ALEXIS

USA

Spacecraft nº:

US spacecraft nº:

ALEXIS= Array of Low-Energy X-ray Imaging Sensors - Also called P89-1

Launch data:

Designation	22638 / 93026A
Launch date	25 Apr 1993
Launch site	Edw, B-52, RW04/22
Launch vehicle:	Pegasus #4
Mission	Scientific:EUV astronomy
Earth orbit on:
Perigee / Apogee	749 x 844 km
Eccentricity
Inclination	70 deg
Period	min

Credit: LANL.

When Alexis was launched, a bracket that had been damaged during ground testing broke loose, leaving one of the satellite's four solar panels dangling by only its electrical cabling and throwing off the carefully determined balance of the spin-stabilized spacecraft. When the Alexis team restored contact with the satellite more than two months after launch they discovered that not only did it have a serious wobble, but the spacecraft's magnetometer, which was intended to determine the satellite's orientation by sensing Earth's magnetic field lines, had broken when the solar paddle came loose. The Los Alamos science team and the manufacturers came up with a way to determine the satellite's orientation in May 1994. Since then it has been quite successful.

Alexis was designed to monitored the sky in three narrow energy bands between 0.06 and 0.1 keV; it also carried the Blackbeard experiment to study ionospheric effects on radio transmissions.

Specifications:

Prime contractor	AeroAstro Inc
Operator	USAF STP (Space Test Program)
Platform
Mase at launch	115 kg
Mass in orbit
Dimension	1 m high x 0.6 m diameter
Equipment	Ultrasoft X-ray monitor (6 compact normal-incidence telescopes, 66, 71 and 93 eV), Blackbeard
Solar array	4 solar panels
Stabilization	spin stabilized (2 rpm)
DCpower	50 W
Design lifetime	1 year

Line drawing of the ALEXIS spacecraft in its deployed configuration - Credit: LANL

S-band telemetry & data (2260.5 MHz)

End of live:

Out of service:	29 April 2005
Cause:

Description:

ALEXIS is a small spacecraft built for the Los Alamos National Laboratory (LANL). Its mission is to provide high resolution maps of low-energy astronomical x-ray sources. The mission is also intended to demonstrate the feasibility of quickly building low cost sensors for arms treaty verification. The spacecraft's cost is reported to be approximately $17 million. An anomaly caused one of the satellite's four solar panels to break away from the vehicle during launch, causing a temporary loss of the spacecraft. Ground controllers were eventually able to control the spacecraft and begin on-orbit operations.
Spacecraft: Four deployed solar panels provide 50 watts average power. Four NiCd batteries. Redundant 80C86 processors. S-Band communications with downlink rate 750 kbps and uplink rate of 9.6 kbps. Mass memory of 100 Mbytes EDAC solid state storage. The bus points the payload in the anti-sun direction and rotates at 2 rpm about that sun-line. Sun sensors and limb sensor provide attitude knowledge of 0.1 deg. Attitude is controlled using magnetic torque coils. Single ground station installed at LANL has 2 m antenna.
Payload: An ultrasoft X-ray monitor which consists of 6 compact normal-incidence telescopes tuned to narrow bands centred on 66, 71, and 93 eV. Status as of 1/21/97: ALEXIS continues to operate nominally, sending the results of EUV telescope observations on every pass. ALEXIS has identified a number of transient outbursts in near real time and sometimes is able to supply such target opportunities to operators of the EUVE satellite which contains instruments that are sensitive on similar wavelengths. During occasional "hot times" when ALEXIS is illuminated by the sun during 100% of it's orbit, the telescopes cease operations to avoid overheating the spacecraft (Normally the spacecraft passes through the earth's shadow.) This anomaly was probably caused by a tear in the thermal insulation, associated with the solar panel damage that occurred during launch.

Mission details:

The spacecraft was launched by a Pegasus air-launched booster (of OSC) on April 25, 1993. During powered flight a solar paddle was damaged; initial attempts to contact ALEXIS were unsuccessful. A deployment hinge bracket failed on the +Y paddle. This left the paddle attached to the spacecraft only by a flexible piece of harness (a bundle of wires) and a restraining cable. The launch damage left a solar array paddle free to flap under the influence of thermal inputs, which causes spin rate and direction changes.
As a consequence, an attitude reconstruction algorithm was developed and tested for the ALEXIS spacecraft. It takes inputs from a fine sun sensor and a horizon crossing indicator and processes them using a square-root-information filter/smoother. The filter/smoother model includes Euler dynamics and random-walk torques to approximate thermal/flexible effects.
The satellite responded in June 1993 and soon was brought under control. By late July 1993, full satellite operations had been restored through implementation of new procedures for attitude control. S/C operations are performed from LANL.
Up to 1999 the ALEXIS mission was operating nominally, it exceeded all expectations in spite of the serious launch incident with the solar panel array. A completely new software system (on the ground) was in place to calculate S/C attitude, critical to the on-board operation of the X-ray telescopes. Over 80 GByte of mission data have been received thus far.
The spacecraft, launched in April 1993, operated for almost 12 years before being decommissioned on April 29, 2005. The design life of one year has been exceeded by far

External links:

Satellite tracking: Real time satellite tracking
EOportal.org Satellite description

Ref.: #1, #7, #8, #14, #242 - update: 07.01.15

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