Erj-145LR flight crew reported an near midair collision on arrival into iah.

Attributes
ACN	1519866
Time
Date	201802
Local Time Of Day	0601-1200
Place
Locale Reference	I90.TRACON
State Reference	TX
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	EMB ERJ 145 ER/LR
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	STAR GESNR ONE
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Person 1
Function	Pilot Flying First Officer
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	ATC Issue All Types Conflict NMAC

Narrative:

We were 20 miles southwest of casst waypoint on the gessner 1 arrival into houston on a downwind at 3;000 ft for runway 8 left when we both noticed on the TCAS a red square [showing an aircraft] 500 ft; then 400 ft; then 300 ft below us and closing in fast. We got a TCAS alert 'traffic; traffic' when it was 300 ft below us and almost underneath us. I clicked off the auto pilot to initiate a climb and the captain decided to take over. The captain initiated a climb to 3;600 ft and I announced to ATC that we were in a climb because we received a traffic alert 300 ft below us. ATC then replied that they could see the target but are not in contact with target. ATC informed us that when able to descend back down to 3;000 ft and fly heading 170 degrees for vectors to the ILS 8L. After clear of traffic captain put the plane back on auto pilot and initiated the localizer and a descent back to 3;000 ft. I was given control of the airplane again and continued with a successful approach and landing.had the captain not taken evasive action we could have hit this target.

Original NASA ASRS Text

Title: ERJ-145LR flight crew reported an NMAC on arrival into IAH.

Narrative: We were 20 miles southwest of CASST waypoint on the Gessner 1 arrival into Houston on a downwind at 3;000 ft for runway 8 left when we both noticed on the TCAS a red square [showing an aircraft] 500 ft; then 400 ft; then 300 ft below us and closing in fast. We got a TCAS alert 'Traffic; Traffic' when it was 300 ft below us and almost underneath us. I clicked off the auto pilot to initiate a climb and the Captain decided to take over. The Captain initiated a climb to 3;600 ft and I announced to ATC that we were in a climb because we received a Traffic alert 300 ft below us. ATC then replied that they could see the target but are not in contact with target. ATC informed us that when able to descend back down to 3;000 ft and fly heading 170 degrees for vectors to the ILS 8L. After clear of traffic Captain put the plane back on auto pilot and initiated the localizer and a descent back to 3;000 ft. I was given control of the airplane again and continued with a successful approach and landing.Had the Captain not taken evasive action we could have hit this target.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.