put on a good show!