Getting warmer: Spotting Near-Earth Asteroids by heat signature

New technique to spot Near-Earth Asteroids using their infrared emissions has been revealed by NASA researchers at the 2019 APS April meeting

On February 15th, 2013, an object broke up in the sky over the Russian city, Chelyabinsk. The blast — detected as far away as Antartica — was more powerful than a nuclear explosion, 25 to 30 times more powerful. It shattered windows and injured approximately 1200 people. In fact, the blast was so intensely bright that it may well have briefly outshone the Sun.

Chelyabinsk fireball recorded by a dashcam from Kamensk-Uralsky north of Chelyabinsk where it was still dawn. (Planetary Society Institute)

The main concern about the Chelyabinsk event is that the meteor involved — which broke off from a larger asteroid —relatively small — with a diameter of 17–20 m. There are many, much larger objects out there. Knowing exactly where would be of great advantage.

The responsibility of locating such objects in Earth’s proximity — Near Earth Objects (NEOs) and the question of how to prevent an impact are being investigated by Amy Mainzer and her colleagues at NASA’s asteroid-hunting mission at the Jet Propulsion Laboratory in Pasadena, California. They have devised a simple yet ingenious way to spot NEOs as they hurtle toward the planet.

This is a collection of images from the WISE spacecraft of the asteroid 2305 King, which is named after Martin Luther King Jr. The asteroid appears as a string of orange dots because this is a set of exposures that have been added together to show its motion across the sky. These infrared pictures have been colour-coded so that we can perceive them with the human eye: 3.4 microns is represented as blue; 4.6 microns is green, 12 microns is yellow, and 22 microns is shown as red. From the WISE data, we can compute that the asteroid is about 12.7 kilometres in diameter, with a 22% reflectivity, indicating a likely stony composition (NASA)

Mainzer, who is the principal investigator of the mission outlined the work of NASA’s Planetary Defense Coordination Office at the American Physical Society April Meeting in Denver — including her team’s NEO recognition method and how it will aid the efforts to prevent future Earth impacts.

Mainzer says: “If we find an object only a few days from impact, it greatly limits our choices, so in our search efforts we’ve focused on finding NEOs when they are further away from Earth, providing the maximum amount of time and opening up a wider range of mitigation possibilities.”

You’re getting warmer!

Locating NEOs isn’t an easy task. Mainzer describes it as like trying to spot a lump of coal in the night sky.

She elaborates: “NEOs are intrinsically faint because they are mostly really small and far away from us in space.

“Add to this the fact that some of them are as dark as printer toner, and trying to spot them against the black of space is very hard.”

This is an image of the proposed Near-Earth Object Camera (NEOCam) mission, which is designed to find, track and characterize Earth-approaching asteroids and comets. Using a thermal infrared camera, the mission would measure the heat signatures of NEOs regardless of whether they are light or dark coloured. The telescope’s housing is painted black to efficiently radiate its own heat into space, and its sun shield allows it to observe close to the Sun where NEOs in the most Earth-like orbits spend much of their time. In the background is a set of images of main belt asteroids collected by the prototype mission NEOWISE; the asteroids appear as red dots against the background stars and galaxies. (NASA)

Instead of using visible light to spot incoming objects, Mainzer and her team at JPL/Caltech worked instead with a characteristic trait of NEOs — their heat.

Asteroids and comets are warmed by the sun and so glow brightly at thermal — infrared — wavelengths. This means they are easier to spot with the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) telescope.

Mainzer explains: “With the NEOWISE mission we can spot objects regardless of their surface colour, and use it to measure their sizes and other surface properties.”

Discovering NEO surface properties provides Mainzer and her colleagues an insight into how big the objects are and what they are made of, both critical details in mounting a defensive strategy against an Earth-threatening NEO.

For instance, one defensive strategy is to physically “nudge” an NEO away from an Earth impact trajectory. The thing is, to calculate the energy required for that nudge, details of NEO mass, and therefore size and composition, are crucial.

The NEOWISE space telescope spotted Comet C/2013 US10 Catalina speeding by Earth on August 28, 2015. This comet swung in from the Oort Cloud, the shell of cold, frozen material that surrounds the Sun in the most distant part of the solar system far beyond the orbit of Neptune. NEOWISE captured the comet as it fizzed with activity caused by the Sun’s heat. On November 15, 2015, the comet made its closest approach to the Sun, dipping inside the Earth’s orbit; it is possible that this is the first time this ancient comet has ever been this close to the Sun. NEOWISE observed the comet in two heat-sensitive infrared wavelengths, 3.4 and 4.6 microns, which are colour-coded as cyan and red in this image. NEOWISE detected this comet a number of times in 2014 and 2015; five of the exposures are shown here in a combined image depicting the comet’s motion across the sky. The copious quantities of gas and dust spewed by the comet appear red in this image because they are very cold, much colder than the background stars. (NASA)

Examining the composition of asteroids will also help astronomers to understand how the circumstances under which the solar system was formed.

Mainzer says: “These objects are intrinsically interesting because some are thought to be as old as the original material that made up the solar system.

“One of the things that we have been finding is that NEOs are pretty diverse in composition.”

Mainzer is now keen to utilise advances in camera technology to assist in the search for NEOs. She says: “We are proposing to NASA a new telescope, the Near-Earth Object Camera (NEOCam), to do a much more comprehensive job of mapping asteroid locations and measuring their sizes.”

Of course, NASA is not the only space agency trying to understand NEOs — the Japan Aerospace Exploration Agency’s (JAXA’s) Hayabusa 2’s mission plans to collect samples from an asteroid. In her presentation, Mainzer explains how NASA works with the global space community in an international effort to defend the planet from NEO impact.