October 30, 2015

Studying Close Approaches of Near-Earth Asteroids

There is a lot of buzz when an asteroid flies by Earth.

You might read a news stories like “NASA Rules Out Earth Impact in 2036 for Asteroid Apophis”, or “Trillion-Dollar Asteroid’ Zooms by Earth as Scientists Watch”.

Organizations like the Catalina Sky Survey, NASA, and Minor Planet Center have vast databases full of information from these fly-bys, tracking around 600,000 asteroids across the Solar System at the time of this post. This database, which grows every day with newly-discovered asteroids, contains information that allows NASA and scientists to determine whether an asteroid is a potential hazard to Earth, or an interesting target for scientific analysis.

Here at Planetary Resources, we utilize this data to determine if a particular near-Earth asteroid is a valuable target for our asteroid prospecting missions. In our recent blog, you can read about what factors we consider when adding a near-Earth asteroid to our target list: “How We Choose our Near-Earth Asteroid Targets”.

What is a near-Earth Asteroid?

Near-Earth asteroids are in a different class than main belt asteroids, as they are much closer energetically to Earth.

There are three main orbits of near-Earth asteroids: Amor, Aten, and Apollo.

There are three main orbits of near-Earth asteroids: Amor, Aten, and Apollo.

Most intersect with the Earth’s orbit at some point during their trip around the sun, making this the prime time to analyze them with a telescope, or even rendezvous with them on a prospecting mission with our Arkyd spacecraft.

Sometimes one of these asteroids comes a bit too close for comfort, which is why humans have been tracking hazardous asteroids since the 1990s. You can find a known list of these in the JPL Near-Earth Object Database.

Famous near-Earth Asteroids

99942 Apophis

Apophis is one of the most infamous near-Earth asteroids in recent memory, as scientists initially thought it was on a deadly collision course with Earth. It was not until February 2013, when it was observed with higher resolution, that the calculation of its orbit was refined. Fortunately, it will not be impacting Earth in 2036 (Phew)!

25143 Itokawa

The Hayabusa mission, led by the Japanese Space Agency (JAXA), was launched in 2003 and rendezvous with asteroid Itokawa in 2005. In 2007, the spacecraft brought samples of the asteroid back to Earth for scientific analysis.

1999 JU3

In 2014, JAXA launched the second of its asteroid sample return missions, Hayabusa 2. This mission is on a 6 year round trip to near-Earth asteroid 1999 JU3, which is a valuable mining target on our list. Being a large carbonaceous asteroid, the amount of water extracted from 1999 JU3 could be turned into enough rocket fuel to fuel every one of the 135 space shuttle missions.

Gathering Data of Close-Approaching Asteroids

There are a few ways to gather the data that scientists, explorers, and asteroid hunters are interested in.

Earth-Based Telescopes:

When you do a telescopic survey, you take images of the night sky, and identify objects moving within frame that are of obvious difference to the stars.

It may seem like an unimpressive image, but the amount of information you can get about a certain asteroid is remarkable! From this data, we can discover:

  • The surface composition: is it rock, carbonaceous material, or metallic?
  • The spin rate: is it spinning like a top (like THIS asteroid), or fairly stable?
  • The size: how many meters long, and what is the diameter?
  • The shape: is it round, oval, rocky, or smooth?
  • The internal structure: is it solid or loose, like a rubble pile?
  • The orbit: what are the origins of the asteroid? Did it come in from the inner main belt, the outer main belt, or is it a dead comet?

Earth-Based Radio:

A radio survey works much like the echolocation of a bat. By bouncing waves off of an asteroid and listening for what reflects back, Radar analysis can help to determine the diameter of the asteroid, as well as a shape model.

Close approaches are the only time that these radar scans can be performed. Two entities who do this today are Goldstone (which is a part of NASA JPL) and Arecibo Observatory in Puerto Rico. These radar systems can tell us a lot, but the asteroid needs to be very close to the Earth and have a very well known orbit in order to be targeted for radar observations.


WISE, or the Wide-field Infrared Survey Explorer, is a tool used to look at the actual thermal emission of an asteroid as opposed to the reflected sunlight (like optical telescopes rely on), so you can more accurately determine the size. Knowing the size allows you to then determine albedo and absolute magnitude, which are very important factors in determining the makeup, size, orbit and density of an asteroid.

Spectrum Analysis

Sometimes scientists dedicate research to solving specific answers about a near-Earth object. In cases like this, spectrum analysis can be used.

