The asteroid 4660 Nereus will make its closest approach to Earth for decades this Saturday. At 3.9 million kilometers (2.4 million miles), the passage is not particularly near – three months ago an asteroid about half the size came twice as close. What makes this visit stand out is that Nereus is considered one of the best asteroid targets for humans to visit, and possibly even to mine – it's estimated worth is almost $5 billion. It will be visible in backyard telescopes, for those under dark skies, and in the Northern Hemisphere.
Nereus was discovered in 1982. In 2003 observations it was measured at 330 meters (1,082 Feet) in diameter, a figure whose close resemblance to the height of the Eiffel tower – 324 meters (1,063 feet) – is frequently mentioned. It’s also about the same size as the asteroid 99942 Apophis, considered the greatest known (although still very low) threats to collide with the Earth. However, asteroids this size are far from perfectly round and Nereus is thought to be 510 meters (1,670 feet) along its longest axis, although this passage may give us a more precise figure.
Consequently, Nereus could do considerable damage if it hit the Earth, while being far short of a “dino-killer”. Since its orbit crosses ours – passing from a little closer to the Sun than us to beyond the orbit of Mars – there is a genuine possibility of eventual collision, earning it the title PHA (Potentially Hazardous Asteroid). However, any such threat is very distant in time – the nearest pass in the next two centuries is in 2060 when Neurus will still be three times further away than the Moon.
Despite the greater distance, Nereus’s low gravity and Earth-like orbit mean it would require less change of orbital velocity, and therefore less fuel, to reach it from low Earth orbit. Most other asteroids that share this property are so small there isn’t much interest in applying this information. Among other things, most don’t contain enough valuable material to be of interest to space miners.
That’s not the case for Nereus. The Hayabusa spacecraft originally planned to visit Nereus before settling on Itokawa. The private Near-Earth Asteroid Prospector project had Nereus among its prime targets had it achieved the funding to fly.
The Asterank database contains 600,000 asteroids with estimates of the factors relevant for mining them. On a “cost-effective” basis, Nereus comes in third. Its suspected $4.7 billion worth of iron, nickel, and most importantly cobalt is far behind such relative giants as 16064 Davidharvey ($54 trillion). However, ease of access would more than make up for that for operations targeting smaller and more convenient amounts of minerals. Close passages give us a chance to study Nereus better to refine those estimates, as well as assess its orbit in more detail for long-term threat potential. Nereus passed at only slightly greater distances in 2002, 1982, and 1961.
The name comes not from any misspelled association with brain cells, but from a sea-god, father to the Nereids.