Preimpact Detection of Chelyabinsk-Type Objects in the Thermal Infrared: Possibilities and Limitations

Abstract

The Chelyabinsk meteor entered Earth’s atmosphere on 15 February 2013, producing a shock wave that injured about 1500 people and damaged thousands of buildings. Despite its relatively large size (∼20 m), the progenitor asteroid approached Earth undetected. Its apparent radiant was too close to the Sun for standard ground-based near-Earth asteroid (NEA) surveys operating in the visible light. In addition, it would have been very faint due to an observing geometry at a large phase angle, and very fast moving. We examine the potential for early detection with current and upcoming infrared (IR) space telescopes, such as NASA’s upcoming NEOSurveyor mission and ESA’s planned NEOMIR mission. We use the 20-m Chelyabinsk progenitor to demonstrate detection possibilities and limitations of an object on a day-side trajectory before impact. IR observations from space offer key advantages like an enhanced Sun-asteroid contrast (compared to visible wavelengths). The small, fast-rotating objects are (nearly) isothermal which make IR detections at high phase angles easier compared to visible-light ones, and allow for radiometric size estimation. The latter is crucial for immediate assessment of the impact risk. The Chelyabinsk asteroid would have entered the field-of-regard about 39 h (NEO Surveyor) or 54 h (NEOMIR) before impact. However, we find that a 20-m object on a Chelyabinsk progenitor orbit could be detected theoretically with a 0.5-m telescope in space (located at the Lagrangian point L1), at mid-IR wavelengths, with a lead time of 5–12 days. The large uncertainty in the calculation of the detection lead-time is mainly related to uncertainties in the flux predictions for small, possibly fast-rotating asteroids seen under very extreme phase angles. However, technical challenges, including detector operations at high sky background due to the low solar elongation, telescope straylight problems for observations close to the Sun, near real-time application of synthetic tracking techniques, and fast orbit determination also must be overcome to achieve reliable early warning capabilities.