September 24, 2000 Laurel, Maryland – Scientists at the Near Earth Asteroid Rendezvous (NEAR) mission in Johns Hopkins University’s Applied Physics Laboratory are recommending to NASA that the NEAR satellite now orbiting asteroid 433 Eros dive down to within five kilometers (3 miles) of the asteroid surface in late October. The South Pole of Eros, which has been pointed away from the sun until now, will be lighted and is the likely target area. There are also many features on Eros that scientists would like to see more closely, including ridges, troughs, large boulders and craters that are square. Detailed survey data has been published inScience and one of the puzzles for geologists and astronomers are many loose boulders laying around on the surface. Eros is a relatively small mass, so it was logical to expect a smoother surface from which debris would have been knocked off into space over its 4.5 billion year evolution from the beginning of the solar system.
In the montage photos above, troughs (top and bottom left) are found within the saddle region of Eros and are very closely aligned. The top right image shows a chain of pits. The pits might result from loose soil falling into cracks on the asteroid’s surface. In the center right photo, a ridge winds from the Eros saddle around the north pole. The center photo shows the north pole ridge coming up from the bottom and splaying out into many small fractures. The bottom right image shows grooves on a relatively smooth area that are evenly spaced.
Geologists say the “likely reason for square craters is that Eros had some surface fractures that were present before the impact events that formed the craters. The result is that the crater rims that run parallel to the fractures are straighter than craters located in nonfractured areas.” On Earth, Arizona’s Barringer Meteor crater is a similar example in which the rim is slightly squared off.
If NEAR is lowered in October to the closest approach any human spacecraft has ever come to an asteroid, the camera and satellite will take pictures with computer instructions designed by engineer Ann Harch at Cornell University in Ithaca, New York.
Ann Harch, Engineer and NEAR Mission Support Specialist, NEAR Project, Cornell University Space Sciences, Ithaca, New York:
“HOW CLOSE TO EROS WILL THE NEAR SATELLITE GO?
The closest we will be on this first low altitude flyover will be about 5 kilometers and that means we will be able to see objects as small as about a meter across.
HAVE WE EVER BEEN IN THIS CLOSE TO AN ASTEROID BEFORE?
No, we’ve never been in this close. Before the NEAR spacecraft arrived at Eros, we had never been closer than about 1200 kilometers. We went into about 1000 kilometer orbit last February 2000 and we have been progressively going lower and lower. For about four or five months, we were in a 50 kilometer mapping orbit. And then just recently, we have gone up to 100 kilometers again to get a more global view of the south side of Eros which was not visible at the first part of the mission because the South Pole was pointing away from the sun when we first arrived.
IT WAS IN DARKNESS?
It was in perpetual darkness. We are in orbit around Eros as Eros is in orbit around the Sun. And we’ve had to wait until this part of Eros’s year in order to see the south side.
So, that’s why we went up to the 100 kilometer orbit. It’s from there we will be diving down. We will go into a 50 kilometer circular orbit briefly and then we’ll go down to the 5 kilometers.
THAT’S IN OCTOBER.
That’s on October 25th.
HOW LONG WILL IT ORBIT IN THAT FIVE KILOMETER ALTITUDE?
Not that long. I think the whole thing is going to take about a day. The time we are the closest is going to be five or six hours. Our sequence that we’re focusing the energy and activities, will run for five or six hours.
WHY IS IT THAT YOU WILL KEEP IT DOWN AT 5 OR 6 KILOMETERS FOR ONLY FIVE OR SIX HOURS?
That kind of a close orbit is difficult to maintain.
BECAUSE OF GRAVITATION?
Right, we don’t want to crash into the asteroid, essentially. (laughs) And so they’ll do a burn that will take us in and then several hours after the closest approach time period, they’ll do another burn that will take us out to a larger orbit right away so there is no chance we’re going to crash into the surface.
Another interesting thing I want to mention about going closer. The higher resolution, the finer the details that you can see, the more you are going to understand the nature of that rock material that makes up Eros. And one of the most interesting features on Eros are boulders. I don’t know if you have seen any of the pictures on the web, but Eros is covered with these various sizes of boulders which represent material that has been excavated. When another asteroid impacts Eros, it creates big craters. The material is thrown out into the vicinity of the earth and ultimately lands on the surface again. So, these boulders do represent the rock material that’s on the inside of Eros. So, the closer we can get, the finer detail we can see of these boulders, the more we can understand about the rock material and the structure of the material that makes up the inside of Eros.
