The Near-Earth Object Camera (NEOCam) is part of a proposed NASA mission to find potentially hazardous asteroids. In a Q&A with Photonics & Imaging Technology, NEOCam principal investigator Amy Mainzer ex plains how the NEOCam chip, a stamp-sized mega pixel infrared sensor, detects the faint heat emitted by near-Earth objects circling the Sun.
Photonics & Imaging Technology: What is NEOCam?
Amy Mainzer: NEOCam is designed to carry out a comprehensive survey of the asteroids and comets in the inner solar system in an effort to understand their origins, evolution, and frequency with which they interact with Earth. NEOCam is an infrared space telescope located at the Sun-Earth L1 Lagrange point, which is a semi-stable region of space about five lunar distances away from Earth. By using a very-wide-angle imager operating at infrared wavelengths, NEOCam quickly and efficiently discovers and tracks asteroids and comets. In addition, it measures their sizes and reflectivities, allowing us to probe how they migrate through the solar system.
P&IT: What kind of sensor does the NEOCam use?
Mainzer: The “breakthrough” that NEOCam relies on is a modification of an existing infrared sensor technology. NEOCam uses mercury-cadmium-telluride (HgCdTe) camera chips that have been optimized to respond to the long infrared wavelengths at which the asteroids and comets are brightest. Thanks to NASA’s investments in this long-wavelength HgCdTe technology, we have been able to produce megapixel versions of these camera chips that have high operability and exceed NEOCam’s requirements.
P&IT: What are the strengths of HgCdTe?
Mainzer: Traditionally, detectors operating at longer infrared wavelengths have had to be kept extremely cold to work with low noise — close to 7-8 Kelvin. But HgCdTe arrays do not need to be brought to such cold temperatures to operate. The NEOCam arrays operate between 35-40 Kelvin, temperatures that are achievable through passively sitting in cold space with an appropriately designed thermal shielding system. This warmer operating point reduces complexity and cost, and increases mission lifetime.
P&IT: How was the infrared sensor modified from the existing sensor?
Mainzer: The infrared sensor we use has a light-sensitive detector layer made of HgCdTe bonded to a Teledyne HAWAII readout integrated circuit. Teledyne HAWAII arrays are widely used in astronomy applications on the ground and in space; the Wide-field Infrared Survey Explorer (WISE), Orbiting Carbon Observatory 2 (OCO-2), and Hubble Space Telescope missions use them, among others. The wavelength of light that the HgCdTe detector layer senses depends on the ratio of Hg to Cd in the material. For NEOCam, we have modified the ratio slightly so that the detectors are sensitive to 10-micron infrared light, because this is the wavelength at which Earth-approaching asteroids are particularly bright.
P&IT: What is the NEOCam designed to track?
Mainzer: NEOCam is optimized for finding, tracking, and characterizing Earth-approaching asteroids, termed near-Earth objects (NEOs). Because NEOs in Earth-like orbits tend to spend much of their time at similar distances to the Sun as the Earth, they are roughly room temperature (300 Kelvin). The Planck equation therefore dictates that much of the energy they absorb from the Sun is reradiated at approximately 10 microns. This is why it’s so important to have a detector capable of working at these wavelengths — the asteroids are very bright around 10 microns. By surveying with infrared light, we are sensitive to asteroids regardless of whether they are highly reflective or extremely dark.
P&IT: What are the specific characteristics of asteroids and comets that the sensor finds?
Mainzer: Asteroids and comets stand out brightly at infrared wavelengths due to their temperatures. We leverage this, plus the fact that they move, to pick them out from background sources such as stars and galaxies, and other transient sources such as cosmic rays.
P&IT: How do you ensure accurate detection of an asteroid?
Mainzer: To reliably detect an asteroid, we need to see repeated observations of it that are consistent with orbital motion. We also carefully characterize the cosmic rays and other image artifacts in the instrument, and we use visual inspections. This process is possible because we send back the entire image captured by the detectors. That allows us to apply finely-tuned software to pick out very faint moving objects. NEOCam is also designed to detect the dust and gas emitted by comets. This allows us to track their history of activity and probe the types and amounts of ices present.
P&IT: What are the challenges of sensing asteroids and comets?
Mainzer: Generally, the objects we seek are faint and spend much of their time in parts of the sky that are close to the Sun, since they are scattered around Earth’s orbit. So we need to be able to search a large area of the sky with great sensitivity. The observational cadence is key for detecting the objects reliably and determining their orbits. The repeated observations are essential for picking them out from the background and for predicting where they are going to go.
Fortunately, our team has had extensive experience at finding asteroids and comets with the WISE/NEOWISE mission. We have learned a lot about using a space-based infrared telescope to discover many objects quickly. We’ve discovered about 34,000 new asteroids (including about 200 NEOs) to date with this [WISE/NEOWISE] mission. While most of these are in the main asteroid belt between Mars and Jupiter, WISE/NEOWISE serves as a prototype for NEOCam, which is a much more capable mission with regards to asteroid discovery.
P&IT: Is the NEOCam purely theoretical right now? What has been developed and tested?
Mainzer: NEOCam is in the second step of a two-part NASA competition. The proposal we are working on now is due August 15, 2016. NASA plans to select at least one mission for flight from this competition. If selected, NEOCam would launch in 2021, depending on the schedule NASA prefers.
Our team has been working on NEOCam for over a decade. We not only have a mature spacecraft and mission design, but we also have designed, tested, and delivered a number of detectors that exceed NEOCam’s mission requirements.