Friday, August 28, 2015

Comet has Faded

The incoming sungrazing comet has faded and will most likely not be visible this afternoon in SDO AIA images. This is common in Kreutz comets, they evaporate before reaching the Sun. The light curve looks like a small comet evaporating.

Here's the latest from Karl Battams: "As I expected, the comet "pulled an ISON". I won't even describe it as a comet any longer. It's a rubble pile." (SunGrazerComets).

The Comet is Still There, Waiting for the Perihelion Time Update

This morning we should get better information about the orbit of the incoming comet. The orbit from last night is shown at left (times are in UTC). The plus signs are spaced 5 minutes apart. They get a little farther apart as the comet moves from lower right to the upper left. This means the comet is speeding up. At perihelion (the closest it gets to the Sun) the comet will be moving at about 600 km/s (1.3 million mph).

We see comets because they evaporate ices and other compounds from their surface. This cools the comet, but makes it disappear. The water ice that comes off the comet quickly turns into Hydrogen and Oxygen atoms. SDO sees comet tails when the oxygen atoms hit the electrons in the corona. We can use the tails to explore the Sun's magnetic field and corona.

Most Kruetz sungrazing comets are too small to make it to perihelion (Comet Lovejoy in December 2012 was the only exception). We can only watch as this comet goes behind the LASCO occulter disks and hope it continues to evaporate and be seen in SDO/AIA images.

Thursday, August 27, 2015

A New Sungrazing Comet, Will SDO See It?

Karl Battams (@SungrazingComets on Twitter) has announced a new sungrazing comet. It is seen in a LASCO image taken earlier today. The image here shows what the comet looked like at 1430 UTC (12:30 pm ET) today.

This is a Kruetz comet, like the first comet seen by SDO back in 2012. The SDO team has decided that our normal pointing at the Sun is all we need to do to capture this comet as it passes across the disk. We used the picture at left to guide us. The comet will reach perihelion (where the plus signs end) just before it goes over the limb of the Sun and disappears behind the Sun. The comet is brightest just before perihelion (if it makes it that far), there is no reason to point SDO to another part of the orbit.

We will try to post images as fast as we can tomorrow afternoon. Perihelion is at 1947 UTC (3:47 pm ET).

Tuesday, August 25, 2015

Welcome to the Fall 2015 Eclipse Season

Today is the beginning of SDO's Fall 2015 eclipse season. From today until September 17, 2015, the Earth will pass between SDO and the Sun once each day. SDO orbits at about the longitude of Boulder, Colorado, so the eclipse is at midnight solar time there. This corresponds to roughly 0700 UTC. Here is the first eclipse, right on schedule at 0710 UTC (1:10 a.m. MDT).

This picture shows the edge of the Earth against the Sun in AIA 193. You can see the edge of the Earth is rough, where the absorption of the Earth's atmosphere dims but does not block the light. This is different from the lunar transits, such as the one coming up on September 13, where the Moon's edge is very crisp. That transit occurs near the end of that day's eclipse and coincides with a solar eclipse in the far southern parts of the Earth.

Eclipse season is a result of our geosynchronous orbit. We don't like missing the Sun for up to 72 minutes each day. But the constant contact with SDO allows the high speed data downlink we use to bring the Sun to everyone.

Wednesday, August 12, 2015

Stationkeeping Maneuver #11 is Today

SDO will execute Stationkeeping Maneuver #11 today from about 2245-2315 UTC (6:45-7:15 p.m. ET). This Delta-V burn is used to keep the observatory within the assigned orbital box. During the maneuver the science data is usually not valid as the spacecraft slews away from the Sun to orient the thrusters in the correct direction.
The next eclipse season starting August 25, 2015, giving us more chances to see the Earth's limb against the Sun. Eclipse season ends September 17.

The next lunar transit on September 13 will have two firsts. The eclipse by the Earth will happen just before the transit and finish while the Moon is still crossing the Sun. At the same time a partial solar eclipse will be visible in the South between Antarctica and Africa. This will be the first time an SDO lunar transit could be seen as a solar eclipse on Earth. The movie of the combined eclipse and transit looks like a race between two spacecraft.

A month later we see another lunar transit on October 12 from 1718 to 1734 UTC (1:18-1:34 pm ET). This is more of a glancing blow but will be cool to watch.

Wednesday, August 5, 2015

Momentum Management Maneuver Today

SDO will perform a Momentum Management maneuver today at 1930 UTC (3:30pm ET). For about 30 minutes from 1920 (3:20 pm ET) science data will not be available. As soon the reaction wheels are at their desired speeds the science data will resume.