Detecting asteroids by Ground-based telescope

Asteroids are constantly entering Earth’s atmosphere due to earth strong gravitational field, we need to be able to detect these planetary objects for us to predict whether these asteroids will be a potential threat to Earth. Telescopes will help us observe and detect these asteroids!

Ground-based telescopes

Optical telescope

A telescope uses mirrors or lenses to reflect or refract light to a single point, called the focal point, forming an image. An eyepiece is then used to view the image. They can be used to make very far away objects appear larger. There are 3 main types of optical telescopes

1. Refracting telescope

Let's start with the main function of these telescopes, they use the lens to bend light to a focal point, which helps us sharpen the image. The primary lens aka the objective lens collects light and focuses it as the lens bends the light waves so that they can meet at the focal point. Lastly, the eyepiece uses a smaller lens to magnify the image formed. This telescope comes with its advantages and disadvantages such as the simple design, high-quality images and menial maintenance. However, the images captured by this telescope face colour distortion (chromatic aberration), which is the failure of a lens to concentrate all colours to the same point, resulting in colour fringing and a blurred image.

Inside a refracting telescope

2. Reflecting telescope

These telescopes make use of mirrors to reflect light to a focal point, which helps us sharpen the image. Similar to the function of a refracting telescope, this telescope consists of a primary mirror that collects light and focuses it. Then, the secondary mirror reflects the focused light to an eyepiece or detector. These telescopes come with their advantages and disadvantages. For example, they do not suffer from chromatic aberration and are cost-effective, allowing a wide range of people to use them. However, they require regular alignment and maintenance to ensure optimal performance

Inside a reflecting telescope

3. Catadioptric Telescope

A combination of lenses (refractive elements) and mirrors (reflective elements) is used to form an image. This design effectively corrects optical aberrations and improves image quality. These telescopes have several advantages, including a compact design and a wide field of view, making them suitable for observing a variety of astronomical objects. The use of lenses and mirrors helps minimize optical aberrations and enhances image quality, allowing us to use these telescopes for various functions like visual observations and astrophotography. However, these telescopes come with disadvantages such as high cost and weight. Additionally, they require a longer cooling time to reach thermal equilibrium compared to simpler designs, which can affect image quality until the telescope temperature stabilizes.

Inside a catadioptric telescope

What is the process by which these telescopes detect asteroids?

Detection:

The detection of asteroids can be done in three processes; sky surveying, imaging and blinking comparison.

Sky surveying involves large optical telescopes that are equipped with wide-field cameras to conduct systematic surveys of the sky and capture images of large areas over time. Then the imaging processes come into place as the movement of asteroids can be observed when observing the same region of sky multiple times with the telescope because points of light representing asteroids differ significantly from the positions of stars. In contrast, the stars remain roughly the same in the background. Lastly, the blinking comparison allows us to easily discover asteroids, as this technique involves a rather fast change of images, taken at different time intervals. When objects like asteroids which are translated in space will appear to shift, their position will be easier to discover.

Tracking:

Tracking the asteroids is a vital part of the discovery of the NEO as we might never know whether it can collide with Earth if they aren’t tracked.

Follow-up observations are essential in the process of tracking asteroids. The next step if an asteroid has been spotted is to watch it with optical telescopes to find out how it moves. Reasoning from such observations, one can correct the asteroid’s orbit, which allows for estimating its further positions. Astrometric Measurements allow precise measurements of the asteroid’s position relative to background stars (astrometry) that are made over time. This data is used to calculate the asteroid’s orbit and predict its trajectory. Lastly, long-term monitoring and continuous monitoring of asteroids ensures that their orbits are well understood. This is particularly important for near-Earth objects (NEOs) that could pose a threat to Earth.

Characterization:

Light Curves are created by astronomers by measuring the brightness of an asteroid over time, the variations in brightness can reveal information about the asteroid’s rotation period, shape, and possible binary nature. Spectroscopy helps us characterize these NEOs, the optical telescopes equipped with spectrographs can analyze the light reflected from an asteroid. The resulting spectrum can provide information about the asteroid’s composition and surface properties. Then by observing asteroids through different optical filters, astronomers can determine colour indices, which give clues about the asteroid’s surface composition and weathering processes.

What are the important tools and techniques that allow us to predict and observe NEOs

• Wide-Field Cameras: These cameras can capture large portions of the sky in a single exposure, making them ideal for survey programs that aim to detect new asteroids
• Automated Detection Software: Advanced software algorithms can automatically detect moving objects in images, greatly speeding up the process of identifying new asteroids.
• Collaboration and Data Sharing: Observatories around the world share data and collaborate on follow-up observations, ensuring that newly discovered asteroids are quickly tracked and characterized

Key Programs and Telescopes that have helped us detect NEOs:

• Catalina Sky Survey (CSS): A leading program in the discovery and tracking of near-Earth objects, utilizing optical telescopes to scan the sky
• Pan-STARRS: Uses a wide-field optical telescope to survey the sky for moving objects, significantly contributing to asteroid discoveries.
• ATLAS: Designed to detect asteroids that could impact Earth, providing early warning of potential threats through systematic sky surveys.

Infrared telescope

Infrared telescopes are used to observe objects in the universe that emit infrared radiation, a type of electromagnetic radiation with wavelengths longer than visible light but shorter than microwaves.

NASA’s infrared telescope facility

Purpose of IR telescope

• Detecting Cool Objects: Infrared telescopes can detect objects that emit little to no visible light but radiate heat. This includes many asteroids that are dark and not easily seen with optical telescopes
• Penetrating Dust Clouds: Infrared light can penetrate dust clouds that block visible light, allowing astronomers to observe objects hidden within these clouds.
• Observing Redshifted Light: Infrared telescopes can observe light that has been red-shifted into the infrared spectrum due to the expansion of the universe, making them useful for studying distant objects.

How do Infrared telescopes function?

• Light Collection: Infrared telescopes use mirrors to collect and focus infrared light from celestial objects.
• Cooling Systems: Detectors are cooled to very low temperatures using liquid helium or mechanical cryocoolers to reduce thermal noise and improve sensitivity
• Infrared Detectors Specialized: Detectors, such as mercury cadmium telluride (HgCdTe) or indium antimonide (InSb), convert incoming infrared radiation into electronic signals.
• Reflective Optics: Mirrors reflect infrared light to minimize absorption and heating of the optical elements, ensuring clear signals.
• Filters and Spectrometers: Filters isolate specific wavelengths, and spectrometers analyze the spectral properties of the observed objects.

How do these IR telescopes detect asteroids?

Detection:

Thermal Emission — Asteroids, especially those with dark surfaces, emit infrared radiation due to their temperature. Infrared telescopes can detect this thermal emission, identifying asteroids that are not easily visible in optical wavelength

Characterization:

Size and Composition — Infrared observations provide accurate estimates of an asteroid’s size by measuring its thermal emission. They also reveal surface properties and composition

Albedo Determination — By comparing infrared and visible light measurements, astronomers can determine an asteroid’s albedo (reflectivity), helping to classify its surface material

Tracking:

Infrared telescopes track asteroids over time, refining their orbits and predicting future positions. This is particularly useful for near-Earth objects (NEOs) that may pose a threat.

Conclusion

Ground-based telescopes are useful in tracking asteroids, but space-based telescopes are more accurate due to less atmospheric disturbance. How do they work? Find out in the next article!