Near-Earth objects (NEOs) are of interest to us for several reasons. One is a matter of safety: if an asteroid or comet were to impact the Earth, the consequences could range from local damage to global catastrophe, depending upon the size of the object and the velocity of impact. Another reason for our interest in these has to do with future human exploration: a human mission to a nearby asteroid could be a dress rehearsal for a later trip to Mars.
Many of these objects have already been found. NASA is actually under a congressional mandate to catalogue all NEOs larger than a kilometer (0.6 miles) in diameter, because of their potential as impact hazards. As of August 2011, a total of 828 Near-Earth asteroids (rocky bodies) had been found. You may see larger numbers cited in some places, but these are based on earlier estimates of the average reflectivity of these objects. Stay with me on this:
• Improved estimates for the average reflectivity of these asteroids have shown them to be more reflective than first thought.
• If we measure the brightness of these objects, and we know their reflectivity, we can then determine their size.
• If they reflect more light than we thought, then they don’t have to be as large as we first thought to appear as bright as they do.
• Higher reflectivities result in smaller sizes.
Asteroid 2010 TK7
All of this is preface to the announcement of an object well below this one-kilometer size threshold: asteroid 2010 TK7, whose discovery was announced in the last few days and whose diameter is estimated at 300 meters. Why so much interest in this particular asteroid?
This is the first Trojan asteroid discovered for the Earth. These asteroids occupy gravitational “sweet spots” near the Earth, specifically ones that precede and follow the Earth in its orbit by 60°. Jupiter, our largest planet, has over 4000 of these.
You can see that these objects are spread out so they are not exactly 60° ahead of and behind the planet. If one were to “freeze” Jupiter in place and show only the motions of these asteroids, you would see some of them move around one point 60° in front of Jupiter and others around another point 60° behind it. What is at these points? Nothing—they are just gravitationally stable locations in the Sun-Jupiter-asteroid system.
Until now, no Earth Trojans had ever been discovered.
2010 TK7’s orbit is complex. Let’s look at it from several perspectives. First, let’s “freeze” the Earth in its orbit and show only the motion of the asteroid. This first image looks down on the Earth’s orbit from above the north pole.
A second image comes from a different perspective. Here we see the Earth and the path of 2010 TK7 above and below the path of the Earth as seen from the sun. Think of the path that the asteroid traces out as covering the surface of an eggshell moving with the Earth as they both orbit the sun.
And it’s even more complicated than that! The asteroid’s position drifts to a point all the way on the other side of the sun from Earth, then drifts back to its current location near us.
What does it look like when you “unfreeze” the motion of the Earth? If you have Java installed on your computer (if you don’t, it’s a free download here: http://java.com/en/download/manual.jsp), you can set both the asteroid and the planets in motion: http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=3548081;orb=1;cov=0;log=0;cad=0#orb
This is a very cool tool! Play with it—here are some suggestions.
1. Slide the right hand scroll bar all the way to the bottom to look down on the orbits from above the north pole.
2. Slide the zoom scroll bar to zoom in just enough so that the asteroid’s orbit nearly fills the screen.
3. Set the interval to 1 day, and click the >> symbol above it to start the animation.
4. While the animation is running, slide the right hand scroll bar halfway up to see the orbits from a different angle, and to see how the asteroid’s orbit is tilted relative to the Earth’s.
5. Play on your own!
Can we go there?
Despite its proximity, 2010 TK7 is not a good candidate for a near-term visit from astronauts because of its high orbital inclination. This means its velocity relative to the Earth is fairly high. The difficulty of getting to some place from the Earth is determined by the ΔV (delta V; change in velocity) required. From low Earth orbit, the requirements to reach each of these solar systems bodies are:
Moon: 6 km/s (kilometers per second)
Mars: 9 km/s
2010 TK7: 9-10 km/s
More typical Near-Earth asteroid: 4 km/s
Of course, energy requirements are not the only factor in human exploration! Mars is not just 50% harder to get to than the moon—the moon is three days away and Mars is at least six months away. A NASA proposal to visit the Near-Earth asteroid 1999 AO10 in 2025 would require a total mission time of five months.
Is it a danger to us?
In a word: no. It’s not on a collision path with us. Even when it comes closest to us, it is generally far above or below the Earth’s orbit. If it did collide with us, a 300-meter diameter rocky body could do serious damage. Just how much damage would depend on its impact velocity, but suffice it to say that an impact over Richmond would devastate Lynchburg. The Near-Earth asteroid with the highest current probability of impact is 1950 DA, which may strike the Earth in 2880. The difficulty of predicting orbits that far into the future, however, could easily lower the probability well before then. In any case, if we haven’t developed the capability to divert such objects by then, we deserve to be hit!