The main thing you discover through this analysis is the Absolute Magnitude (H) of the asteroid. You need both the measured brightness and the distance to calculate absolute magnitude (H).

What is Absolute Magnitude (H)? The Key Factors of Asteroid Analysis.

Knowing the Absolute Magnitude (H) of an asteroid can tell us how large it is and its potential composition. This data is critical for our asteroid prospecting missions, as it tells us if the target is worth investing in a prospecting mission.

Absolute magnitude (H) has 2 factors: reflectivity (albedo), size (reflective properties).

For a summary of the Absolute Magnitude (H) equation and calculator click HERE, and JPL also has a nice explanation HERE.

Often times, you may not have all the data for a given near-Earth asteroid to determine the absolute magnitude, but knowing part of the equation can help you determine the missing pieces.

How to get the Size: Size can be determined by radar observation and occultation.

Here is an example from a radar image of an asteroid, taken from an observation done by Goldstone’s Solar System Radar on December 12th and 13th of 2012.


Image Credit NASA JPL: https://neo.jpl.nasa.gov/images/toutatis.html

You can watch the full video of Toutatis from NASA/JPL here.

When calculating size with Occultation, astronomers from all over the world are told about the opportunity to provide an occultation observation from different points on Earth. Below is a chart showing the data from these astronomers. Each line represents the location where the asteroid goes in front of a star, and you watch the star to measure when the star blinks out, and then when it comes back. Thus, producing these lines which give us the size of the object. Here is an occultation observation from asteroid Tercidina.

Image Credit: https://www.euraster.net/results/2002/20020917-Tercidina-crd.gif

Image Credit: https://www.euraster.net/results/2002/20020917-Tercidina-crd.gif

On this website you can join in with the thousands of astronomers on these types of observations, and understand more about what you need in order to make an occultation observation.

Albedo: Albedo is the determination of the type of the asteroid, based on the reflectivity of light emitted from the sun off the surface of the object.

It is a tough measurement to get from Earth-based telescopes, and it’s typically the unknown factor in the Absolute Magnitude (H) equation. The WISE telescope is one of the best sources for determining Albedo from the Earth’s surface. Since it can look at the asteroid in the infrared, it can see a more accurate view of its relative brightness. It will take this infrared data, and compare it to the point of light observation (what we see with our own eyes), and can come up with a range for Albedo which helps narrow the uncertainty of the asteroid’s composition.

In this infographic from NASA/JPL, you can learn about this calculation.

Image Credit: https://www.nasa.gov/mission_pages/WISE/multimedia/gallery/neowise/pia14733.html

Image Credit: https://www.nasa.gov/mission_pages/WISE/multimedia/gallery/neowise/pia14733.html

Using the Albedo range, you can determine the expected composition of the asteroid:

Albedo = 5% (darker than asphalt on the road)

Probable type: C Type, Carbonaceous Chondrite. Planetary Resources first targets are C-Types as we can create rocket fuel, consumable water, and radiation shielding with the H2O we extract from the asteroid. Read more on this in our blog “The Trillion Dollar Market: Fuel in Space from Asteroids”

Albedo = 15- 25% (the color of a stainless steel kitchen appliance)

Probable Type: X Type, Metallic. X-types have dense quantities of rare-Earth metals or precious metals like iron, nickel and platinum. Planetary Resources is planning missions to X-Type asteroids on our target list, and will extract metals from them for building structures in space.

Albedo = 15 – 35% (light colored stones)

Probable Type: S Type , Stony. These are not super interesting targets for Planetary Resources, as they would not produce useful materials like metals or water in large quantities, but can be intriguing to visit for scientific analysis.

How do we know where the asteroid is going? Determining Orbit.

Often times when you hear probability statement about “if an asteroid will impact Earth or not” it’s based on ongoing observations of the asteroids orbit. In order to narrow the uncertainty of the orbit around a particular asteroid, you must observe it more than once to plot its definite trajectory as its orbits around the Sun. The more it is observed, the more accurate your orbit prediction path is.

This happened with Apophis. At first, scientists thought it was almost definite to impact Earth in 2036. But after more observations from astronomers were logged, the orbit trajectory was more well known, and it was concluded that while Apophis will come very close to Earth in 2036 – it will not impact us.

As we continue to learn more about near-Earth asteroids through these observations, our list of prospecting targets will grow, more scientific and exploration targets will be found, and new data on how to defend Earth will be discovered. All good reasons to keep looking up!

-Caitlin O’Keefe, Director of Marketing