We want to search for asteroids thought to exist along the Earth's orbit that may be leftover material from the formation of our planet. These asteroids always appear close to - or even behind - the Sun in the sky and are therefore difficult to detect from Earth. We will use images taken from the two STEREO probes which have been studying the Sun and its vicinity since 2009. The spacecraft have been slowly drifting along the Earth's orbit and are able to image the sky from different vantage points around the Sun. This way, we will constrain the abundance of asteroids in Earth-like orbits, including any large, hundred-m to km size, objects in unstable paths that are not picked up by surveys and present a long-term impact hazard to our planet.

The objective of the exercise is to use the SECCHI images to search for asteroids along the Earth’s orbit, i.e. with a ~ 1 AU and small e, small i.
This dataset has already been used to study individual asteroids and to survey the region close to the Sun. The asteroids we are looking for here are smaller and much closer to the spc than in previous studies that use STEREO data.

The perfect result would be to identify all asteroids in the images, including Earth-like asteroids if any. This requires to discriminate between real moving objects and noise or other artifacts on the detector.

The technical risk is that the search is negative.

This will happen if

• there are no objects to be found,
• the objects are near or above the limiting magntitude,
• the objects are there and are bright enough to be detected but on-board processing removes them from the images.

The scientific risk is that the search does not place a meaningful constraint on the existence of these asteroids.

Which spacecraft? STEREO A(head), STEREO B(ehind)

The twin STEREO spacecraft were launched aboard a single Delta II rocket from Cape Canaveral Air Force Base at 01:52 UT on 26 October 2007 (8:52 pm EDT on 25 October). Contact with STEREO-B was lost in 2014, but STEREO-A is still operational.

Which insturments? SECCHI Heliospheric Imagers (HI-1, HI-2)

SECCHI is a set of remote sensing instruments designed to follow Coronal Mass Ejections (CMEs) from their origins on the Sun, out through the corona and the interplanetary medium and to possible impact with the Earth. HI1 and HI2 are externally occulted coronagraphs that image the inner heliosphere between the Sun and the Earth between 12-215 solar radii.

One of the scientific goals of the instrument is to observe asteroids. Papers on asteroid observations by SECCHI HI:

• Phaethon: Jewitt & Li 2010; Jewitt et al. 2013; Li & Jewitt 2013; Hui & Li 2016
• Vulcanoids: Steffl et al, 2013

The HI detectors are CCDs with 2048 x 2048 pixels, where each pixel has a size of 13.5 x 13.5 μm. These are usually binned on board to 1024 x 1024 bins, resulting in image bin angular sizes of 70 arcsec and 4 arcmin, for HI-1 and HI-2 , respectively. Short exposures are taken and cleaned of cosmic rays on board, and a number of such exposures are then summed to produce an image to be down-linked.

HI-1 images are recorded in sequences of 30 exposures of 40 sec each. These are summed to produce a single image with 40 min cadence. HI-2 images are recorded in sequences of 99 exposures of 50 sec each. These are summed to produce a single image with 2 hr cadence. Exposure cadence is 60 sec. A total of 36 summed images for HI-1, and 12 for HI-2, are taken every 24 hr.

Cosmic ray rejection

The individual exposures are scrubbed to remove cosmic ray events. The algorithm used compares each new image with the previous image on a pixel-by- pixel basis – if the signal in a given pixel exceeds that in the previous image by more than a defined threshold, then the value for that pixel is replaced by the previous value. The threshold normally used is 5σ, where σ is the predicted noise standard deviation based on the number of photoelectrons detected.

In addition to the image processing described above for HI science operations, the SEB software also permits other types of images to be down-linked for calibration and engineering purposes, including (i) single full-resolution CCD exposures with 2048 x 2048 pixels, (ii) single 2 x 2 binned exposures, and (iii) exposures which still include the under-scan and over-scan regions. The SEB also produces more highly compressed and binned versions of the summed images which are downlinked in real-time via the STEREO Space Weather Beacon (see Section 9.4).

The HI-A door was opened on 13 December 2006, two days before STEREO-A lunar swing-by. The HI-B door was deployed on 11 January 2007. The science operations phase for HI commenced at the beginning of April 2007, although observations of scientific importance were made before this time

Technical Description of SECCHI HI-1 and HI-2: http://www.stereo.rl.ac.uk/Documents/firstHI_Instrument_Paper_Revised_v2.pdf

HI data analysis User Guide: https://www.stereo.rl.ac.uk/Documents/HI_user_guidejuly.pdf

How to browse data: https://stereo.nascom.nasa.gov/cgi-bin/images (browse images in jpg format, can choose the resolution from 128×128 to 2048×2048)

Retrieve data: https://stereo.nascom.nasa.gov/data/ins_data/secchi/L0/b/img/hi_2/
(example above for STEREO Behind Level 0 data from HI-2; data is sorted by date,so directory 20071105
contains data obtained on that date)

The data may also be retrieved from the JHU APL mirror site, e.g. http://sd-www.jhuapl.edu/secchi/lz/L0/b/img/hi_2

Other ways of retrieving the data

• Using wget and curl:

https://stereo-ssc.nascom.nasa.gov/ftp_deactivation.shtml

https://stereo.gsfc.nasa.gov/curl.shtml

• Through IDL, Using the SolarSoft (SSW) package

Example script:

files =VSO_SEARCH('YYYY-MM-DD hh:mm:ss','YYYY-MM-DD
hh:mm:ss',source='STEREO_A',instrument='SECCHI',detector='COR2')
(where the first date string is the start of the period of interest, and the second one is the end of the period of interest)

datafiles = VSO_GET(files, /FORCE, /QUIET)