GOCE Satellite Is Falling To Earth But Nobody Knows Where It Will Land

Virtucon writes "The Gravity field and steady-state Ocean Circulation Explorer or GOCE Satellite is expected to fall to Earth this weekend. It weighs over a ton and unfortunately the Scientists don't exactly know where it will land. You can track it here. It should re-enter sometime between Sunday night and Monday morning. Makr Hopkins, chair of the National Society's Executive Committee said: 'The satellite is one of the few satellites in a Polar Orbit. Consequently, it could land almost anywhere.' The GOCE mission was to create an accurate gravity map of the Earth."

Re:Define "irony"

[Immerman]

It's not gravity that's the problem - it's air resistance. Earth's atmosphere doesn't have a distinct edge, and you have to get pretty frelling far out before the particle count drops low enough not to matter to things going 10,000+mph. Certainly a lot farther than the measly few dozen miles to low Earth orbit.

Re:fall to Earth

[qvatch]

Some, but not all of it. http://www.spaceflight101.com/goce-re-entry.html [spaceflight101.com] : With its fins and aerodynamic shape, GOCE will maintain a stable position in orbit as it approaches entry. During entry, the spacecraft will likely remain in that position for the initial phase of re-entry until it breaks up. Following the destruction of the spacecraft, most of its components will harmlessly burn up in the atmosphere. However, it is known that about 20 to 40% of a re-entering satellite's total mass reach Earth's surface. Dense components of satellites usually impact 800 to 1,300 Kilometers downrange from the Orbital Decay Point. Their journey back to Earth is strongly influenced by atmospheric properties like crosswinds that play a major role during atmospheric descent.

Re:fall to Earth

[qvatch]

and http://www.spacesafetymagazine.com/2013/09/14/european-satellite-goce-uncontrolled-reentry/ [spacesafetymagazine.com] his will be the first uncontrolled reentry of an ESA satellite since Isee-2, in 1987. Unfortunately, it will not be the last, considering that the bus-size Envisat’s altitude is gradually decaying in Low-Earth Orbit without control. According to ESA, up to 25% of GOCE’s mass will survive the extreme reentry conditions to fall to the ground. However, the risk for populated areas is very small since the majority of the Earth is covered by oceans. “The major part of what survives to the surface will be the core instrument,” says Dr. Floberghagen. “From the original mass which we have now in space, we have estimated that about 25%, about 250 kilos, will reach the surface, and these 250 kilos will be distributed over between 40 and 50 fragments.” The fragments that survive will hit the ground in a 900 km long footprint. The reentry will be a good test for debris monitoring systems and fragmentation models.

Re:Define "irony"

[Deadstick]

What Immerman said. A satellite deals with gravity just peachy, but air kills it.

No artificial satellite is completely outside the atmosphere. There are still traces of air even hundreds of miles out, and every time a satellite hits an air molecule it loses an eensy-teensy bit of energy. Each loss makes the orbit a little bit lower, and a little bit faster. (Yes, orbital mechanics is a curious realm where you can slow down by applying thrust and speed up by applying the brakes.) The lower it gets, the more often it hits a molecule, and the energy loss gradually begins to snowball.

You can't predict the precise impact point without precise knowledge of the air density the satellite is encountering, and we don't have that information because it varies with all manner of factors, like solar wind and terrestrial weather. The principal means of prediction is the change in the length of an orbit. When you start seeing a measurable time difference from one orbit to the next, things are starting to happen.At that point, you can predict the time of impact with a precision on the order of weeks, and as time goes on you can narrow it down further.

Right now, we know when GOCE will come in give or take a handful of hours -- and since it can circle the world a couple of times in that interval, we have very little idea of where it will hit. As time passes, the error factor shrinks...when Skylab came in, NASA knew it would hit "somewhere in Australia" three or four hours before it hit.

An intentional reentry is different, because you use a retro-rocket to dump a nice big packet of energy and skip right over the protracted decay time, and make it land where you want.

In the interest of perspective, keep in mind that Nature throws rocks at us from space all the time -- meteors big enough to survive the trip through the atmosphere hit the earth dozens of times per day. Yet there are only a handful of cases on record where a person was injured, or even saw one hit -- simply because you and I and all the other people cover a VERY tiny fraction of the earth's surface. We are little bitty spots on a great big dartboard.

Find out if you might get hit

[ral]

Check out the prediction web site [n2yo.com] to see if it might land near you. Be sure to click the "show all passes" button to see the daylight passes in addition to the night passes. It calculates your lat/long from your ip address, then builds a table of overhead passes in the next 5 days. Look at the "El" column. That's the maximum degrees from the nearest horizon. If you see a number near 90 between Sunday night and Monday morning, watch out. Otherwise, rest easy.