The Spitzer First Look Survey (FLS) provided an initial characterization of the infrared sky at Spitzer wavelengths and sensitivities. The ecliptic plane component (EPC) of the FLS concentrated on two 0.13 square deg fields at a solar elongation of 115 degrees and ecliptic latitudes (b) of 0 degrees and +5 degrees. The FLS-EPC explored the small asteroid counts at 8 and 24 microns, with a detection limit down to 0.08 and 0.8 mJy, respectively, and a completeness limit almost twice as deep as the 8 micron equivalent flux density of the previous deepest mid-IR survey. The FLS-EPC also provided initial characterization of the zodiacal light near the ecliptic plane. Fifteen known and 19 unknown asteroids were identified, and asteroids detected at both wavelengths displayed similar 8 to 24 micron flux ratios of ~0.1. Comparing number counts for the b=0 and +5 degree fields indicates a slower-than-anticipated drop-off in contrast to predicted scale heights, possibly due to the presence of higher inclination objects in the small population sampled by Spitzer. The measured zodiacal light background was found to be within 5% of Spitzer model predictions at 24 microns.

For more information, please see these documents:

• The Spitzer First Look Survey -- Ecliptic Plane Component: Asteroids and Zodiacal Background, Meadows et al., 2004, ApJS, 154, 469
• Thermal Observations of Six Asteroids (PDF, 38 KB) (December 31, 2003) While not technically part of the ecliptic FLS fields, we've included these first results here nonetheless.
• Ecliptic FLS results report: Thermal Observations and Preliminary Number Counts (PDF, 18 KB) (2 Feb 04)

Survey Requirements

• IRAC and MIPS characterization of Ecliptic Plane and Zodiacal Light.
• Two strips at ecliptic latitudes between +5 deg and 0 deg
• Survey 3 times with IRAC, separated by several hours to detect moving objects
• Survey once with MIPS to obtain color of objects
• Ancillary data: Near-simultaneous ground-based support observations are required. Community interest was strongly welcomed.

Goals

The main goal was to characterize the population of moving objects, at 8 and 24 microns, and to explore smaller members of the asteroid population (< 1 km diameter). This survey was designed to target asteroids in the main belt region between 2 and 4 AU, to determine number counts and ecliptic plane scale heights. With supporting ground-based observations at visible wavelengths asteroid sizes and orbits were also provided to enable follow-up observations. This survey also included a component to characterize the zodiacal light as a function of distance from the ecliptic plane.

Observational Strategy

• Consisted of two 0.13 square degree fields
• at b=0 and +5 degrees (scale height determination)
• between 110 and 120 degrees solar elongation (tunes asteroid motion)
• pointing back towards the Earth (increases "follow-up season")
• Detection of asteroids performed with 3 passes with IRAC
• lower background and smaller pixel size
• 120s exposure per position gives 0.06 mJy@8 microns (~80 astroids in b=0)
• 2-3 hours between each pass
• One wider-area pass performed consecutively with MIPS
• wider area compensates for possible loss of IRAC sources due to motion
• 200s exposure per position gives 1.3 mJy at 24 microns (~40 asteroids in b=0)
• IRAC and MIPS detection allows crude determination of T, diameter
• more accurate diameters obtained with supporting ground-based followup data
• MIPS fast scan from 0 to +6 degrees ecliptic latitude to map zodiacal light
• Observing date was Jan 21, 2004
• Fields shifted when launch date changed to August.
• Optical followup is required to maximize science return.
• Now above the galactic plane, and observable at new moon.

To determine pointing direction for this survey, we looked at the total rate of apparent motion in arcseconds per hour for main belt asteroids as a function of solar elongation. A planning constraint was that we also wanted to look back towards the Earth, to ensure accessibility of the same region of sky by ground-based observers, and to allow a long follow up season for observing. This would favor a survey direction that pointed away from the Sun towards the outer limit of the OPZ. Combined with the fact that we wanted a relatively slow motion for the asteroids across our frames (10" per hour or less, given a 3 hour separation between field revisits) then this dictates that our pointing direction will be near the maximum solar elongation for the OPZ, i.e. in the 110-120° solar elongation range. We chose a nominal pointing direction of 115 ° solar elongation for the two Ecliptic plane fields.