There is one more piece of information we have gained. We have a magnetometer on board which would measure the presence of metallic materials. Some asteroids are very heavily concentrated with metallic material. We have not seen any magnetic fields. They have not detected the presence of a magnetic field from Eros which is a scientific result that says there is probably not much metal inside of Eros.
HOW WILL TRYING TO ANALYZE THE STRUCTURE OF EROS OR ANY ASTEROID HELP US IN TERMS OF DEALING WITH ASTEROIDS THAT MIGHT BE COMING TOWARDS THE EARTH CLOSE ENOUGH TO IMPACT. AND I’M THINKING IN TERMS OF SEPTEMBER 2000 IN WHICH THERE HAVE BEEN AT LEAST FIVE SPACE ROCKS THAT HAVE FLOWN PAST THE EARTH SINCE THE FIRST OF SEPTEMBER AND A COUPLE OF THEM WERE NOT MORE THAN 12 TIMES THE DISTANCE TO THE MOON?
Large asteroid impacts with the earth occur very rarely. We’re talking once every 30 million years. But you never know when it’s going to happen. It could happen any day. If it does happen, then it might be good to understand what asteroids are made of, how the rock material – whether it’s one solid piece, what the density is, how strong the material is. And some of the questions we can answer with our orbital mission to Eros.
Then for the remainder of the year, we will spend time in orbits that will be about 35 kilometers or less from the center of Eros. Then in January and February, we are going to start some unusual operations where we will do some more of these low altitude passes. At the very end of the mission, we will gently bring the spacecraft down to the surface taking pictures as we go and hopefully try to lay the spacecraft down on the surface with the objective that if we can lay it down in just the right way we might be able to keep transmitting data from the surface to the earth of pictures while we’re laying on the surface.
THIS MEANS AN ACTUAL LANDING ON EROS IN FEBRUARY. AND WOULD THE SPACECRAFT LEAVE EROS OR STAY THERE?
I think if they could lift it off again that would be a fun thing to do! But it’s not clear that’s possible. This spacecraft was not designed as a lander, so it’s going to be tricky to land it intact.
ARE YOU GOING TO BE BEHIND THE CONTROLS IN SOME WAY WHEN THEY START TRYING TO LAND THE SPACECRAFT ON EROS?
My part of it will be to design the sequences that will tell the spacecraft how and when to take pictures. There will be other teams of people who are in charge of the thruster control, the maneuvers that will bring the spacecraft close to the surface. We’ll all be working together to build the sequences that will operate the spacecraft during those last few hours.
DO YOU HAVE YOURSELF ANY SENSE OF WHERE THE LANDING SITE WILL BE ON THE ASTEROID AND WHAT THE ENVIRONMENT, THE SURROUNDINGS, MIGHT BE?
That is being worked out right now. We’re going to have a series of meetings over the next few months that will determine where the best place is to bring the spacecraft down. There are a lot of factors that will go into that. Some of the factors are where is an area that will provide good viewing. You have to remember that Eros is spinning. It’s a 35 kilometer-long object that is spinning once every 5.3 hours. So, the place that we land is likely going to be somewhere near the South Pole because if you can imagine trying to land on the tip of the thing as it’s spinning once every 5.3 hours, there is too much movement to bring the spacecraft in safely. So the place on the planet that is going to be the most stable is going to be somewhere near one of the poles. It will probably be the South Pole since that is the illuminated side at the time of the year we’re going to land.
WHICH IS FEBRUARY 2001.
WHAT IS DOWN THERE THAT YOU KNOW NOW THAT WOULD BE INTERESTING TO TRY TO LAND CLOSE TO?
With the way our camera optics work, they are designed as a remote sensing instrument that is supposed to take pictures from a distance. So, there is a minimum distance we have. We can’t take pictures like the little Pathfinder camera where you can see something a few meters away. We can only take pictures say that are a half a kilometer or so away. So, what we’ll be looking at on the surface will be the horizon essentially. And as we’re coming down, we’ll be looking at the part of Eros we’ll be able to see. There are a lot of spacecraft constraints when you do something like this.
We get our electrical power from solar panels, so it’s a big game of trying to keep the solar panels on the sun, trying to keep the transmitters on the earth so we can continue to send pictures back and there are a lot of these things. We can’t turn the camera and look straight down. We’ll probably look to the side a little bit whatever direction we’re allowed to point the camera in that will allow us to satisfy all those other engineering constraints at the same time.”
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