Bubbling Spacecraft in the SA skies
Oct 18th 2009
Hundreds of South Africans saw a star-like point moving across the sky while emitting "haloes" or "bubbles" of light just before 9pm on Sunday evening October 18th. Auke Slotegraaf has posted a few of the descriptions.
The photo on the left was taken by Mitchell Krog, who was out photographing lightning at the time!
and an explanation of the pics, from Case Rijsdijk
This strange sight turns out to have been a Centaur rocket carrying out tests as it passed over South Africa nearly three hours after launch.
More detail
The mission (AV-017) is a project of United Launch Alliance - see ulalaunch.com for a nice video of the launch and a 23-page "mission overview".
The purpose was to launch a US Defence Force weather satellite (DMSP F18) into orbit.
After the successful release of the weather satellite into orbit 860km above Earth, the Centaur rocket used its remaining fuel to carry out some tests - these tests were in progress as the Centaur passed northwards over South Africa towards Europe. While passing over Europe, the rocket apparently dumped fuel, causing some excitement among sky-watchers there - see spaceweather.com for photos.
Event timeline (all times are South
African time)
6:12pm - Atlas V rocket launches from Vandenberg Airforce Base
6:16pm - booster rocket shuts down and is jettisoned over the Pacific
- Centaur main engine starts
- payload fairing jettisoned
6:27pm - Centaur main engine shuts down
6:30pm - satellite released
- Collision and Contamination Avoidance Maneuver
6:46pm - testing begins
8:57pm - testing ends
9:10pm - Centaur main engine fires for 4min to send the rocket away from Earth (into an Earth-escape trajectory)
9:17pm - "blowdown" of the fuel tanks
9:56pm - burn off of residual hydrazine
What caused the bubbles?
Possibly venting of fuel during the tests, or firing of "reaction control" motors used to change the orientation or spin of the vehicle.
Tests carried out on the Centaur
Since the DMSP satellite was relatively light, the Centaur rocket had fuel left over after completing its mission of putting the satellite into orbit. The collision and contamination avoidance manoeuvre gets the rocket a safe distance from the satellite.
After this, the Centaur is in free-fall - a (almost) zero-gravity state that can be created on Earth for a short time by dropping something from e.g. a tall tower, or in a plane for a few minutes (e.g. NASA's "Vomit Comet"). The test phase of the Centaur lasted about two hours.
The tests carried out during this time were described by ULA as:
"zero-g, long-coast propellant management . . . we do zero-g settling by very reduced motion of the settling thrusters . . . [and then] settle the propellants against the wall of the tank by spinning the stage with no settling thrusters . . . [and] some venting during zero-g."
Before re-starting the engine, the engineers also planned "a pulsed chilldown demo . . . then some experiments related to the depletion of [the engine]"
One interesting use for these tests is to see how feasible orbiting "filling stations" are. These would be useful for future manned spacecraft missions to the Moon or Mars, that would have to carry large (heavy) amounts of equipment into space. Some of the experiments that would be useful for the design of these orbiting "propellant depots" include spinning the spacecraft to settle the fuel against the sides of the container, and firing small rockets or venting small amounts of fuel to push the fuel against one end of the container. A large fuel tanker in zero-gravity has problems that include leakage of the hydrogen fuel (hydrogen molecules are very small and eventually leak through the walls of containers), and keeping the fuel cool.
United Launch Alliance has a great collection of (fairly technical) publications related to the future development of space exploration - see their publications page, especially this one which mentions using the DMSP-18 mission for zero-g propellant tests.
Last updated: 22 Oct 2009