Rapid Response Astronomy

Nothing in astronomy ever changes.  

That is to say, nearly every thing we study in astronomy is effectively unchanging. The stars and galaxies we look at through our telescopes are just about as constant and eternal as you can get. Even a massive star with a "fast" life cycle takes millions of years to exhibit any visible change, far longer than the time available to human observers. So for most astronomical observations we can really take our time; there's never any hurry to catch a galaxy or a cluster of stars before it disappears.

For an observatory like the Hubble Space Telescope, this means that most of the observations being done are fully designed and scheduled well in advance. The typical process for observing with HST is spread over many months. First, astronomers prepare a proposal describing the science they want to do, and how they'll use HST to do it.  These are submitted each year around the first week of March. Then in May a panel of volunteer astronomers is gathered in Baltimore at the Space Telescope Science Institute (STScI) to review all the proposals and select the ones that will be awarded time on HST. The successful proposers then go through another round of preparation, where they pin down the details of exactly how the observations will be done. The observations can happen anytime over the next year or so, and special large programs like CANDELS get spread out over multiple years. 

The Hubble Space Telescope.
 Image Credit:  NASA, Z. Levay

The specifics of when HST actually collects the data are hammered out by a dedicated team of STScI research support staff. These program coordinators and calendar builders have the job of piecing together the puzzle of many hundreds of different HST observations. Each observation has a unique set of constraints to consider: When is the target visible? What does the rotation of the telescope need to be? Are there bright stars near the target that HST can use to lock its position? Each week the calendar builders balance these competing requirements and put together a very detailed schedule for exactly what HST will do two weeks in the future. Efficiently packing and organizing those observations is a big task, and one with real significance. Astronomers and telescope operators know that every observation with HST is a precious resource, and represents a substantial investment in this science. The total cost of HST divided by its lifetime works out to about $15 per second, or $54,000 per hour. All the careful advance planning is really critical for maximizing the science return from that investment. 

For CANDELS, however, we can't plan out all of our observations many months in advance. One of our primary science goals is to detect and analyze distant supernovae: stars reaching the end of their life-cycle with a violent explosion. The explosion itself occurs without warning in a fraction of a second, and we can observe the after-glow for weeks and months afterward. There is no way to predict when and where these explosions may appear, but when we do spot one, we often need to quickly mobilize HST for follow-up observations, while the supernova is still bright enough to see. For this type of object, the HST operators allow a special mode for submitting observation plans, called the Target Of Opportunity (ToO) mode.

Here's how it works:

When we discover a new supernova of interest, like the record-setting SN Wilson, we sift through all the available data and decide that we want to get a quick follow-up observation, maybe as soon as next week. We quickly contact our program coordinators at STScI and tell them that we're going to trigger a ToO observation. Then we plan out the observations and submit them for review. To make room for our new ToO supernova, the calendar builders then pull out some of the pre-planned observations from other programs (they'll get put back in sometime later in the year). 

The bright star in the lower left is SN 1994D
in the galaxy NGC 4526.
Image Credit: High-z SN Search Team, HST, NASA

With experience and good organization, the detailed observing plan for a new SN can be arranged in a few hours - but sometimes we only have a few hours to spare. For most ground-based observatories, normal ToO observations can be slotted in on the same night that a supernova is discovered. For HST, however, it is much more complex and risky to make sudden changes, so each week the HST schedule gets locked in place on Wednesday morning. We need to give the program coordinators and calendar builders at least 4-5 hours to process a new observation, so that means that we have a weekly deadline of 12 noon each Tuesday for any new ToO interruptions.   

This brings us to the peculiar situation that if we happen to discover a new supernova on a Wednesday, then we have almost a week to leisurely examine the data and decide whether it warrants a ToO trigger. If that same supernova is found on a Monday, though, we are scrambling to get all our decisions made and plans in place before the Tuesday noon deadline. This can lead to some long hours on Monday nights when CANDELS observations are coming down from HST - but its exciting and rare to get any kind of astronomy in rapid action. We supernova hunters really appreciate what a unique privilege it is to get to push around an orbiting space telescope at the last minute when our science requires it.