Wednesday, December 14, 2011

Will SDO see another comet?

We are setting up a special website to watch Comet Lovejoy pass by the Sun tomorrow night. The URL will be published tomorrow afternoon. You will be able to examine the special off-point images for signatures of the comet! The Helioviewer website is also available for making movies and looking for the comet.

Comet Lovejoy was discovered on November 27, 2011. It is a Kreutz comet, one of a family of comets that move along the same orbit---much like the meteoroids in a meteor shower. These comets come so close to the Sun that they are called sun-grazing comets.

Comet Lovejoy may become one of the brightest Kreutz comets ever seen. The latest estimates are a visible magnitude of -3. (Bright stars are magnitude -1 or dimmer.) We are making special efforts to see Comet Lovejoy this week.

Thursday night at 7:30 pm ET Comet Lovejoy will move through its closest approach to the Sun. Starting at 23:30 UTC (6:30 pm ET) Thursday we will point the SDO spacecraft a little to the left of its usual position. The images will look like this figure, where the circle is the edge of the Sun and the X’s are where we estimate the comet will be and the red arrow shows the direction the comet moves. Watching from SDO the comet goes behind at 00:22 UTC (7:22 pm ET).

All three instruments on SDO will be watching the comet. The EUV images from AIA would show the second comet seen in these ultraviolet wavelengths (the first was in July). HMI and AIA can use the comet to understand the roll of the spacecraft. EVE might see some of the atomic ions responsible for making the comet bright in the EUV.

In July we saw a Kreutz comet disappear as it passed across the face of the Sun, why does it go behind the Sun this time?

The way we see a Kruetz comet depends on where we look from. We have constructed a 3-D model of the comet orbit from blue wire and a yellow sphere. The top picture shows how the comet will move Thursday night; up from below the ecliptic, behind the Sun, and then back down, generally moving from left to right.

A view from the July position of the Earth is shown in the lower picture. The comet seen then moved up and across the face of the Sun (from right to left) because the Earth and SDO were on the other side of our orbit.

Amazing fact #1: A Kreutz comet takes 800-900 years to orbit the Sun. Because its closest approach (or perihelion) is only 0.2 solar radii above the Sun’s photosphere, each comet must travel 150-200 AU (out near Sedna) out into space before returning!

Amazing fact #2: No Kreutz comet seen by SoHO has survived perihelion passage. The comets go behind the occulting disk and do not re-appear. They probably evaporate in the fierce heat of the Sun.

Amazing fact #3: SDO is not the only satellite that will be watching Comet Lovejoy. Coronagraphs on STEREO and SOHO are already seeing the comet move toward the Sun.

It should a great show!