Asteroid 2004 MN4 will come scarily
close to Earth on April 13, 2029, but it will not hit.
by Dr Tony Phillips
Friday the 13th is supposed to be
an unlucky day, the sort of day you trip on your shoe laces or lose
your wallet or get bad news.
But maybe it's not
so bad. Consider this: On April 13th - Friday the 13th - 2029,
millions of people are going to go outside, look up and marvel at
their good luck. A point of light will be gliding across the sky,
faster than many satellites, brighter than most stars.
What's so lucky about that? It's
asteroid 2004 MN4 ... not hitting Earth.
For a while astronomers thought
it might. On Christmas Eve 2004, Paul Chodas, Steve Chesley and
Don Yeomans at NASA's Near Earth Object Program office calculated
a 1-in-60 chance that 2004 MN4 would collide with Earth. Impact
date: April 13, 2029.
orbits of Earth and asteroid 2004 MN4.
The asteroid is about 320 meters
wide. "That's big enough to punch through Earth's atmosphere,"
devastating a region the size of, say, Texas, if it hit land, or
causing widespread tsunamis if it hit ocean, says Chodas. So much
for holiday cheer.
Asteroid 2004 MN4 had been discovered
in June 2004, lost, then discovered again six months later. With
such sparse tracking data it was difficult to say, precisely, where
the asteroid would go. A collision with Earth was theoretically
possible. "We weren't too worried," Chodas says, "but
the odds were disturbing."
This is typical, by the way, of
newly-discovered asteroids. Step 1: An asteroid is discovered. Step
2: Uncertain orbits are calculated from spotty tracking data. Step
3: Possible Earth impacts are noted. Step 4: Astronomers watch the
asteroid for a while, then realize that it's going to miss our planet.
Killer Asteroid! headlines
generally appear between steps 3 and 4, but that's another
Astronomers knew 2004 MN4 would
miss Earth when they found pictures of the asteroid taken, unwittingly,
in March 2004, three months before its official discovery. The extra
data ruled out a collision in 2029.
Instead, what we're going to have
is an eye-popping close encounter:
On April 13, 2029, asteroid 2004
MN4 will fly past Earth only 18,600 miles (30,000 km) above the
ground. For comparison, geosynchronous satellites orbit at 22,300
miles (36,000 km). "At closest approach, the asteroid will
shine like a 3rd magnitude star, visible to the unaided eye from
Africa, Europe and Asia--even through city lights," says Jon
Giorgini of JPL. This is rare. "Close approaches by objects
as large as 2004 MN4 are currently thought to occur at 1000-year
intervals, on average."
trajectory (blue) of asteroid 2004 MN4 past Earth on April
13, 2029. Uncertainty in the asteroid's close-approach
distance is represented by the short white bar.
The asteroid's trajectory will bend
approximately 28 degrees during the encounter, "a result of
Earth's gravitational pull," explains Giorgini. What happens
next is uncertain. Some newspapers have stated that the asteroid
might swing around and hit Earth after all in 2035 or so, but Giorgini
discounts that: "Our ability to 'see' where 2004 MN4 will go
(by extrapolating its orbit) is so blurred out by the 2029 Earth
encounter, it can't even be said for certain what side of the sun
2004 MN4 will be on in 2035. Talk of Earth encounters in 2035 is
In January 2004, a team of astronomers
led by Lance Benner of JPL pinged 2004 MN4 using the giant Arecibo
radar in Puerto Rico. (Coincidentally, the Arecibo dish is about
the same size as the asteroid.) Echoes revealed the asteroid's precise
distance and velocity, "allowing us to calculate the details
of the 2029 flyby," says Giorgini, who was a member of the
team along with Benner, Mike Nolan (NAIC) and Steve Ostro (JPL).
More data are needed to forecast
2004 MN4's motion beyond 2029. "The next good opportunities
are in 2013 and 2021," Giorgini says. The asteroid will be
about 9 million miles (14 million km) from Earth, invisible to the
naked eye, but close enough for radar studies. "If we get radar
ranging in 2013, we should be able to predict the location of 2004
MN4 out to at least 2070."
Arecibo radar in Puerto Rico is coincidentally about the
size of asteroid 2004 MN4.
The closest encounter of all, Friday
the 13th, 2029, will be a spectacular opportunity to explore this
asteroid via radar. During this encounter, says Giorgini, "radar
could detect the distortion of 2004 MN4's shape and spin as it passes
through Earth's gravity field. How the asteroid changes (or not)
would provide information about its internal structure and material
composition." Beautifully-detailed surface maps are possible,
The view through an optical telescope
won't be so impressive. The asteroid's maximum angular diameter
is only 2 to 4 arcseconds, which means it will be a starlike point
of light in all but the very largest telescopes.
But to the naked eye--wow! No one
in recorded history has ever seen an asteroid in space so bright.
Friday the 13th might not be so
bad after all.