The actual geocentric J2000 coordinates for the centers of the fields at 0h UT on 21 Jan 2004 are :

• RA=12:03:22.03, Dec= -00:21:53.8 for the 0 degree field
• RA=12:11:20.10, Dec= +04:13:18.7 for the +5 degree field

IRAC mapping

Two rectangular fields of approximately 10' x 0.8 degree (0.13 square degrees) were observed at 0 and +5° ecliptic latitude, to probe scale height drop-off in the asteroid population. These fields are a thin rectangle with the long axis running perpendicular to the ecliptic plane. This field shape was chosen as a compromise between instrument observing mode efficiency, which favors long scan strips, and the desire for greater spread parallel to the ecliptic plane to minimize loss of asteroids off the detection area.

The IRAC map strategy consisted of taking 30 second exposures and stepping in columns by almost a full array, and dithering twice at each position, and then stepping by half the array in row, and dithering twice at each position. This results in a 120 second exposure at each position, for a totally covered map of 9.7' by 2900'. With the above mapping strategy and using the High Dynamic Range option to avoid saturating on bright sources, we were able to map each 0.13 sq. deg field in approximately 1.1 hours to a sensitivity depth for a 5-sigma detection of a 0.06 mJy source at 8 microns against a high background. This 0.06 mJy limit corresponded to an anticipated detection rate of 30-80 asteroids in the ecliptic plane field, many with diameters as low as 0.5 km. To detect moving objects, these fields were revisited three times with time separation between visits of 2-3 hrs. This time separation was dictated by the time required to do one full map of each of the two ecliptic latitude fields.

Visualization of planned observations on an all-sky image. Click for un-cropped image.

Closer view of visualization of planned observations on an 25 micron ISSA image. Blue is IRAC, pink is MIPS. RA/Dec grid overlaid. Click for larger image.

MIPS Mapping

To compensate for possible loss of objects from the edges of the IRAC fields, the MIPS field was slightly larger than the IRAC fields by an amount corresponding to the "loss margin" dictated by the apparent rate of motion in arcsec/hr of the main belt population and the total number of hours taken to perform the IRAC observations and implement the instrument changeover to MIPS. This was approximately an extra 1-2' on a side.

The MIPS observations required 200 seconds integration per position, allowing a 5-sigma detection of a 1.3 mJy source at 24 microns against a high background. Anticipated source counts were approximately 56 or more asteroids in the ecliptic plane field. The sensitivity limits for the survey made it highly likely that all the asteroids detected in the MIPS observations will also be seen by IRAC, and therefore would be positively identified as asteroids. This would eliminate thermal selection effects from using a shorter wavelength as the detection mechanism. The three MIPS scan strips (covered twice) for each field were 1° long, running perpendicular to the ecliptic plane and with a 2 pixel overlap between consecutive scan strip positions.

Zodiacal Light Strip

A single 6° long MIPS fast scan strip was also taken extending from 0° to +6° ecliptic latitude. This strip passed through the two deeper asteroid fields and characterized the zodiacal light as a function of ecliptic latitude while providing context for the zodiacal light observed in the deeper fields.

Ground-based Supporting Observations
Near-simultaneous ground-based supporting observations greatly enhanced the scientific products available from this component of the Spitzer First Look Survey. They did this by providing visual astrometry and photometry, which can be used in conjunction with the Spitzer data to determine object distances (good to ~0.01 to 0.1 AU), sizes, albedos, and preliminary orbits for a large fraction of the observed sample. This information (1) could not be provided by the Spitzer data alone, (2) was essential to expand the science legacy of this Spitzer data set beyond asteroid number counts, and (3) enabled Spitzer and ground-based follow-up of the objects found in this survey.